A.18.3 The Generic Package Containers.Doubly_Linked_Lists
{
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The language-defined generic package Containers.Doubly_Linked_Lists provides
private types List and Cursor, and a set of operations for each type.
A list container is optimized for insertion and deletion at any position.
{
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A doubly-linked list container object manages a linked
list of internal
nodes, each of which contains an element and
pointers to the next (successor) and previous (predecessor) internal
nodes. A cursor designates a particular node within a list (and by extension
the element contained in that node). A cursor keeps designating the same
node (and element) as long as the node is part of the container, even
if the node is moved in the container.
{
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The
length of a list is the number of elements it contains.
Static Semantics
{
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The generic library package Containers.Doubly_Linked_Lists has the following
declaration:
{
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{
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{
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with Ada.Iterator_Interfaces;
generic
type Element_Type
is private;
with function "=" (Left, Right : Element_Type)
return Boolean
is <>;
package Ada.Containers.Doubly_Linked_Lists
is
with Preelaborate, Remote_Types,
Nonblocking, Global => in out synchronized is
pragma Preelaborate(Doubly_Linked_Lists);
pragma Remote_Types(Doubly_Linked_Lists);Discussion: {
AI12-0112-1}
For discussion on the reasons and meaning of the
specifications of the Global and Nonblocking aspects in this generic
package, see the notes on the equivalent operations in the specification
of the Containers.Vectors package (see A.18.2).
{
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{
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{
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{
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{
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{
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{
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{
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type List
is tagged private
with Constant_Indexing => Constant_Reference,
Variable_Indexing => Reference,
Default_Iterator => Iterate,
Iterator_Element => Element_Type
,
Iterator_View => Stable.List,
Aggregate => (Empty => Empty,
Add_Unnamed => Append),
Stable_Properties => (Length,
Tampering_With_Cursors_Prohibited,
Tampering_With_Elements_Prohibited),
Default_Initial_Condition =>
Length (List) = 0 and then
(not Tampering_With_Cursors_Prohibited (List)) and then
(not Tampering_With_Elements_Prohibited (List)),;
pragma Preelaborable_Initialization
(List);
{
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type Cursor
is private;
with pragma Preelaborable_Initialization
(Cursor);
Empty_List :
constant List;
No_Element :
constant Cursor;
{
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{
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function Has_Element (Position : Cursor)
return Boolean
with Nonblocking, Global => in all, Use_Formal => null;
{
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function Has_Element (Container : List; Position : Cursor)
return Boolean
with Nonblocking, Global => null, Use_Formal => null;{
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package List_Iterator_Interfaces
is new
Ada.Iterator_Interfaces (Cursor, Has_Element);
function "=" (Left, Right : List) return Boolean;
{
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function Tampering_With_Cursors_Prohibited
(Container : List) return Boolean
with Nonblocking, Global => null, Use_Formal => null;{
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function Tampering_With_Elements_Prohibited
(Container : List) return Boolean
with Nonblocking, Global => null, Use_Formal => null;{
AI12-0339-1}
function Empty return List
is (Empty_List)
with Post =>
not Tampering_With_Elements_Prohibited (Empty'Result) and then
not Tampering_With_Cursors_Prohibited (Empty'Result) and then
Length (Empty'Result) = 0;{
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function Length (Container : List)
return Count_Type
with Nonblocking, Global => null, Use_Formal => null;
{
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function Is_Empty (Container : List)
return Boolean
with Nonblocking, Global => null, Use_Formal => null,
Post => Is_Empty'Result = (Length (Container) = 0);
{
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procedure Clear (Container :
in out List)
with Pre => not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error,
Post => Length (Container) = 0;
{
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function Element (Position : Cursor)
return Element_Type
with Pre => Position /= No_Element or else raise Constraint_Error,
Nonblocking, Global => in all, Use_Formal => Element_Type;
{
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function Element (Container : List;
Position : Cursor) return Element_Type
with Pre => (Position /= No_Element or else
raise Constraint_Error) and then
(Has_Element (Container, Position)
or else raise Program_Error),
Nonblocking, Global => null, Use_Formal => Element_Type;{
AI12-0112-1}
procedure Replace_Element (Container :
in out List;
Position :
in Cursor;
New_item :
in Element_Type)
with Pre => (not Tampering_With_Elements_Prohibited (Container)
or else raise Program_Error) and then
(Position /= No_Element
or else raise Constraint_Error) and then
(Has_Element (Container, Position)
or else raise Program_Error);
{
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procedure Query_Element
(Position :
in Cursor;
Process :
not null access procedure (Element :
in Element_Type))
with Pre => Position /= No_Element or else raise Constraint_Error,
Global => in all;
{
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procedure Query_Element
(Container : in List;
Position : in Cursor;
Process : not null access procedure (Element : in Element_Type))
with Pre => (Position /= No_Element
or else raise Constraint_Error) and then
(Has_Element (Container, Position)
or else raise Program_Error);{
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procedure Update_Element
(Container :
in out List;
Position :
in Cursor;
Process :
not null access procedure
(Element :
in out Element_Type))
with Pre => (Position /= No_Element
or else raise Constraint_Error) and then
(Has_Element (Container, Position)
or else raise Program_Error);
{
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{
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type Constant_Reference_Type
(Element :
not null access constant Element_Type)
is private
with Implicit_Dereference => Element
,
Nonblocking, Global => in out synchronized,
Default_Initial_Condition => (raise Program_Error);
{
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{
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type Reference_Type (Element :
not null access Element_Type)
is private
with Implicit_Dereference => Element
,
Nonblocking, Global => in out synchronized,
Default_Initial_Condition => (raise Program_Error);
{
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{
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function Constant_Reference (Container :
aliased in List;
Position :
in Cursor)
return Constant_Reference_Type
with Pre => (Position /= No_Element or else
raise Constraint_Error) and then
(Has_Element (Container, Position) or else
raise Program_Error),
Post => Tampering_With_Cursors_Prohibited (Container),
Nonblocking, Global => null, Use_Formal => null;
{
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{
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function Reference (Container :
aliased in out List;
Position :
in Cursor)
return Reference_Type
with Pre => (Position /= No_Element or else
raise Constraint_Error) and then
(Has_Element (Container, Position) or else
raise Program_Error),
Post => Tampering_With_Cursors_Prohibited (Container),
Nonblocking, Global => null, Use_Formal => null;
{
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{
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procedure Assign (Target :
in out List; Source :
in List)
with Pre => not Tampering_With_Cursors_Prohibited (Target)
or else raise Program_Error,
Post => Length (Source) = Length (Target);
{
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{
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function Copy (Source : List)
return List
with Post =>
Length (Copy'Result) = Length (Source) and then
not Tampering_With_Elements_Prohibited (Copy'Result) and then
not Tampering_With_Cursors_Prohibited (Copy'Result);
{
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procedure Move (Target :
in out List;
Source :
in out List)
with Pre => (not Tampering_With_Cursors_Prohibited (Target)
or else raise Program_Error) and then
(not Tampering_With_Cursors_Prohibited (Source)
or else raise Program_Error),
Post => (if not Target'Has_Same_Storage (Source) then
Length (Target) = Length (Source'Old) and then
Length (Source) = 0);
{
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procedure Insert (Container :
in out List;
Before :
in Cursor;
New_Item :
in Element_Type;
Count :
in Count_Type := 1)
with Pre => (not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error) and then
(Before = No_Element or else
Has_Element (Container, Before)
or else raise Program_Error) and then
(Length (Container) <= Count_Type'Last - Count
or else raise Constraint_Error),
Post => Length (Container)'Old + Count = Length (Container);
{
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procedure Insert (Container :
in out List;
Before :
in Cursor;
New_Item :
in Element_Type;
Position :
out Cursor;
Count :
in Count_Type := 1)
with Pre => (not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error) and then
(Before = No_Element or else
Has_Element (Container, Before)
or else raise Program_Error) and then
(Length (Container) <= Count_Type'Last - Count
or else raise Constraint_Error),
Post => Length (Container)'Old + Count = Length (Container)
and then Has_Element (Container, Position);
{
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procedure Insert (Container :
in out List;
Before :
in Cursor;
Position :
out Cursor;
Count :
in Count_Type := 1)
with Pre => (not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error) and then
(Before = No_Element or else
Has_Element (Container, Before)
or else raise Program_Error) and then
(Length (Container) <= Count_Type'Last - Count
or else raise Constraint_Error),
Post => Length (Container)'Old + Count = Length (Container)
and then Has_Element (Container, Position);
{
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procedure Prepend (Container :
in out List;
New_Item :
in Element_Type;
Count :
in Count_Type := 1)
with Pre => (not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error) and then
(Length (Container) <= Count_Type'Last - Count
or else raise Constraint_Error),
Post => Length (Container)'Old + Count = Length (Container);
{
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{
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procedure Append (Container :
in out List;
New_Item :
in Element_Type;
Count :
in Count_Type
:= 1)
with Pre => (not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error) and then
(Length (Container) <= Count_Type'Last - Count
or else raise Constraint_Error),
Post => Length (Container)'Old + Count = Length (Container);
{
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{
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procedure Append (Container : in out List;
New_Item : in Element_Type)
with Pre => (not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error) and then
(Length (Container) <= Count_Type'Last - 1
or else raise Constraint_Error),
Post => Length (Container)'Old + 1 = Length (Container);{
AI12-0112-1}
procedure Delete (Container :
in out List;
Position :
in out Cursor;
Count :
in Count_Type := 1)
with Pre => (not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error) and then
(Position /= No_Element
or else raise Constraint_Error) and then
(Has_Element (Container, Position)
or else raise Program_Error),
Post => Length (Container)'Old - Count <= Length (Container)
and then Position = No_Element;
{
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procedure Delete_First (Container :
in out List;
Count :
in Count_Type := 1)
with Pre => not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error,
Post => Length (Container)'Old - Count <= Length (Container);
{
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procedure Delete_Last (Container :
in out List;
Count :
in Count_Type := 1)
with Pre => not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error,
Post => Length (Container)'Old - Count <= Length (Container);
{
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procedure Reverse_Elements (Container :
in out List)
with Pre => not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error;
{
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procedure Swap (Container :
in out List;
I, J :
in Cursor)
with Pre => (not Tampering_With_Elements_Prohibited (Container)
or else raise Program_Error) and then
(I /= No_Element or else Constraint_Error) and then
(J /= No_Element or else Constraint_Error) and then
(Has_Element (Container, I)
or else raise Program_Error) and then
(Has_Element (Container, J)
or else raise Program_Error);
{
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procedure Swap_Links (Container :
in out List;
I, J :
in Cursor)
with Pre => (not Tampering_With_Elements_Prohibited (Container)
or else raise Program_Error) and then
(I /= No_Element or else Constraint_Error) and then
(J /= No_Element or else Constraint_Error) and then
(Has_Element (Container, I)
or else raise Program_Error) and then
(Has_Element (Container, J)
or else raise Program_Error);
{
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procedure Splice (Target :
in out List;
Before :
in Cursor;
Source :
in out List)
with Pre => (not Tampering_With_Cursors_Prohibited (Target)
or else raise Program_Error) and then
(not Tampering_With_Cursors_Prohibited (Source)
or else raise Program_Error) and then
(Before = No_Element or else
Has_Element (Target, Before)
or else raise Program_Error) and then
(Target'Has_Same_Storage (Source) or else
Length (Target) <= Count_Type'Last - Length (Source)
or else raise Constraint_Error),
Post => (if not Target'Has_Same_Storage (Source) then
(declare
Result_Length : constant Count_Type :=
Length (Source)'Old + Length (Target)'Old;
begin
Length (Source) = 0 and then
Length (Target) = Result_Length));
{
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procedure Splice (Target :
in out List;
Before :
in Cursor;
Source :
in out List;
Position :
in out Cursor)
with Pre => (not Tampering_With_Cursors_Prohibited (Target)
or else raise Program_Error) and then
(not Tampering_With_Cursors_Prohibited (Source)
or else raise Program_Error) and then
(Position /= No_Element
or else raise Constraint_Error) and then
(Has_Element (Source, Position)
or else raise Program_Error) and then
(Before = No_Element or else
Has_Element (Target, Before)
or else raise Program_Error) and then
(Target'Has_Same_Storage (Source) or else
Length (Target) <= Count_Type'Last - 1
or else raise Constraint_Error),
Post => (declare
Org_Target_Length : constant Count_Type :=
Length (Target)'Old;
Org_Source_Length : constant Count_Type :=
Length (Source)'Old;
begin
(if Target'Has_Same_Storage (Source) then
Position = Position'Old
else
Length (Source) = Org_Source_Length - 1 and then
Length (Target) = Org_Target_Length + 1 and then
Has_Element (Target, Position)));
procedure Splice (Container:
in out List;
Before :
in Cursor;
Position :
in Cursor)
with Pre => (not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error) and then
(Position /= No_Element
or else raise Constraint_Error) and then
(Has_Element (Container, Position)
or else raise Program_Error) and then
(Before = No_Element or else
Has_Element (Container, Before)
or else raise Program_Error),
Post => Length (Container) = Length (Container)'Old;
{
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function First (Container : List)
return Cursor
with Nonblocking, Global => null, Use_Formal => null,
Post => (if not Is_Empty (Container)
then Has_Element (Container, First'Result)
else First'Result = No_Element);
{
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function First_Element (Container : List)
return Element_Type
with Pre => (not Is_Empty (Container)
or else raise Constraint_Error);
{
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function Last (Container : List)
return Cursor
with Nonblocking, Global => null, Use_Formal => null,
Post => (if not Is_Empty (Container)
then Has_Element (Container, Last'Result)
else Last'Result = No_Element);
{
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function Last_Element (Container : List)
return Element_Type
with Pre => (not Is_Empty (Container)
or else raise Constraint_Error);
{
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function Next (Position : Cursor)
return Cursor
with Nonblocking, Global => in all, Use_Formal => null,
Post => (if Position = No_Element then Next'Result = No_Element);
{
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function Next (Container : List;
Position : Cursor) return Cursor
with Nonblocking, Global => null, Use_Formal => null,
Pre => Position = No_Element or else
Has_Element (Container, Position)
or else raise Program_Error,
Post => (if Position = No_Element then Next'Result = No_Element
elsif Next'Result = No_Element then
Position = Last (Container)
else Has_Element (Container, Next'Result));{
AI12-0112-1}
function Previous (Position : Cursor)
return Cursor
with Nonblocking, Global => in all, Use_Formal => null,
Post => (if Position = No_Element then
Previous'Result = No_Element);
{
AI12-0112-1}
function Previous (Container : List;
Position : Cursor) return Cursor
with Nonblocking, Global => null, Use_Formal => null,
Pre => Position = No_Element or else
Has_Element (Container, Position)
or else raise Program_Error,
Post => (if Position = No_Element then
Previous'Result = No_Element
elsif Previous'Result = No_Element then
Position = First (Container)
else Has_Element (Container, Previous'Result));{
AI12-0112-1}
procedure Next (Position :
in out Cursor)
with Nonblocking, Global => in all, Use_Formal => null;
{
AI12-0112-1}
procedure Next (Container : in List;
Position : in out Cursor)
with Nonblocking, Global => null, Use_Formal => null,
Pre => Position = No_Element or else
Has_Element (Container, Position)
or else raise Program_Error,
Post => (if Position /= No_Element
then Has_Element (Container, Position));{
AI12-0112-1}
procedure Previous (Position :
in out Cursor)
with Nonblocking, Global => in all, Use_Formal => null;
{
AI12-0112-1}
procedure Previous (Container : in List;
Position : in out Cursor)
with Nonblocking, Global => null, Use_Formal => null,
Pre => Position = No_Element or else
Has_Element (Container, Position)
or else raise Program_Error,
Post => (if Position /= No_Element then
Has_Element (Container, Position));{
AI12-0112-1}
function Find (Container : List;
Item : Element_Type;
Position : Cursor := No_Element)
return Cursor
with Pre => Position = No_Element or else
Has_Element (Container, Position)
or else raise Program_Error,
Post => (if Find'Result /= No_Element
then Has_Element (Container, Find'Result));
{
AI12-0112-1}
function Reverse_Find (Container : List;
Item : Element_Type;
Position : Cursor := No_Element)
return Cursor
with Pre => Position = No_Element or else
Has_Element (Container, Position)
or else raise Program_Error,
Post => (if Reverse_Find'Result /= No_Element
then Has_Element (Container, Reverse_Find'Result));
function Contains (Container : List;
Item : Element_Type)
return Boolean;
{
AI12-0112-1}
procedure Iterate
(Container :
in List;
Process :
not null access procedure (Position :
in Cursor))
with Allows_Exit;
{
AI12-0112-1}
procedure Reverse_Iterate
(Container :
in List;
Process :
not null access procedure (Position :
in Cursor))
with Allows_Exit;
{
AI05-0212-1}
{
AI12-0112-1}
{
AI12-0266-1}
function Iterate (Container :
in List)
return List_Iterator_Interfaces.
