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.
The declaration of the generic library package
Containers.Bounded_Hashed_Sets has the same contents and semantics as
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;
The type Set needs finalization
if and only if type Element_Type needs finalization.
The type Set cannot depend on package Ada.Finalization
unless the element or key 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.
description of Reserve_Capacity is replaced with:
If the specified Capacity
is larger than the capacity of Container, then Reserve_Capacity propagates
Capacity_Error. Otherwise, the operation has no effect.
additional operation is added immediately following Reserve_Capacity:
function Default_Modulus (Capacity : Count_Type) return Hash_Type;
an implementation-defined value for the number of distinct hash values
to be used for the given capacity (maximum number of elements).
function Copy is replaced with:
function Copy (Source : Set;
Capacity : Count_Type := 0;
Modulus : Hash_Type := 0) return Set;
Returns a set whose elements are initialized from
the values in Source. If Capacity is 0, then the set capacity is the
length of Source; if Capacity is equal to or greater than the length
of Source, the set capacity is the value of the Capacity parameter; otherwise,
the operation propagates Capacity_Error. If the Modulus argument is 0,
then the set modulus is the value returned by a call to Default_Modulus
with the set capacity as its argument; otherwise, the set modulus is
the value of the Modulus parameter.
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.
with elements includes tampering with cursors, so we only really need
to talk about tampering with elements here; we mention cursors for clarity.
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.
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.
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
Bounded hashed set objects should be implemented
without implicit pointers or dynamic allocation.
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.
The implementation advice for procedure
Move to minimize copying does not apply to bounded hashed sets.
Extensions to Ada 2005
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe