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A.18.32 The Generic Package Containers.Bounded_Indefinite_Holders

1/5
{AI12-0254-1} The language-defined generic package Containers.Bounded_Indefinite_Holders provides a private type Holder and a set of operations for that type. It provides the same operations as the package Containers.Indefinite_Holders (see A.18.18), with the difference that the maximum storage is bounded. 

Static Semantics

2/5
{AI12-0254-1} The declaration of the generic library package Containers.Bounded_Indefinite_Holders has the same contents and semantics as Containers.Indefinite_Holders except:
3/5
The following is added to the context clause:
4/5
   with System.Storage_Elements; use System.Storage_Elements;
5/5
An additional generic parameter follows Element_Type:
6/5
   Max_Element_Size_in_Storage_Elements : Storage_Count;
6.a/5
Reason: This value is in Storage_Elements so that it can be used as a discriminant on a storage pool object in the implementation of the object; Ada doesn't allow discriminant dependent components to use formulas.
6.b/5
This is a generic parameter as it is a property of the Element_Type; the largest possible object of Element_Type is unlikely to be different for different containers, so making it a discriminant (as Capacity is) provides no useful capability.
7/5
Add to the precondition of To_Holder and Replace_Element:
8/5
  and then (New_Item'Size <=
               Max_Element_Size_in_Storage_Elements * System.Storage_Unit
            or else raise Program_Error)
8.a/5
Reason: This ensures that an object that won't fit is not inserted into the container.

Bounded (Run-Time) Errors

9/5
{AI12-0254-1} It is a bounded error to assign from a bounded holder object while tampering with elements of that object is prohibited. Either Program_Error is raised by the assignment, execution proceeds with the target object prohibiting tampering with elements, or execution proceeds normally. 

Implementation Requirements

10/5
{AI12-0254-1} For each instance of Containers.Indefinite_Holders and each instance of Containers.Bounded_Indefinite_Holders, if the two instances meet the following conditions, then the output generated by the Holder'Output or Holder'Write subprograms of either instance shall be readable by the Holder'Input or Holder'Read of the other instance, respectively:
11/5
the Element_Type parameters of the two instances are statically matching subtypes of the same type; and
12/5
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

13/5
{AI12-0254-1} Bounded holder objects should be implemented without dynamic allocation and any finalization should be trivial unless Element_Type needs finalization. 
13.a.1/5
Implementation Advice: Bounded holder objects should be implemented without dynamic allocation.
13.a/5
To be honest: Implementation of this container in Ada will probably require the use of a special storage pool. When we say "without dynamic allocation", we mean that this pool does not use heap memory and has a trivial finalization routine (that is, procedures Adjust and Finalize are null procedures). All storage pools are controlled, so we can't reasonably say that a bounded holder will not need finalization. 
13.b/5
Implementation Note: If the implementation supports discontiguous objects that require multiple calls to Allocate in a storage pool, the storage pool will need to support such allocations. The storage pool implementation can assume that all Allocate calls occur together, and similarly for Deallocate calls, thus simplifying the pool implementation so that allocation only occurs at a high-water mark location. 
14/5
{AI12-0350-1} The Implementation Advice about the Move and Swap operations is deleted for bounded holders; these operations can copy elements as needed.
14.a/5
Reason: The memory of a bounded indefinite holder belongs directly to the container, so it cannot be moved with the element. If the element type contains any internal pointers, moving it without calling Adjust would leave such pointers pointing to the wrong holder object. Thus, a full copy is needed, including any associated finalization and adjustments. 

Extensions to Ada 2012

14.b/5
{AI12-0254-1} The generic package Containers.Bounded_Indefinite_Holders is new. 

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