Parallel_Reversible_Iterator Reversible_Iterator'Class
with Post => Tampering_With_Cursors_Prohibited (Container);
{
AI05-0212-1}
{
AI12-0112-1}
function Iterate (Container :
in List; Start :
in Cursor)
return List_Iterator_Interfaces.Reversible_Iterator'Class
with Pre => (Start /= No_Element
or else raise Constraint_Error) and then
(Has_Element (Container, Start)
or else raise Program_Error),
Post => Tampering_With_Cursors_Prohibited (Container);
{
AI12-0112-1}
generic
with function "<" (Left, Right : Element_Type)
return Boolean is <>;
package Generic_Sorting
with Nonblocking, Global => null is function Is_Sorted (Container : List)
return Boolean;
{
AI12-0112-1}
procedure Sort (Container :
in out List)
with Pre => not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error;
{
AI12-0112-1}
procedure Merge (Target :
in out List;
Source :
in out List)
with Pre => (not Tampering_With_Cursors_Prohibited (Target)
or else raise Program_Error) and then
(not Tampering_With_Elements_Prohibited (Source)
or else raise Program_Error) and then
(Length (Target) <= Count_Type'Last - Length (Source)
or else raise Constraint_Error) and then
((Length (Source) = 0 or else
not Target'Has_Same_Storage (Source))
or else raise Constraint_Error),
Post => (declare
Result_Length : constant Count_Type :=
Length (Source)'Old + Length (Target)'Old;
begin
(Length (Source) = 0 and then
Length (Target) = Result_Length));
end Generic_Sorting;
{
AI12-0111-1}
{
AI12-0339-1}
{
AI12-0391-1}
{
AI12-0400-1}
{
AI12-0407-1}
type List (Base : not null access Doubly_Linked_Lists.List) is
tagged limited private
with Constant_Indexing => Constant_Reference,
Variable_Indexing => Reference,
Default_Iterator => Iterate,
Iterator_Element => Element_Type,
Stable_Properties => (Length),
Global => null,
Default_Initial_Condition => Length (List) = 0,
Preelaborable_Initialization;{
AI12-0111-1}
type Cursor is private
with Preelaborable_Initialization;{
AI12-0111-1}
function Has_Element (Position : Cursor) return Boolean
with Nonblocking, Global => in all, Use_Formal => null;{
AI12-0111-1}
package List_Iterator_Interfaces is new
Ada.Iterator_Interfaces (Cursor, Has_Element);{
AI12-0111-1}
procedure Assign (Target : in out Doubly_Linked_Lists.List;
Source : in List)
with Post => Length (Source) = Length (Target);{
AI12-0111-1}
function Copy (Source : Doubly_Linked_Lists.List) return List
with Post => Length (Copy'Result) = Length (Source);{
AI12-0111-1}
type Constant_Reference_Type
(Element : not null access constant Element_Type) is private
with Implicit_Dereference => Element,
Nonblocking, Global => null, Use_Formal => null,
Default_Initial_Condition => (raise Program_Error);{
AI12-0111-1}
type Reference_Type
(Element : not null access Element_Type) is private
with Implicit_Dereference => Element,
Nonblocking, Global => null, Use_Formal => null,
Default_Initial_Condition => (raise Program_Error);{
AI12-0111-1}
-- Additional subprograms as described in the text
-- are declared here.private
... -- not specified by the language
end Ada.Containers.Doubly_Linked_Lists;
{
AI95-00302-03}
The actual function for the generic formal function "=" on
Element_Type values is expected to define a reflexive and symmetric relationship
and return the same result value each time it is called with a particular
pair of values. If it behaves in some other manner, the functions Find,
Reverse_Find, and "=" on list values return an unspecified
value. The exact arguments and number of calls of this generic formal
function by the functions Find, Reverse_Find, and "=" on list
values are unspecified.
Ramification: If the actual function
for "=" is not symmetric and consistent, the result returned
by the listed functions cannot be predicted. The implementation is not
required to protect against "=" raising an exception, or returning
random results, or any other “bad” behavior. And it can call
"=" in whatever manner makes sense. But note that only the
results of Find, Reverse_Find, and List "=" are unspecified;
other subprograms are not allowed to break if "=" is bad (they
aren't expected to use "=").
{
AI95-00302-03}
The type List is used to represent lists. The type List needs finalization
(see
7.6).
{
AI95-00302-03}
Empty_List represents the empty List object. It has a length of 0. If
an object of type List is not otherwise initialized, it is initialized
to the same value as Empty_List.
{
AI95-00302-03}
No_Element represents a cursor that designates no element. If an object
of type Cursor is not otherwise initialized, it is initialized to the
same value as No_Element.
{
AI95-00302-03}
{
AI12-0434-1}
The
primitive predefined
"=" operator for type Cursor returns True if both cursors are
No_Element, or designate the same element in the same container.
To be honest: {
AI12-0434-1}
“The primitive "=" operator”
is the one with two parameters of type Cursor which returns Boolean.
We're not talking about some other (hidden) primitive function named
"=".
{
AI95-00302-03}
Execution of the default implementation of the Input, Output, Read, or
Write attribute of type Cursor raises Program_Error.
Reason: A cursor will probably be implemented
in terms of one or more access values, and the effects of streaming access
values is unspecified. Rather than letting the user stream junk by accident,
we mandate that streaming of cursors raise Program_Error by default.
The attributes can always be specified if there is a need to support
streaming.
{
AI05-0001-1}
{
AI05-0262-1}
{
AI12-0437-1}
List'Write for a List object
L writes Length(
L) elements
of the list to the stream. It
may also
may write additional information about the list.
{
AI05-0001-1}
{
AI05-0262-1}
List'Read reads the representation of a list from the stream, and assigns
to
Item a list with the same length and elements as was written
by List'Write.
Ramification: Streaming more elements
than the container length is wrong. For implementation implications of
this rule, see the Implementation Note in
A.18.2.
{
AI95-00302-03}
{
AI12-0111-1}
{
AI12-0112-1}
[Some operations
of this generic package have access-to-subprogram parameters. To ensure
such operations are well-defined, they guard against certain actions
by the designated subprogram. In particular, some operations check
for “tampering with cursors” of a container because they
depend on the set of elements of the container remaining constant, and
others check for “tampering with elements” of a container
because they depend on elements of the container not being replaced.]
When tampering with cursors is prohibited
for a particular list object L, Program_Error
is propagated by the finalization of L[, as well as by a call
that passes L to certain of the operations of this package, as
indicated by the precondition of such an operation]. Similarly, when
tampering with elements is prohibited for L, Program_Error
is propagated by a call that passes L to certain of the other
operations of this package, as indicated by the precondition of such
an operation.Paragraphs
62 through 69 are removed as preconditions now describe these rules.
it inserts or deletes elements
of L, that is, it calls the Insert, Clear, Delete, or Delete_Last
procedures with L as a parameter; or
To be honest: Operations
which are defined to be equivalent to a call on one of these operations
also are included. Similarly, operations which call one of these as part
of their definition are included.
it reorders the elements
of L, that is, it calls the Splice, Swap_Links, or Reverse_Elements
procedures or the Sort or Merge procedures of an instance of Generic_Sorting
with L as a parameter; or
it finalizes L; or
{
AI05-0001-1}
it calls the Assign procedure with L as
the Target parameter; orRamification: We don't need to explicitly
mention
assignment_statement,
because that finalizes the target object as part of the operation, and
finalization of an object is already defined as tampering with cursors.
it calls the Move procedure
with L as a parameter.
Reason: Swap copies
elements rather than reordering them, so it doesn't tamper with cursors.
it tampers with cursors of
L; or
it replaces one or more elements
of L, that is, it calls the Replace_Element or Swap procedures
with L as a parameter.
Reason: Complete
replacement of an element can cause its memory to be deallocated while
another operation is holding onto a reference to it. That can't be allowed.
However, a simple modification of (part of) an element is not a problem,
so Update_Element does not cause a problem.
{
AI05-0265-1}
{
AI12-0110-1}
{
AI12-0111-1}
{
AI12-0112-1}
When tampering
with cursors is prohibited for a particular list object L,
Program_Error is propagated by a call of any language-defined subprogram
that is defined to tamper with the cursors of L, leaving L
unmodified. Similarly, when tampering with elements is prohibited
for a particular list object L, Program_Error is propagated by
a call of any language-defined subprogram that is defined to tamper with
the elements of L [(or tamper with the cursors of L)],
leaving L unmodified. These checks are made before any other defined behavior of the body of
the language-defined subprogram.
Proof: Tampering
with elements includes tampering with cursors, so we mention it only
from completeness in the second sentence.
function Has_Element (Position : Cursor) return Boolean
with Nonblocking, Global => in all, Use_Formal => null;
{
AI05-0212-1}
Returns True if Position designates an element, and returns False otherwise.
To be honest: {
AI05-0005-1}
{
AI05-0212-1}
This function might not detect cursors that designate deleted elements;
such cursors are invalid (see below) and the result of calling Has_Element
with an invalid cursor is unspecified (but not erroneous).
function Has_Element (Container : List; Position : Cursor)
return Boolean
with Nonblocking, Global => null, Use_Formal => null;
{
AI12-0112-1}
Returns True if Position designates an element
in Container, and returns False otherwise.Ramification: If
Position is No_Element, Has_Element returns False.
function "=" (Left, Right : List) return Boolean;
{
AI95-00302-03}
{
AI05-0264-1}
If Left and Right denote the same list object, then the function returns
True. If Left and Right have different lengths, then the function returns
False. Otherwise, it compares each element in Left to the corresponding
element in Right using the generic formal equality operator. If any such
comparison returns False, the function returns False; otherwise, it returns
True. Any exception raised during evaluation of element equality is propagated.
Implementation Note: This wording describes
the canonical semantics. However, the order and number of calls on the
formal equality function is unspecified for all of the operations that
use it in this package, so an implementation can call it as many or as
few times as it needs to get the correct answer. Specifically, there
is no requirement to call the formal equality additional times once the
answer has been determined.
function Tampering_With_Cursors_Prohibited
(Container : List) return Boolean
with Nonblocking, Global => null, Use_Formal => null;
{
AI12-0112-1}
Returns True if tampering with cursors or tampering
with elements is currently prohibited for Container, and returns False
otherwise.Reason: {
AI12-0112-1}
Prohibiting tampering with elements also needs
to prohibit tampering with cursors, as deleting an element is similar
to replacing it.
