A.18.19 The Generic Package Containers.Bounded_Vectors
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The language-defined generic package Containers.Bounded_Vectors
provides a private type Vector and a set of operations. It provides the
same operations as the package Containers.Vectors (see
A.18.2),
with the difference that the maximum storage is bounded.
Static Semantics
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The declaration of the generic library package Containers.Bounded_Vectors
has the same contents and semantics as Containers.Vectors except:
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The aspect Preelaborate is replaced with aspect Pure. Aspect Global is
deleted.
Ramification: {
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The Global aspect for a Pure package is
null (see
6.1.2),
so we don't need to give it explicitly.
The type Vector is
declared with a discriminant that specifies the capacity:
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type Vector (Capacity : Count_Type)
is tagged private...
Preelaborable_Initialization =>
Element_Type'Preelaborable_Initialization
The type Vector needs finalization if and only
if type Element_Type needs finalization.
Implementation Note: {
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The type Vector 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.
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Capacity is omitted from the Stable_Properties of type Vector.
Reason: The capacity is a discriminant
here, so it can't be changed by most routines; thus including it in the
stable properties adds no information.
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In function Empty, the postcondition is altered to:
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;
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In function Copy, the postcondition is altered to:
Post => Length (Copy'Result) = Length (Source) and then
(if Capacity > Length (Source) then
Copy'Result.Capacity = Capacity
else Copy'Result.Capacity >= Length (Source));
The description of
Reserve_Capacity is replaced with:
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procedure Reserve_Capacity (Container :
in out Vector;
Capacity :
in Count_Type)
with Pre => Capacity <= Container.Capacity
or else raise Capacity_Error;
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This operation has no effect, [other than checking the precondition].
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The portion of the postcondition checking the capacity is omitted from
subprograms Set_Length, Assign, Insert, Insert_Space, Prepend, Append,
and Delete.
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For procedures Insert, Insert_Space, Prepend, and Append, the part of
the precondition reading:
(<some length> <= Maximum_Length - <some other length>
or else raise Constraint_Error)
is replaced
by:
(<some length> <= Maximum_Length - <some other length>
or else raise Constraint_Error) and then
(<some length> <= Container.Capacity - <some other length>
or else raise Capacity_Error)
Bounded (Run-Time) Errors
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It is a bounded error to assign from a bounded vector
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 vector object
V is finalized, if tampering with
cursors is prohibited for
V other than due to an assignment from
another vector, 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.Vectors and each instance of Containers.Bounded_Vectors,
if the two instances meet the following conditions, then the output generated
by the Vector'Output or Vector'Write subprograms of either instance shall
be readable by the Vector'Input or Vector'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);
and
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the preceding two conditions also hold for the Index_Type parameters
of the instances.
Implementation Advice
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Bounded vector objects should be implemented without implicit pointers
or dynamic allocation.
Implementation Advice: Bounded vector
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
vectors.
Extensions to Ada 2005
Inconsistencies With Ada 2012
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Correction: 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
2022. Needless to say, this shouldn't happen outside of test programs.
See Inconsistencies With Ada 2012 in
A.18.2
for more details.
Incompatibilities With Ada 2012
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Correction:A bounded vector now only has Preelaborable_Initialization
(abbreviated PI in this note) when the actual for the Element_Type has
PI. If an program used a vector 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.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
