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A.18.24 The Generic Package Containers.Bounded_Ordered_Sets

{AI05-0001-1} The language-defined generic package Containers.Bounded_Ordered_Sets provides a private type Set and a set of operations. It provides the same operations as the package Containers.Ordered_Sets (see A.18.9), with the difference that the maximum storage is bounded. 

Static Semantics

{AI05-0001-1} The declaration of the generic library package Containers.Bounded_Ordered_Sets has the same contents and semantics as Containers.Ordered_Sets except:
{AI12-0112-1} The aspect pragma Preelaborate is replaced with aspect pragma Pure. Aspect Global is deleted.
The type Set is declared with a discriminant that specifies the capacity (maximum number of elements) as follows: 
{AI12-0112-1}   type Set (Capacity : Count_Type) is tagged private...;
{AI12-0409-1} The aspect_definition for Preelaborable_Initialization for type Set is changed to: 
  Preelaborable_Initialization =>
The type Set needs finalization if and only if type Element_Type needs finalization.
Implementation Note: {AI05-0212-1} The type Set 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 Set
      with Post =>
            Empty'Result.Capacity = Capacity and then
            not Tampering_With_Cursors_Prohibited (Empty'Result) and then
            Length (Empty'Result) = 0;
{AI12-0112-1} For procedures Insert and Include, the part of the precondition reading: If Insert (or Include) adds an element, a check is made 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   : Set;
                 Capacity : Count_Type := 0) return Map
     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_Cursors_Prohibited (Copy'Result) and then
             Copy'Result.Capacity = (if Capacity = 0 then
                Length (Source) else Capacity)
{AI12-0112-1} Returns a set with key/element pairs initialized from the values in Source. If Capacity is 0, then the set capacity is the length of Source; if Capacity is equal to or greater than the length of Source, the set capacity is the specified value; otherwise, the operation propagates Capacity_Error.

Bounded (Run-Time) Errors

{AI05-0160-1} {AI05-0265-1} It is a bounded error to assign from a bounded set 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 set object S is finalized, if tampering with cursors is prohibited for S other than due to an assignment from another set, 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.Ordered_Sets and each instance of Containers.Bounded_Ordered_Sets, if the two instances meet the following conditions, then the output generated by the Set'Output or Set'Write subprograms of either instance shall be readable by the Set'Input or Set'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} {AI05-0269-1} Bounded ordered set objects should be implemented without implicit pointers or dynamic allocation. 
Implementation Advice: Bounded ordered set 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 ordered sets.

Extensions to Ada 2005

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

Incompatibilities With Ada 2012

{AI12-0409-1} Correction:A bounded set now only has Preelaborable_Initialization (abbreviated PI in this note) when the actual for the Element_Type has PI. If an program used a set 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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