Implementation Note:
{
AI12-0112-1}
Various contracts elsewhere in this specification
require that this function be implemented with synchronized data. Moreover,
it is possible for tampering to be prohibited by multiple operations
(sequentially or in parallel). Therefore, tampering needs to be implemented
with an atomic or protected counter. The counter is initialized to zero,
and is incremented when tampering is prohibited, and decremented when
leaving an area that prohibited tampering. Function Tampering_With_Cursors_Prohibited
returns True if the counter is nonzero. (Note that any case where the
result is not well-defined for one task is incorrect use of shared variables
and would be erroneous by the rules of 9.10,
so no special protection is needed to read the counter.)
function Tampering_With_Elements_Prohibited
(Container : List) return Boolean
with Nonblocking, Global => null, Use_Formal => null;
{
AI12-0112-1}
Always returns False[, regardless of whether tampering
with elements is prohibited].Reason: {
AI12-0111-1}
A definite element cannot change size, so we allow
operations that tamper with elements even when tampering with elements
is prohibited. That's not true for the indefinite containers, which is
why this kind of tampering exists.
function Length (Container : List) return Count_Type
with Nonblocking, Global => null, Use_Formal => null;
function Is_Empty (Container : List) return Boolean
with Nonblocking, Global => null, Use_Formal => null,
Post => Is_Empty'Result = (Length (Container) = 0);
procedure Clear (Container : in out List)
with Pre => not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error,
Post => Length (Container) = 0;
function Element (Position : Cursor) return Element_Type
with Pre => Position /= No_Element or else raise Constraint_Error,
Nonblocking, Global => in all, Use_Formal => Element_Type;
{
AI95-00302-03}
{
AI12-0112-1}
If Position equals No_Element, then Constraint_Error
is propagated. Otherwise, Element returns the element designated
by Position.
function Element (Container : List;
Position : Cursor) return Element_Type
with Pre => (Position /= No_Element or else
raise Constraint_Error) and then
(Has_Element (Container, Position)
or else raise Program_Error),
Nonblocking, Global => null, Use_Formal => Element_Type;
{
AI12-0112-1}
Element returns the element designated by Position
in Container.procedure Replace_Element (Container : in out List;
Position : in Cursor;
New_item : in Element_Type)
with Pre => (not Tampering_With_Elements_Prohibited (Container)
or else raise Program_Error) and then
(Position /= No_Element
or else raise Constraint_Error) and then
(Has_Element (Container, Position)
or else raise Program_Error);
{
AI95-00302-03}
{
AI05-0264-1}
{
AI12-0112-1}
{
AI12-0196-1}
If Position equals No_Element, then Constraint_Error
is propagated; if Position does not designate an element in Container,
then Program_Error is propagated. Otherwise, Replace_Element assigns
the value New_Item to the element designated by Position.
For the purposes of determining whether the parameters overlap in a call
to Replace_Element, the Container parameter is not considered to overlap
with any object [(including itself)].procedure Query_Element
(Position : in Cursor;
Process : not null access procedure (Element : in Element_Type))
with Pre => Position /= No_Element or else raise Constraint_Error,
Global => in all;
{
AI95-00302-03}
{
AI05-0021-1}
{
AI05-0265-1}
{
AI12-0112-1}
If Position equals No_Element, then Constraint_Error
is propagated. Otherwise, Query_Element calls Process.
all
with the element designated by Position as the argument. Tampering with
the elements of the list that contains the element designated by Position
is prohibited during the execution of the call on Process.
all.
Any exception raised by Process.
all is propagated.
procedure Query_Element
(Container : in List;
Position : in Cursor;
Process : not null access procedure (Element : in Element_Type))
with Pre => (Position /= No_Element
or else raise Constraint_Error) and then
(Has_Element (Container, Position)
or else raise Program_Error);
{
AI12-0112-1}
Query_Element calls Process.all with the
element designated by Position as the argument. Tampering with the elements
of Container is prohibited during the execution of the call on Process.all.
Any exception raised by Process.all is propagated.procedure Update_Element
(Container : in out List;
Position : in Cursor;
Process : not null access procedure
(Element : in out Element_Type))
with Pre => (Position /= No_Element
or else raise Constraint_Error) and then
(Has_Element (Container, Position)
or else raise Program_Error);
{
AI95-00302-03}
{
AI05-0264-1}
{
AI05-0265-1}
{
AI12-0112-1}
If Position equals No_Element, then Constraint_Error
is propagated; if Position does not designate an element in Container,
then Program_Error is propagated. Otherwise, Update_Element calls
Process.
all with the element designated by Position as the argument.
Tampering with the elements of Container is prohibited during the execution
of the call on Process.
all. Any exception raised by Process.
all
is propagated.
If Element_Type
is unconstrained and definite, then the actual Element parameter of Process.all
shall be unconstrained.
Ramification: This means that the elements
cannot be directly allocated from the heap; it must be possible to change
the discriminants of the element in place.
{
AI12-0112-1}
type Constant_Reference_Type
(Element :
not null access constant Element_Type)
is private
with Implicit_Dereference => Element
,
Nonblocking, Global => in out synchronized,
Default_Initial_Condition => (raise Program_Error);
{
AI12-0112-1}
type Reference_Type (Element :
not null access Element_Type)
is private
with Implicit_Dereference => Element
,
Nonblocking, Global => in out synchronized,
Default_Initial_Condition => (raise Program_Error);
{
AI05-0212-1}
The types Constant_Reference_Type and Reference_Type need finalization.
This paragraph
was deleted.{
AI12-0112-1}
The default initialization of an object of type
Constant_Reference_Type or Reference_Type propagates Program_Error.
Reason: It is expected that Reference_Type
(and Constant_Reference_Type) will be a controlled type, for which finalization
will have some action to terminate the tampering check for the associated
container. If the object is created by default, however, there is no
associated container. Since this is useless, and supporting this case
would take extra work, we define it to raise an exception.
{
AI12-0112-1}
function Constant_Reference (Container :
aliased in List;
Position :
in Cursor)
return Constant_Reference_Type
with Pre => (Position /= No_Element or else
raise Constraint_Error) and then
(Has_Element (Container, Position) or else
raise Program_Error),
Post => Tampering_With_Cursors_Prohibited (Container),
Nonblocking, Global => null, Use_Formal => null;
{
AI05-0212-1}
{
AI05-0269-1}
This function (combined with the Constant_Indexing and Implicit_Dereference
aspects) provides a convenient way to gain read access to an individual
element of a list given a cursor.
{
AI05-0212-1}
{
AI05-0265-1}
{
AI12-0112-1}
If Position equals No_Element, then Constraint_Error
is propagated; if Position does not designate an element in Container,
then Program_Error is propagated. Otherwise, Constant_Reference
returns an object whose discriminant is an access value that designates
the element designated by Position. Tampering with the elements of Container
is prohibited while the object returned by Constant_Reference exists
and has not been finalized.
{
AI12-0112-1}
function Reference (Container :
aliased in out List;
Position :
in Cursor)
return Reference_Type
with Pre => (Position /= No_Element or else
raise Constraint_Error) and then
(Has_Element (Container, Position) or else
raise Program_Error),
Post => Tampering_With_Cursors_Prohibited (Container),
Nonblocking, Global => null, Use_Formal => null;
{
AI05-0212-1}
{
AI05-0269-1}
This function (combined with the Variable_Indexing and Implicit_Dereference
aspects) provides a convenient way to gain read and write access to an
individual element of a list given a cursor.
{
AI05-0212-1}
{
AI05-0265-1}
{
AI12-0112-1}
If Position equals No_Element, then Constraint_Error
is propagated; if Position does not designate an element in Container,
then Program_Error is propagated. Otherwise, Reference returns
an object whose discriminant is an access value that designates the element
designated by Position. Tampering with the elements of Container is prohibited
while the object returned by Reference exists and has not been finalized.
{
AI12-0112-1}
procedure Assign (Target :
in out List; Source :
in List)
with Pre => not Tampering_With_Cursors_Prohibited (Target)
or else raise Program_Error,
Post => Length (Source) = Length (Target);
{
AI05-0001-1}
{
AI05-0248-1}
If Target denotes the same object as Source, the operation has no effect.
Otherwise, the elements of Source are copied to Target as for an
assignment_statement
assigning Source to Target.
Discussion: {
AI05-0005-1}
This routine exists for compatibility with the bounded list container.
For an unbounded list,
Assign(A, B) and
A := B behave
identically. For a bounded list, := will raise an exception if the container
capacities are different, while Assign will not raise an exception if
there is enough room in the target.
{
AI12-0112-1}
function Copy (Source : List)
return List
with Post =>
Length (Copy'Result) = Length (Source) and then
not Tampering_With_Elements_Prohibited (Copy'Result) and then
not Tampering_With_Cursors_Prohibited (Copy'Result);
{
AI05-0001-1}
Returns a list whose elements match the elements of Source.
