A.18.20 The Generic Package Containers.Bounded_Doubly_Linked_Lists
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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
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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 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: {
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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
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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
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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
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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:
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the Element_Type parameters of the two instances are statically matching
subtypes of the same type; and
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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
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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.
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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
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
