A.18.20 The Generic Package Containers.Bounded_Doubly_Linked_Lists
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The language-defined generic package Containers.Bounded_Doubly_Linked_Lists
provides a private type List and a set of operations. It provides the
same operations as the package Containers.Doubly_Linked_Lists (see
A.18.3),
with the difference that the maximum storage is bounded.
Static Semantics
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The declaration of the generic library package Containers.Bounded_Doubly_Linked_Lists
has the same contents and semantics as Containers.Doubly_Linked_Lists
except:
Ramification: {
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The Global aspect for a Pure package is null
(see 6.1.2), so we don't need to give it
explicitly.
The type List is
declared with a discriminant that specifies the capacity (maximum number
of elements) as follows:
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type List (Capacity : Count_Type)
is tagged private...; Preelaborable_Initialization =>
Element_Type'Preelaborable_Initialization
The type List needs finalization if and only if
type Element_Type needs finalization.
Implementation Note: {
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The type List cannot depend on package Ada.Finalization unless the element
type depends on that package. The objects returned from the Iterator
and Reference functions probably do depend on package Ada.Finalization.
Restricted environments may need to avoid use of those functions and
their associated types.
function Empty (Capacity : Count_Type := implementation-defined)
return List
with Post =>
Empty'Result.Capacity = Capacity and then
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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For procedures Insert,
Prepend, Append, Merge, and the three-parameter Splice whose parameter
Source has type List, the part of the precondition reading: The
allocation of internal storage includes a check that the capacity is
not exceeded, and Capacity_Error is raised if this check fails. (<some length> <= Count_Type'Last - <some other length>
or else raise Constraint_Error)
is
replaced by:
(<some length> <= Count_Type'Last - <some other length>
or else raise Constraint_Error) and then
(<some length> <= Container.Capacity - <some other length>
or else raise Capacity_Error)
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In procedure Assign,
the
precondition is altered to: if Source length
is greater than Target capacity, then Capacity_Error is propagated.
Pre => (not Tampering_With_Cursors_Prohibited (Target)
or else raise Program_Error) and then
(Length (Source) <= Target.Capacity
or else raise Capacity_Error),
The function Copy
is replaced with:
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function Copy (Source : List; Capacity : Count_Type := 0)
return List
with Pre => Capacity = 0 or else Capacity >= Length (Source)
or else raise Capacity_Error,
Post =>
Length (Copy'Result) = Length (Source) and then
not Tampering_With_Elements_Prohibited (Copy'Result) and then
not Tampering_With_Cursors_Prohibited (Copy'Result) and then
Copy'Result.Capacity = (if Capacity = 0 then
Length (Source) else Capacity);
Returns a list whose elements
have the same values as the elements of Source If
Capacity is 0, then the list capacity is the length of Source; if Capacity
is equal to or greater than the length of Source, the list capacity equals
the value of the Capacity parameter; otherwise, the operation propagates
Capacity_Error.
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In the three-parameter procedure Splice whose Source
has type List, if the sum of the length of Target and the length of Source
is greater than the capacity of Target, then Splice propagates Capacity_Error.
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In the four-parameter procedure Splice,
the precondition is altered to: if
the length of Target equals the capacity of Target, then Splice propagates
Capacity_Error. 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
or else raise Constraint_Error) and then
(Target'Has_Same_Storage (Source) or else
Length (Target) /= Target.Capacity
or else raise Capacity_Error),
Bounded (Run-Time) Errors
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It is a bounded error to assign from a bounded list
object while tampering with elements [or cursors] of that object is prohibited.
Either Program_Error is raised by the assignment, execution proceeds
with the target object prohibiting tampering with elements [or cursors],
or execution proceeds normally.
Proof: Tampering with elements includes
tampering with cursors, so we only really need to talk about tampering
with elements here; we mention cursors for clarity.
Erroneous Execution
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When a bounded list object
L is finalized, if tampering with cursors
is prohibited for
L other than due to an assignment from another
list, then execution is erroneous.
Reason: This is a tampering event, but
since the implementation is not allowed to use Ada.Finalization, it is
not possible in a pure Ada implementation to detect this error. (There
is no Finalize routine that will be called that could make the check.)
Since the check probably cannot be made, the bad effects that could occur
(such as an iterator going into an infinite loop or accessing a nonexistent
element) cannot be prevented and we have to allow anything. We do allow
re-assigning an object that only prohibits tampering because it was copied
from another object as that cannot cause any negative effects.
Implementation Requirements
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For each instance of Containers.Doubly_Linked_Lists and each instance
of Containers.Bounded_Doubly_Linked_Lists, if the two instances meet
the following conditions, then the output generated by the List'Output
or List'Write subprograms of either instance shall be readable by the
List'Input or List'Read of the other instance, respectively:
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the Element_Type parameters of the two instances are statically matching
subtypes of the same type; and
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the output generated by Element_Type'Output or Element_Type'Write is
readable by Element_Type'Input or Element_Type'Read, respectively (where
Element_Type denotes the type of the two actual Element_Type parameters).
Implementation Advice
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Bounded list objects should be implemented without implicit pointers
or dynamic allocation.
Implementation Advice: Bounded list objects
should be implemented without implicit pointers or dynamic allocation.
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The implementation advice for procedure Move to minimize copying does
not apply.
Implementation Advice: The implementation
advice for procedure Move to minimize copying does not apply to bounded
lists.
Extensions to Ada 2005
Inconsistencies With Ada 2012
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Correction: Tampering
with elements is now defined to be equivalent to tampering with cursors
for bounded 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
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Correction:A bounded
list now only has Preelaborable_Initialization (abbreviated PI in this
note) when the actual for the Element_Type has PI. If an program used
a list whose actual Element_Type does not have PI in a context when PI
is required (such as a library-level object in a preelaborated unit or
as a component of a type with PI), the program would be illegal in Ada
2022 but legal in original Ada 2012. This situation is unlikely, especially
as some existing Ada 2012 implementations reject the instance in this
case.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