{
AI12-0112-1}
procedure Move (Target :
in out List;
Source :
in out List)
with Pre => (not Tampering_With_Cursors_Prohibited (Target)
or else raise Program_Error) and then
(not Tampering_With_Cursors_Prohibited (Source)
or else raise Program_Error),
Post => (if not Target'Has_Same_Storage (Source) then
Length (Target) = Length (Source'Old) and then
Length (Source) = 0);
{
AI95-00302-03}
{
AI05-0001-1}
{
AI05-0248-1}
{
AI05-0262-1}
If Target denotes the same object as Source, then the operation has no
effect. Otherwise, the operation is equivalent to Assign (Target, Source)
followed by Clear (Source).
procedure Insert (Container : in out List;
Before : in Cursor;
New_Item : in Element_Type;
Count : in Count_Type := 1)
with Pre => (not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error) and then
(Before = No_Element or else
Has_Element (Container, Before)
or else raise Program_Error) and then
(Length (Container) <= Count_Type'Last - Count
or else raise Constraint_Error),
Post => Length (Container)'Old + Count = Length (Container);
{
AI95-00302-03}
{
AI12-0112-1}
If Before is not No_Element, and does not designate
an element in Container, then Program_Error is propagated. Otherwise,
Insert inserts Count copies of New_Item prior to the element designated
by Before. If Before equals No_Element, the new elements are inserted
after the last node (if any). Any exception raised during allocation
of internal storage is propagated, and Container is not modified.
Ramification: The check on Before checks
that the cursor does not belong to some other Container. This check implies
that a reference to the container is included in the cursor value. This
wording is not meant to require detection of dangling cursors; such cursors
are defined to be invalid, which means that execution is erroneous, and
any result is allowed (including not raising an exception).
procedure Insert (Container : in out List;
Before : in Cursor;
New_Item : in Element_Type;
Position : out Cursor;
Count : in Count_Type := 1)
with Pre => (not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error) and then
(Before = No_Element or else
Has_Element (Container, Before)
or else raise Program_Error) and then
(Length (Container) <= Count_Type'Last - Count
or else raise Constraint_Error),
Post => Length (Container)'Old + Count = Length (Container)
and then Has_Element (Container, Position);
{
AI95-00302-03}
{
AI05-0257-1}
{
AI12-0112-1}
If Before is not No_Element, and does not designate
an element in Container, then Program_Error is propagated. Otherwise,
Insert allocates Count copies of New_Item, and inserts them prior
to the element designated by Before. If Before equals No_Element, the
new elements are inserted after the last element (if any). Position designates
the first newly-inserted element, or if Count equals 0, then Position
is assigned the value of Before. Any exception raised during allocation
of internal storage is propagated, and Container is not modified.
procedure Insert (Container : in out List;
Before : in Cursor;
Position : out Cursor;
Count : in Count_Type := 1)
with Pre => (not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error) and then
(Before = No_Element or else
Has_Element (Container, Before)
or else raise Program_Error) and then
(Length (Container) <= Count_Type'Last - Count
or else raise Constraint_Error),
Post => Length (Container)'Old + Count = Length (Container)
and then Has_Element (Container, Position);
{
AI95-00302-03}
{
AI05-0257-1}
{
AI12-0112-1}
If Before is not No_Element, and does not designate
an element in Container, then Program_Error is propagated. Otherwise,
Insert inserts Count new elements prior to the element designated
by Before. If Before equals No_Element, the new elements are inserted
after the last node (if any). The new elements are initialized by default
(see
3.3.1). Position designates the first
newly-inserted element, or if Count equals 0, then Position is assigned
the value of Before. Any exception raised during allocation of internal
storage is propagated, and Container is not modified.
{
AI12-0112-1}
procedure Prepend (Container :
in out List;
New_Item :
in Element_Type;
Count :
in Count_Type := 1)
with Pre => (not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error) and then
(Length (Container) <= Count_Type'Last - Count
or else raise Constraint_Error),
Post => Length (Container)'Old + Count = Length (Container);
{
AI95-00302-03}
Equivalent to Insert (Container, First (Container), New_Item, Count).
{
AI12-0112-1}
{
AI12-0400-1}
procedure Append (Container :
in out List;
New_Item :
in Element_Type;
Count :
in Count_Type
:= 1)
with Pre => (not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error) and then
(Length (Container) <= Count_Type'Last - Count
or else raise Constraint_Error),
Post => Length (Container)'Old + Count = Length (Container);
{
AI95-00302-03}
Equivalent to Insert (Container, No_Element, New_Item, Count).
{
AI12-0391-1}
{
AI12-0400-1}
procedure Append (Container : in out List;
New_Item : in Element_Type)
with Pre => (not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error) and then
(Length (Container) <= Count_Type'Last - 1
or else raise Constraint_Error),
Post => Length (Container)'Old + 1 = Length (Container);{
AI12-0391-1}
Equivalent to Insert (Container, No_Element, New_Item,
1).procedure Delete (Container : in out List;
Position : in out Cursor;
Count : in Count_Type := 1)
with Pre => (not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error) and then
(Position /= No_Element
or else raise Constraint_Error) and then
(Has_Element (Container, Position)
or else raise Program_Error),
Post => Length (Container)'Old - Count <= Length (Container)
and then Position = No_Element;
{
AI95-00302-03}
{
AI05-0264-1}
{
AI12-0112-1}
If Position equals No_Element, then Constraint_Error
is propagated. If Position does not designate an element in Container,
then Program_Error is propagated. Otherwise, Delete removes (from
Container) Count elements starting at the element designated by Position
(or all of the elements starting at Position if there are fewer than
Count elements starting at Position). Finally, Position is set to No_Element.
{
AI12-0112-1}
procedure Delete_First (Container :
in out List;
Count :
in Count_Type := 1)
with Pre => not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error,
Post => Length (Container)'Old - Count <= Length (Container);
{
AI95-00302-03}
{
AI05-0021-1}
If Length (Container) <= Count, then Delete_First is equivalent to
Clear (Container). Otherwise, it removes the first Count nodes from Container.
{
AI12-0112-1}
procedure Delete_Last (Container :
in out List;
Count :
in Count_Type := 1)
with Pre => not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error,
Post => Length (Container)'Old - Count <= Length (Container);
{
AI95-00302-03}
{
AI05-0264-1}
If Length (Container) <= Count, then Delete_Last is equivalent to
Clear (Container). Otherwise, it removes the last Count nodes from Container.
{
AI12-0112-1}
procedure Reverse_Elements (Container :
in out List)
with Pre => not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error;
{
AI95-00302-03}
Reorders the elements of Container in reverse order.
Discussion: Unlike the similar routine
for a vector, elements should not be copied; rather, the nodes should
be exchanged. Cursors are expected to reference the same elements afterwards.
procedure Swap (Container : in out List;
I, J : in Cursor)
with Pre => (not Tampering_With_Elements_Prohibited (Container)
or else raise Program_Error) and then
(I /= No_Element or else Constraint_Error) and then
(J /= No_Element or else Constraint_Error) and then
(Has_Element (Container, I)
or else raise Program_Error) and then
(Has_Element (Container, J)
or else raise Program_Error);
{
AI95-00302-03}
{
AI12-0112-1}
If either I or J is No_Element, then Constraint_Error
is propagated. If either I or J do not designate an element in Container,
then Program_Error is propagated. Otherwise, Swap exchanges the
values of the elements designated by I and J.
Ramification: After a call to Swap, I
designates the element value previously designated by J, and J designates
the element value previously designated by I. The cursors do not become
ambiguous from this operation.
To be honest: The implementation is not
required to actually copy the elements if it can do the swap some other
way. But it is allowed to copy the elements if needed.
procedure Swap_Links (Container : in out List;
I, J : in Cursor)
with Pre => (not Tampering_With_Elements_Prohibited (Container)
or else raise Program_Error) and then
(I /= No_Element or else Constraint_Error) and then
(J /= No_Element or else Constraint_Error) and then
(Has_Element (Container, I)
or else raise Program_Error) and then
(Has_Element (Container, J)
or else raise Program_Error);
{
AI95-00302-03}
{
AI12-0112-1}
If either I or J is No_Element, then Constraint_Error
is propagated. If either I or J do not designate an element in Container,
then Program_Error is propagated. Otherwise, Swap_Links exchanges
the nodes designated by I and J.
Ramification: Unlike Swap, this exchanges
the nodes, not the elements. No copying is performed. I and J designate
the same elements after this call as they did before it. This operation
can provide better performance than Swap if the element size is large.
procedure Splice (Target : in out List;
Before : in Cursor;
Source : in out List)
with Pre => (not Tampering_With_Cursors_Prohibited (Target)
or else raise Program_Error) and then
(not Tampering_With_Cursors_Prohibited (Source)
or else raise Program_Error) and then
(Before = No_Element or else
Has_Element (Target, Before)
or else raise Program_Error) and then
(Target'Has_Same_Storage (Source) or else
Length (Target) <= Count_Type'Last - Length (Source)
or else raise Constraint_Error),
Post => (if not Target'Has_Same_Storage (Source) then
(declare
Result_Length : constant Count_Type :=
Length (Source)'Old + Length (Target)'Old;
begin
Length (Source) = 0 and then
Length (Target) = Result_Length));
{
AI95-00302-03}
{
AI12-0112-1}
If
Before is not No_Element, and does not designate
an element in Target, then Program_Error is propagated. Otherwise, if
Source denotes the same object as Target, the operation has no
effect. Otherwise, Splice reorders elements such that they are removed
from Source and moved to Target, immediately prior to Before. If Before
equals No_Element, the nodes of Source are spliced after the last node
of Target.
