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A.18.22 The Generic Package Containers.Bounded_Ordered_Maps

1/3
{AI05-0001-1} The language-defined generic package Containers.Bounded_Ordered_Maps provides a private type Map and a set of operations. It provides the same operations as the package Containers.Ordered_Maps (see A.18.6), with the difference that the maximum storage is bounded.

Static Semantics

2/3
{AI05-0001-1} The declaration of the generic library package Containers.Bounded_Ordered_Maps has the same contents and semantics as Containers.Ordered_Maps except:
3/5
{AI12-0112-1} The aspect pragma Preelaborate is replaced with aspect pragma Pure.
4/3
The type Map is declared with a discriminant that specifies the capacity (maximum number of elements) as follows: 
5/5
{AI12-0112-1}   type Map (Capacity : Count_Type) is tagged private...;
6/3
The type Map needs finalization if and only if type Key_Type or type Element_Type needs finalization.
6.a/3
Implementation Note: {AI05-0212-1} The type Map 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. 
6.1/5
{AI12-0339-1} The function Empty is replaced by:
6.2/5
   function Empty (Capacity : Count_Type := implementation-defined)
      return Map
      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;
7/5
{AI12-0112-1} For procedures Insert and Include, 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.
7.1/5
     (if <some length> > Count_Type'Last - <some other length>
      then raise Constraint_Error)
7.2/5
is replaced by:
7.3/5
     (if <some length> > Count_Type'Last - <some other length>
         then raise Constraint_Error
      elsif <some length> > Container.Capacity - <some other length>
         then raise Capacity_Error)
8/5
{AI12-0112-1} In procedure Assign, the precondition is altered to: if Source length is greater than Target capacity, then Capacity_Error is propagated.
8.1/5
   Pre => (not Tampering_With_Cursors_Prohibited (Target)
              or else raise Program_Error) and then
           (Length (Source) <= Target.Capacity
              or else raise Capacity_Error),
9/3
The function Copy is replaced with: 
10/5
{AI12-0112-1}   function Copy (Source   : Map;
                 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_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)
;
11/5
{AI12-0112-1} Returns a map with key/element pairs initialized from the values in Source. If Capacity is 0, then the map capacity is the length of Source; if Capacity is equal to or greater than the length of Source, the map capacity is the specified value; otherwise, the operation propagates Capacity_Error.

Bounded (Run-Time) Errors

12/3
{AI05-0160-1} {AI05-0265-1} It is a bounded error to assign from a bounded map 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. 
12.a/3
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

13/3
{AI05-0265-1} When a bounded map object M is finalized, if tampering with cursors is prohibited for M other than due to an assignment from another map, then execution is erroneous.
13.a/3
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

14/3
{AI05-0184-1} {AI05-0264-1} For each instance of Containers.Ordered_Maps and each instance of Containers.Bounded_Ordered_Maps, if the two instances meet the following conditions, then the output generated by the Map'Output or Map'Write subprograms of either instance shall be readable by the Map'Input or Map'Read of the other instance, respectively:
15/3
{AI05-0184-1} {AI05-0248-1} the Element_Type parameters of the two instances are statically matching subtypes of the same type; and
16/3
{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); and
17/3
{AI05-0184-1} the preceding two conditions also hold for the Key_Type parameters of the instances. 

Implementation Advice

18/3
{AI05-0001-1} {AI05-0269-1} Bounded ordered map objects should be implemented without implicit pointers or dynamic allocation. 
18.a.1/3
Implementation Advice: Bounded ordered map objects should be implemented without implicit pointers or dynamic allocation.
19/3
{AI05-0001-1} The implementation advice for procedure Move to minimize copying does not apply. 
19.a.1/3
Implementation Advice: The implementation advice for procedure Move to minimize copying does not apply to bounded ordered maps.

Extensions to Ada 2005

19.a/3
{AI05-0001-1} {AI05-0160-1} {AI05-0184-1} The generic package Containers.Bounded_Ordered_Maps is new. 

Inconsistencies With Ada 2012

19.b/5
{AI12-0111-1} Corrigendum: 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 202x. Needless to say, this shouldn't happen outside of test programs. See Inconsistencies With Ada 2012 in A.18.2 for more details. 

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