A.18.23 The Generic Package Containers.Bounded_Hashed_Sets
The language-defined generic package Containers.Bounded_Hashed_Sets
provides a private type Set and a set of operations. It provides the
same operations as the package Containers.Hashed_Sets (see
A.18.8),
with the difference that the maximum storage is bounded.
Static Semantics
The declaration of
the generic library package Containers.Bounded_Hashed_Sets has the same
contents and semantics as Containers.Hashed_Sets except:
The aspect Preelaborate is replaced with aspect
Pure. Aspect Global is deleted.
The type Set is declared
with discriminants that specify both the capacity (number of elements)
and modulus (number of distinct hash values) of the hash table as follows:
type Set (Capacity : Count_Type;
Modulus : Hash_Type) is tagged private...
Preelaborable_Initialization =>
Element_Type'Preelaborable_Initialization
The type Set needs finalization if and only if
type Element_Type needs finalization.
In function Empty,
the postcondition is altered to:
Post =>
Empty'Result.Capacity = Capacity and then
Empty'Result.Modulus = Default_Modulus (Capacity) and then
not Tampering_With_Cursors_Prohibited (Empty'Result) and then
Length (Empty'Result) = 0;
The description of
Reserve_Capacity is replaced with:
procedure Reserve_Capacity (Container : in out Set;
Capacity : in Count_Type)
with Pre => Capacity <= Container.Capacity
or else raise Capacity_Error;
This operation has no effect, other than checking
the precondition.
An additional operation
is added immediately following Reserve_Capacity:
function Default_Modulus (Capacity : Count_Type)
return Hash_Type;
Default_Modulus returns an implementation-defined
value for the number of distinct hash values to be used for the given
capacity (maximum number of elements).
For procedures Insert
and Include, the part of the precondition reading:
(<some length> <= Count_Type'Last - <some other length>
or else raise Constraint_Error)
is replaced
by:
(<some length> <= Count_Type'Last - <some other length>
or else raise Constraint_Error) and then
(<some length> <= Container.Capacity - <some other length>
or else raise Capacity_Error)
In procedure Assign,
the precondition is altered to:
Pre => (not Tampering_With_Cursors_Prohibited (Target)
or else raise Program_Error) and then
(Length (Source) <= Target.Capacity
or else raise Capacity_Error),
The function Copy
is replaced with:
function Copy (Source : Set;
Capacity : Count_Type := 0;
Modulus : Hash_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_Cursors_Prohibited (Copy'Result)
and then
Copy'Result.Capacity = (
if Capacity = 0
then
Length (Source)
else Capacity)
and then
Copy'Result.Modulus = (
if Modulus = 0
then
Default_Modulus (Capacity)
else Modulus);
Returns a set with key/element pairs initialized
from the values in Source.
Bounded (Run-Time) Errors
It is a bounded error to assign
from a bounded set 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.
Erroneous Execution
When a bounded set object
S is finalized,
if tampering with cursors is prohibited for
S other than due to
an assignment from another set, then execution is erroneous.
Implementation Requirements
For each instance of
Containers.Hashed_Sets and each instance of Containers.Bounded_Hashed_Sets,
if the two instances meet the following conditions, then the output generated
by the Set'Output or Set'Write subprograms of either instance shall be
readable by the Set'Input or Set'Read of the other instance, respectively:
the Element_Type parameters of the two instances
are statically matching subtypes of the same type; and
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
Bounded hashed set objects should be implemented
without implicit pointers or dynamic allocation.
The implementation advice for procedure Move to minimize
copying does not apply.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