The length of Target is incremented
by the number of nodes in Source, and the length of Source is set to
0.procedure Splice (Target : in out List;
Before : in Cursor;
Source : in out List;
Position : in out Cursor)
with Pre => (not Tampering_With_Cursors_Prohibited (Target)
or else raise Program_Error) and then
(not Tampering_With_Cursors_Prohibited (Source)
or else raise Program_Error) and then
(Position /= No_Element
or else raise Constraint_Error) and then
(Has_Element (Source, Position)
or else raise Program_Error) and then
(Before = No_Element or else
Has_Element (Target, Before)
or else raise Program_Error) and then
(Target'Has_Same_Storage (Source) or else
Length (Target) <= Count_Type'Last - 1
or else raise Constraint_Error),
Post => (declare
Org_Target_Length : constant Count_Type :=
Length (Target)'Old;
Org_Source_Length : constant Count_Type :=
Length (Source)'Old;
begin
(if Target'Has_Same_Storage (Source) then
Position = Position'Old
else Length (Source) = Org_Source_Length - 1 and then
Length (Target) = Org_Target_Length + 1 and then
Has_Element (Target, Position)));
{
AI95-00302-03}
{
AI05-0264-1}
{
AI12-0112-1}
If
Position is No_Element, then Constraint_Error
is propagated. If Before does not equal No_Element, and does not designate
an element in Target, then Program_Error is propagated. If Position does
not equal No_Element, and does not designate a node in Source, then Program_Error
is propagated. If Source denotes the same object as Target, then
there is no effect if Position equals Before, else the element designated
by Position is moved immediately prior to Before, or, if Before equals
No_Element, after the last element.
In both cases,
Position and the length of Target are unchanged. Otherwise, the
element designated by Position is removed from Source and moved to Target,
immediately prior to Before, or, if Before equals No_Element, after the
last element of Target.
The length of Target is
incremented, the length of Source is decremented, and Position
is updated to represent an element in Target.
Ramification: If Source is the same as
Target, and Position = Before, or Next(Position) = Before, Splice has
no effect, as the element does not have to move to meet the postcondition.
procedure Splice (Container: in out List;
Before : in Cursor;
Position : in Cursor)
with Pre => (not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error) and then
(Position /= No_Element
or else raise Constraint_Error) and then
(Has_Element (Container, Position)
or else raise Program_Error) and then
(Before = No_Element or else
Has_Element (Container, Before)
or else raise Program_Error),
Post => Length (Container) = Length (Container)'Old;
{
AI95-00302-03}
{
AI05-0264-1}
{
AI12-0112-1}
If
Position is No_Element, then Constraint_Error
is propagated. If Before does not equal No_Element, and does not designate
an element in Container, then Program_Error is propagated. If Position
does not equal No_Element, and does not designate a node in Container,
then Program_Error is propagated. If Position equals Before there
is no effect. Otherwise, the element designated by Position is moved
immediately prior to Before, or, if Before equals No_Element, after the
last element.
The length of Container is unchanged.{
AI12-0112-1}
function First (Container : List)
return Cursor
with Nonblocking, Global => null, Use_Formal => null,
Post => (if not Is_Empty (Container)
then Has_Element (Container, First'Result)
else First'Result = No_Element);
{
AI95-00302-03}
If Container is empty, First returns No_Element. Otherwise, it returns
a cursor that designates the first node in Container.
{
AI12-0112-1}
function First_Element (Container : List)
return Element_Type
with Pre => (not Is_Empty (Container)
or else raise Constraint_Error);
{
AI95-00302-03}
Equivalent to Element (Container, First_Index (Container)).
{
AI12-0112-1}
function Last (Container : List)
return Cursor
with Nonblocking, Global => null, Use_Formal => null,
Post => (if not Is_Empty (Container)
then Has_Element (Container, Last'Result)
else Last'Result = No_Element);
{
AI95-00302-03}
If Container is empty, Last returns No_Element. Otherwise, it returns
a cursor that designates the last node in Container.
{
AI12-0112-1}
function Last_Element (Container : List)
return Element_Type
with Pre => (not Is_Empty (Container)
or else raise Constraint_Error);
{
AI12-0112-1}
function Next (Position : Cursor)
return Cursor
with Nonblocking, Global => in all, Use_Formal => null,
Post => (if Position = No_Element then Next'Result = No_Element);
{
AI95-00302-03}
If Position equals No_Element or designates the last element of the container,
then Next returns the value No_Element. Otherwise, it returns a cursor
that designates the successor of the element designated by Position.
function Next (Container : List;
Position : Cursor) return Cursor
with Nonblocking, Global => null, Use_Formal => null,
Pre => Position = No_Element or else
Has_Element (Container, Position)
or else raise Program_Error,
Post => (if Position = No_Element then Next'Result = No_Element
elsif Next'Result = No_Element then
Position = Last (Container)
else Has_Element (Container, Next'Result));
{
AI12-0112-1}
Returns a cursor designating the successor of the
element designated by Position in Container.{
AI12-0112-1}
function Previous (Position : Cursor)
return Cursor
with Nonblocking, Global => in all, Use_Formal => null,
Post => (if Position = No_Element then
Previous'Result = No_Element);
{
AI95-00302-03}
If Position equals No_Element or designates the first element of the
container, then Previous returns the value No_Element. Otherwise, it
returns a cursor that designates the predecessor of the element designated
by Position.
function Previous (Container : List;
Position : Cursor) return Cursor
with Nonblocking, Global => null, Use_Formal => null,
Pre => Position = No_Element or else
Has_Element (Container, Position)
or else raise Program_Error,
Post => (if Position = No_Element then
Previous'Result = No_Element
elsif Previous'Result = No_Element then
Position = First (Container)
else Has_Element (Container, Previous'Result));
{
AI12-0112-1}
Returns a cursor designating the predecessor of
the element designated by Position in Container, if any.{
AI12-0112-1}
procedure Next (Position :
in out Cursor)
with Nonblocking, Global => in all, Use_Formal => null;
procedure Next (Container : in List;
Position : in out Cursor)
with Nonblocking, Global => null, Use_Formal => null,
Pre => Position = No_Element or else
Has_Element (Container, Position)
or else raise Program_Error,
Post => (if Position /= No_Element
then Has_Element (Container, Position));
{
AI12-0112-1}
Equivalent to Position := Next (Container, Position).{
AI12-0112-1}
procedure Previous (Position :
in out Cursor)
with Nonblocking, Global => in all, Use_Formal => null;
procedure Previous (Container : in List;
Position : in out Cursor)
with Nonblocking, Global => null, Use_Formal => null,
Pre => Position = No_Element or else
Has_Element (Container, Position)
or else raise Program_Error,
Post => (if Position /= No_Element
then Has_Element (Container, Position));
{
AI12-0112-1}
Equivalent to Position := Previous (Container,
Position).function Find (Container : List;
Item : Element_Type;
Position : Cursor := No_Element)
return Cursor
with Pre => Position = No_Element or else
Has_Element (Container, Position)
or else raise Program_Error,
Post => (if Find'Result /= No_Element
then Has_Element (Container, Find'Result));
{
AI95-00302-03}
{
AI12-0112-1}
If Position is not No_Element, and does not designate
an element in Container, then Program_Error is propagated. Find
searches the elements of Container for an element equal to Item (using
the generic formal equality operator). The search starts at the element
designated by Position, or at the first element if Position equals No_Element.
It proceeds towards Last (Container). If no equal element is found, then
Find returns No_Element. Otherwise, it returns a cursor designating the
first equal element encountered.
function Reverse_Find (Container : List;
Item : Element_Type;
Position : Cursor := No_Element)
return Cursor
with Pre => Position = No_Element or else
Has_Element (Container, Position)
or else raise Program_Error,
Post => (if Reverse_Find'Result /= No_Element
then Has_Element (Container, Reverse_Find'Result));
{
AI95-00302-03}
{
AI12-0112-1}
If Position is not No_Element, and does not designate
an element in Container, then Program_Error is propagated. Find
searches the elements of Container for an element equal to Item (using
the generic formal equality operator). The search starts at the element
designated by Position, or at the last element if Position equals No_Element.
It proceeds towards First (Container). If no equal element is found,
then Reverse_Find returns No_Element. Otherwise, it returns a cursor
designating the first equal element encountered.
function Contains (Container : List;
Item : Element_Type) return Boolean;
{
AI95-00302-03}
Equivalent to Find (Container, Item) /= No_Element.
Paragraphs 139
and 140 were moved above.
{
AI12-0112-1}
procedure Iterate
(Container :
in List;
Process :
not null access procedure (Position :
in Cursor))
with Allows_Exit;
{
AI95-00302-03}
{
AI05-0265-1}
Iterate calls Process.
all with a cursor that designates each node
in Container, starting with the first node and moving the cursor as per
the Next function. Tampering with the cursors of Container is prohibited
during the execution of a call on Process.
all. Any exception raised
by Process.
all is propagated.
