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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:
The pragma Preelaborate is replaced with pragma Pure.
The type List is declared with a discriminant that specifies the capacity (maximum number of elements) as follows: 
  type List (Capacity : Count_Type) is tagged private;
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. 
The allocation of internal storage includes a check that the capacity is not exceeded, and Capacity_Error is raised if this check fails.
In procedure Assign, if Source length is greater than Target capacity, then Capacity_Error is propagated.
The function Copy is replaced with: 
  function Copy (Source : List; Capacity : Count_Type := 0)
     return List;
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.
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.
In the four-parameter procedure Splice, if the length of Target equals the capacity of Target, then Splice propagates 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. 

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