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A.18.12 The Generic Package Containers.Indefinite_Doubly_Linked_Lists

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{AI95-00302-03} The language-defined generic package Containers.Indefinite_Doubly_Linked_Lists provides private types List and Cursor, and a set of operations for each type. It provides the same operations as the package Containers.Doubly_Linked_Lists (see A.18.3), with the difference that the generic formal Element_Type is indefinite. 

Static Semantics

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{AI95-00302-03} {AI05-0092-1} The declaration of the generic library package Containers.Indefinite_Doubly_Linked_Lists has the same contents and semantics as Containers.Doubly_Linked_Lists except:
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The generic formal Element_Type is indefinite.
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The procedure with the profile: 
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procedure Insert (Container : in out List;
                  Before    : in     Cursor;
                  Position  :    out Cursor;
                  Count     : in     Count_Type := 1);
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is omitted.
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Discussion: This procedure is omitted because there is no way to create a default-initialized object of an indefinite type. We considered having this routine insert an empty element similar to the empty elements of a vector, but rejected this possibility because the semantics are fairly complex and very different from the existing definite container. That would make it more error-prone to convert a container from a definite type to an indefinite type; by omitting the routine completely, any problems will be diagnosed by the compiler. 
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The actual Element parameter of access subprogram Process of Update_Element may be constrained even if Element_Type is unconstrained.
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{AI12-0035-1} The operations Append, Insert, Prepend, and Replace_Element that have a formal parameter of type Element_Type perform indefinite insertion (see A.18).
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{AI12-0111-1} {AI12-0112-1} The description of Tampering_With_Elements_Prohibited is replaced by:
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Returns True if tampering with elements is prohibited for Container, and False otherwise.
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Reason: Complete replacement of an element can cause its memory to be deallocated while another operation is holding onto a reference to it. That can't be allowed. However, a simple modification of (part of) an element is not a problem.
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{AI12-0111-1} {AI12-0112-1} Tampering_With_Cursors_Prohibited is replaced by Tampering_With_Elements_Prohibited in the postcondition for the operations Reference and Constant_Reference.
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{AI12-0111-1} The operations Replace_Element and Swap are omitted from the nested package Stable.

Extensions to Ada 95

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{AI95-00302-03} The generic package Containers.Indefinite_Doubly_Linked_Lists is new. 

Inconsistencies With Ada 2012

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{AI12-0035-1} Corrigendum: Defined some routines to “perform indefinite insertion”. This could mean that some calls to those routines would now raise Program_Error where they previously worked. However, this is extremely unlikely; see Inconsistencies With Ada 2012 in A.18.11 for details.
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{AI12-0111-1} Defined the Iterator_View aspect, so that the stable view is used for container element iterators. This leads to a rare situation where Program_Error will be raised in Ada 2022 for code that would have worked in Ada 2012. See Inconsistencies With Ada 2012 in A.18.11 for details. 
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