Implementation Note: The purpose of the
tamper with cursors check is to prevent erroneous execution from the
Position parameter of Process.all becoming invalid. This check
takes place when the operations that tamper with the cursors of the container
are called. The check cannot be made later (say in the body of Iterate),
because that could cause the Position cursor to be invalid and potentially
cause execution to become erroneous -- defeating the purpose of the check.
See Iterate for vectors (
A.18.2)
for a suggested implementation of the check.
{
AI12-0112-1}
procedure Reverse_Iterate
(Container :
in List;
Process :
not null access procedure (Position :
in Cursor))
with Allows_Exit;
{
AI95-00302-03}
{
AI05-0212-1}
Iterates over the nodes in Container as per procedure Iterate, except
that elements are traversed in reverse order, starting with the last
node and moving the cursor as per the Previous function.
{
AI12-0112-1}
{
AI12-0266-1}
function Iterate (Container :
in List)
return List_Iterator_Interfaces.
Parallel_Reversible_Iterator Reversible_Iterator'Class
with Post => Tampering_With_Cursors_Prohibited (Container);
{
AI05-0212-1}
{
AI05-0265-1}
{
AI05-0269-1}
{
AI12-0266-1}
Iterate returns
an a
reversible iterator object (see
5.5.1)
that will generate a value for a loop parameter (see
5.5.2)
designating each node in Container, starting with the first node and
moving the cursor as per the Next function when used as a forward iterator,
and starting with the last node and moving the cursor as per the Previous
function when used as a reverse iterator
, and processing
all nodes concurrently when used as a parallel iterator. Tampering
with the cursors of Container is prohibited while the iterator object
exists (in particular, in the
sequence_of_statements
of the
loop_statement
whose
iterator_specification
denotes this object). The iterator object needs finalization.
function Iterate (Container : in List; Start : in Cursor)
return List_Iterator_Interfaces.Reversible_Iterator'Class
with Pre => (Start /= No_Element
or else raise Constraint_Error) and then
(Has_Element (Container, Start)
or else raise Program_Error),
Post => Tampering_With_Cursors_Prohibited (Container);
{
AI05-0212-1}
{
AI05-0262-1}
{
AI05-0265-1}
{
AI05-0269-1}
{
AI12-0112-1}
If Start is not No_Element and does not designate
an item in Container, then Program_Error is propagated. If Start is No_Element,
then Constraint_Error is propagated. Otherwise, Iterate returns
a reversible iterator object (see
5.5.1)
that will generate a value for a loop parameter (see
5.5.2)
designating each node in Container, starting with the node designated
by Start and moving the cursor as per the Next function when used as
a forward iterator, or moving the cursor as per the Previous function
when used as a reverse iterator. Tampering with the cursors of Container
is prohibited while the iterator object exists (in particular, in the
sequence_of_statements
of the
loop_statement
whose
iterator_specification
denotes this object). The iterator object needs finalization.
Discussion:
Exits are allowed from the loops created using the iterator objects.
In particular, to stop the iteration at a particular cursor, just add
exit when Cur = Stop;
in the body of
the loop (assuming that Cur is the loop parameter and Stop
is the cursor that you want to stop at).
{
AI05-0044-1}
{
AI05-0262-1}
The actual function for the generic formal function "<"
of Generic_Sorting is expected to return the same value each time it
is called with a particular pair of element values. It should define
a strict weak ordering relationship (see
A.18);
it should not modify Container. If the actual for "<" behaves
in some other manner, the behavior of the subprograms of Generic_Sorting
are unspecified. The number of times the subprograms of Generic_Sorting
call "<" is unspecified.
function Is_Sorted (Container : List) return Boolean;
{
AI95-00302-03}
Returns True if the elements are sorted smallest first as determined
by the generic formal "<" operator; otherwise, Is_Sorted
returns False. Any exception raised during evaluation of "<"
is propagated.
{
AI12-0112-1}
procedure Sort (Container :
in out List)
with Pre => not Tampering_With_Cursors_Prohibited (Container)
or else raise Program_Error;
{
AI95-00302-03}
Reorders the nodes of Container such that the elements are sorted smallest
first as determined by the generic formal "<" operator provided.
The sort is stable. Any exception raised during evaluation of "<"
is propagated.
Ramification: Unlike array sorts, we
do require stable sorts here. That's because algorithms in the merge
sort family (as described by Knuth) can be both fast and stable. Such
sorts use the extra memory as offered by the links to provide better
performance.
Note that list sorts never copy elements; it
is the nodes, not the elements, that are reordered.
procedure Merge (Target : in out List;
Source : in out List)
with Pre => (not Tampering_With_Cursors_Prohibited (Target)
or else raise Program_Error) and then
(not Tampering_With_Elements_Prohibited (Source)
or else raise Program_Error) and then
(Length (Target) <= Count_Type'Last - Length (Source)
or else raise Constraint_Error) and then
((Length (Source) = 0 or else
not Target'Has_Same_Storage (Source))
or else raise Constraint_Error),
Post => (declare
Result_Length : constant Count_Type :=
Length (Source)'Old + Length (Target)'Old;
begin
(Length (Source) = 0 and then
Length (Target) = Result_Length));
{
AI95-00302-03}
{
AI05-0021-1}
{
AI12-0112-1}
If Source is empty, then Merge does nothing. If
Source and Target are the same nonempty container object, then Program_Error
is propagated. Otherwise, Merge removes elements from Source and
inserts them into Target; afterwards, Target contains the union of the
elements that were initially in Source and Target; Source is left empty.
If Target and Source are initially sorted smallest first, then Target
is ordered smallest first as determined by the generic formal "<"
operator; otherwise, the order of elements in Target is unspecified.
Any exception raised during evaluation of "<" is propagated.
Ramification: It is a bounded error if
either of the lists is unsorted, see below. The bounded error can be
recovered by sorting Target after the merge call, or the lists can be
pretested with Is_Sorted.
{
AI12-0111-1}
The nested package Doubly_Linked_Lists.Stable provides
a type Stable.List that represents a stable list,
which is one that cannot grow and shrink. Such a list can be created
by calling the Copy function, or by establishing a stabilized view
of an ordinary list. {
AI12-0111-1}
The subprograms of package Containers.Doubly_Linked_Lists
that have a parameter or result of type List are included in the nested
package Stable with the same specification, except that the following
are omitted:Tampering_With_Cursors_Prohibited,
Tampering_With_Elements_Prohibited, Assign, Move, Insert, Append, Prepend,
Clear, Delete, Delete_First, Delete_Last, Splice, Swap_Links, and Reverse_Elements
Ramification: The
names List and Cursor mean the types declared in the nested package in
these subprogram specifications.
Reason: The omitted
routines are those that tamper with cursors or elements (or test that
state). The model is that it is impossible to tamper with cursors or
elements of a stable view since no such operations are included. Thus
tampering checks are not needed for a stable view, and we omit the operations
associated with those checks.
The Generic_Sorting generic
is omitted entirely, as only function Is_Sorting does not tamper with
cursors. It isn't useful enough by itself to include.
{
AI12-0111-1}
The operations of this package are equivalent to
those for ordinary lists, except that the calls to Tampering_With_Cursors_Prohibited
and Tampering_With_Elements_Prohibited that occur in preconditions are
replaced by False, and any that occur in postconditions are replaced
by True. {
AI12-0111-1}
{
AI12-0439-1}
If a stable list is declared with the Base discriminant
designating a pre-existing ordinary list, the stable list represents
a stabilized view of the underlying ordinary list, and any operation
on the stable list is reflected on the underlying ordinary list. While
a stabilized view exists, any operation that tampers with elements performed
on the underlying list is prohibited. The finalization of a stable list
that provides such a view removes this restriction on the underlying
ordinary list [(though some other restriction can exist due to other
concurrent iterations or stabilized views)]. {
AI12-0111-1}
{
AI12-0438-1}
If a stable list is declared without specifying
Base, the object is necessarily initialized. The initializing expression
of the stable list, [typically a call on Copy], determines the Length
of the list. The Length of a stable list never changes after initialization.Proof: {
AI12-0438-1}
Initialization is required as the type is indefinite,
see 3.3.1.
Bounded (Run-Time) Errors
{
AI95-00302-03}
Calling Merge in an instance of Generic_Sorting with
either Source or Target not ordered smallest first using the provided
generic formal "<" operator is a bounded error. Either Program_Error
is raised after Target is updated as described for Merge, or the operation
works as defined.
{
AI05-0022-1}
{
AI05-0248-1}
It is a bounded error for the actual function associated
with a generic formal subprogram, when called as part of an operation
of this package, to tamper with elements of any List parameter of the
operation. Either Program_Error is raised, or the operation works as
defined on the value of the List either prior to, or subsequent to, some
or all of the modifications to the List.
{
AI05-0027-1}
It is a bounded error to call any subprogram declared
in the visible part of Containers.Doubly_Linked_Lists when the associated
container has been finalized. If the operation takes Container as an
in out parameter, then it raises Constraint_Error or Program_Error.
