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A.18.20 The Generic Package Containers.Bounded_Doubly_Linked_Lists

{AI05-0001-1} 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

{AI05-0001-1} The declaration of the generic library package Containers.Bounded_Doubly_Linked_Lists has the same contents and semantics as Containers.Doubly_Linked_Lists except:
{AI12-0112-1} The aspect pragma Preelaborate is replaced with aspect pragma Pure. Aspect Global is deleted.
Ramification: {AI12-0112-1} 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: 
{AI12-0112-1}   type List (Capacity : Count_Type) is tagged private...;
{AI12-0409-1} The aspect_definition for Preelaborable_Initialization for type List is changed to:
  Preelaborable_Initialization =>
The type List needs finalization if and only if type Element_Type needs finalization.
Implementation Note: {AI05-0212-1} 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. 
{AI12-0339-1} The function Empty is replaced by:
   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;
{AI12-0112-1} 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)
{AI12-0112-1} 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: 
{AI12-0112-1}   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
This paragraph was deleted.{AI12-0112-1} 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.
{AI12-0112-1} 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

{AI05-0160-1} {AI05-0265-1} 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

{AI05-0265-1} 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

{AI05-0184-1} {AI05-0264-1} 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:
{AI05-0184-1} {AI05-0248-1} the Element_Type parameters of the two instances are statically matching subtypes of the same type; and
{AI05-0184-1} 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

{AI05-0001-1} 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.
{AI05-0001-1} 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

{AI05-0001-1} {AI05-0160-1} {AI05-0184-1} The generic package Containers.Bounded_Doubly_Linked_Lists is new. 

Inconsistencies With Ada 2012

{AI12-0111-1} 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

{AI12-0409-1} 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. 

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