Otherwise, the operation either proceeds as it would for an empty container,
or it raises Constraint_Error
or Program_Error.
Erroneous Execution
{
AI95-00302-03}
A Cursor value is
invalid if any of the following have occurred
since it was created:
The list that contains the element it designates
has been finalized;
{
AI05-0160-1}
The list that contains the element it designates has been used as the
Target of a call to Assign, or as the target of an
assignment_statement;
[The list that contains the element it designates
has been used as the Source or Target of a call to Move;] or
Proof: {
AI05-0001-1}
Move has been reworded in terms of Assign and Clear, which are covered
by other bullets, so this text is redundant.
{
AI05-0160-1}
{
AI05-0262-1}
The element it designates has been removed from the list that previously
contained the element.
To be honest: {
AI05-0160-1}
The cursor modified by the four parameter Splice is not invalid, even
though the element it designates has been removed from the source list,
because that cursor has been modified to designate that element in the
target list – the cursor no longer designates an element in the
source list.
Ramification: {
AI05-0160-1}
This can happen directly via calls to Delete, Delete_Last, Clear, Splice
with a Source parameter, and Merge; and indirectly via calls to Delete_First,
Assign, and Move.
{
AI95-00302-03}
The result of "=" or Has_Element is unspecified if it is called
with an invalid cursor parameter. Execution is erroneous if any other
subprogram declared in Containers.Doubly_Linked_Lists is called with
an invalid cursor parameter.
Discussion: The list above is intended
to be exhaustive. In other cases, a cursor value continues to designate
its original element. For instance, cursor values survive the insertion
and deletion of other nodes.
While it is possible to check for these cases,
in many cases the overhead necessary to make the check is substantial
in time or space. Implementations are encouraged to check for as many
of these cases as possible and raise Program_Error if detected.
{
AI05-0212-1}
Execution is erroneous if the list associated with the result of a call
to Reference or Constant_Reference is finalized before the result object
returned by the call to Reference or Constant_Reference is finalized.
Reason: Each object of Reference_Type
and Constant_Reference_Type probably contains some reference to the originating
container. If that container is prematurely finalized (which is only
possible via Unchecked_Deallocation, as accessibility checks prevent
passing a container to Reference that will not live as long as the result),
the finalization of the object of Reference_Type will try to access a
nonexistent object. This is a normal case of a dangling pointer created
by Unchecked_Deallocation; we have to explicitly mention it here as the
pointer in question is not visible in the specification of the type.
(This is the same reason we have to say this for invalid cursors.)
Implementation Requirements
{
AI95-00302-03}
{
AI12-0437-1}
No storage associated with a doubly-linked
list List
object shall be lost upon assignment or scope exit.
{
AI95-00302-03}
{
AI05-0262-1}
The execution of an
assignment_statement
for a list shall have the effect of copying the elements from the source
list object to the target list object and changing the length of the
target object to that of the source object.
Implementation Note: {
AI05-0298-1}
An assignment of a List is a “deep” copy; that is the elements
are copied as well as the data structures. We say “effect of”
in order to allow the implementation to avoid copying elements immediately
if it wishes. For instance, an implementation that avoided copying until
one of the containers is modified would be allowed. (Note that this implementation
would require care, see
A.18.2 for more.)
Implementation Advice
{
AI95-00302-03}
Containers.Doubly_Linked_Lists should be implemented similarly to a linked
list. In particular, if
N is the length of a list, then the worst-case
time complexity of Element, Insert with Count=1, and Delete with Count=1
should be
O(log
N).
Implementation Advice: The worst-case
time complexity of Element, Insert with Count=1, and Delete with Count=1
for Containers.Doubly_Linked_Lists should be O(log N).
Reason: We do not mean to overly constrain
implementation strategies here. However, it is important for portability
that the performance of large containers has roughly the same factors
on different implementations. If a program is moved to an implementation
that takes O(N) time to access elements, that program could
be unusable when the lists are large. We allow O(log N)
access because the proportionality constant and caching effects are likely
to be larger than the log factor, and we don't want to discourage innovative
implementations.
{
AI95-00302-03}
The worst-case time complexity of a call on procedure Sort of an instance
of Containers.Doubly_Linked_Lists.Generic_Sorting should be
O(
N**2),
and the average time complexity should be better than
O(
N**2).
Implementation Advice: A call on procedure
Sort of an instance of Containers.Doubly_Linked_Lists.Generic_Sorting
should have an average time complexity better than O(N**2)
and worst case no worse than O(N**2).
Ramification: In other words, we're requiring
the use of a better than O(N**2) sorting algorithm, such
as Quicksort. No bubble sorts allowed!
{
AI95-00302-03}
Move should not copy elements, and should minimize copying of internal
data structures.
Implementation Advice: Containers.Doubly_Linked_Lists.Move
should not copy elements, and should minimize copying of internal data
structures.
Implementation Note: Usually that can
be accomplished simply by moving the pointer(s) to the internal data
structures from the Source container to the Target container.
{
AI95-00302-03}
If an exception is propagated from a list operation, no storage should
be lost, nor any elements removed from a list unless specified by the
operation.
Implementation Advice: If an exception
is propagated from a list operation, no storage should be lost, nor any
elements removed from a list unless specified by the operation.
Reason: This is important so that programs
can recover from errors. But we don't want to require heroic efforts,
so we just require documentation of cases where this can't be accomplished.
NOTE {
AI95-00302-03}
{
AI12-0442-1}
Sorting a list never copies elements, and is a stable sort (equal elements
remain in the original order). This is different than sorting an array
or vector, which
will often may
need to copy elements, and
hence is
probably not a stable sort.
Extensions to Ada 95
{
AI95-00302-03}
The generic package Containers.Doubly_Linked_Lists
is new.
Inconsistencies With Ada 2005
{
AI05-0248-1}
{
AI05-0257-1}
Correction: The Insert versions that return
a Position parameter are now defined to return Position = Before if Count
= 0. This was unspecified for Ada 2005; so this will only be inconsistent
if an implementation did something else and a program depended on that
something else — this should be very rare.
Incompatibilities With Ada 2005
{
AI05-0001-1}
Subprograms Assign and Copy are added to Containers.Doubly_Linked_Lists.
If an instance of Containers.Doubly_Linked_Lists is referenced in a
use_clause,
and an entity
E with the same
defining_identifier
as a new entity in Containers.Doubly_Linked_Lists is defined in a package
that is also referenced in a
use_clause,
the entity
E may no longer be use-visible, resulting in errors.
This should be rare and is easily fixed if it does occur.
Extensions to Ada 2005
{
AI05-0212-1}
Added iterator, reference, and indexing support to
make list containers more convenient to use.
Wording Changes from Ada 2005
{
AI05-0001-1}
Generalized the definition of Move. Specified which elements are read/written
by stream attributes.
{
AI05-0022-1}
Correction: Added a Bounded (Run-Time) Error to cover tampering
by generic actual subprograms.
{
AI05-0027-1}
Correction: Added a Bounded (Run-Time) Error to cover access to
finalized list containers.
{
AI05-0044-1}
Correction: Redefined "<" actuals to require a strict
weak ordering; the old definition allowed indeterminant comparisons that
would not have worked in a container.
{
AI05-0084-1}
Correction: Added a pragma Remote_Types so that containers can
be used in distributed programs.
{
AI05-0160-1}
Correction: Revised the definition of invalid cursors to cover
missing (and new) cases.
{
AI05-0257-1}
Correction: Added missing wording to describe the Position after
Inserting 0 elements.
{
AI05-0265-1}
Correction: Defined when a container prohibits tampering in order
to more clearly define where the check is made and the exception raised.
Inconsistencies With Ada 2012
{
AI12-0111-1}
Correction: Tampering
with elements is now defined to be equivalent to tampering with cursors
for ordinary containers. If a program requires tampering detection to
work, it might fail in Ada 2022. Needless to say, this shouldn't happen
outside of test programs. See Inconsistencies With Ada 2012 in A.18.2
for more details. Incompatibilities With Ada 2012
{
AI12-0111-1}
{
AI12-0112-1}
{
AI12-0339-1}
{
AI12-0391-1}
A number of new subprograms,
types, and even a nested package were added to Containers.Doubly_Linked_Lists
to better support contracts and stable views. Therefore, a use clause
conflict is possible; see the introduction of Annex
A for more on this topic. Extensions to Ada 2012
{
AI12-0196-1}
Replace_Element is now defined
such that it can be used concurrently so long as it operates on different
elements. This allows some container operations to be used in parallel
without separate synchronization.{
AI12-0266-1}
The iterator for the entire container now can return
a parallel iterator which can be used to process the container in parallel.
Wording Changes from Ada 2012
{
AI12-0110-1}
Corrigendum: Clarified that tampering checks
precede all other checks made by a subprogram (but come after those associated
with the call).{
AI12-0112-1}
Added contracts to this container. This includes
describing some of the semantics with pre- and postconditions, rather
than English text. Note that the preconditions can be Suppressed (see
11.5).{
AI12-0400-1}
Correction: Split the Append routine into
two routines rather than having a single routine with a default parameter,
in order that a routine with the appropriate profile for the Aggregate
aspect exists. This change should not change the behavior of any existing
code.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
