A.18.21 The Generic Package Containers.Bounded_Hashed_Maps
The language-defined generic package Containers.Bounded_Hashed_Maps
provides a private type Map and a set of operations. It provides the
same operations as the package Containers.Hashed_Maps (see A.18.5),
with the difference that the maximum storage is bounded.
The declaration of the generic library package
Containers.Bounded_Hashed_Maps has the same contents and semantics as
type Map 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 Map (Capacity : Count_Type;
Modulus : Hash_Type) is tagged private;
The type Map needs finalization
if and only if type Key_Type or type Element_Type needs finalization.
The type Map 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 : Map;
Capacity : Count_Type := 0;
Modulus : Hash_Type := 0) return Map;
Returns a map with key/element pairs initialized
from the values in Source. If Capacity is 0, then the map capacity is
the length of Source; if Capacity is equal to or greater than the length
of Source, the map capacity is the value of the Capacity parameter; otherwise,
the operation propagates Capacity_Error. If the Modulus argument is
0, then the map modulus is the value returned by a call to Default_Modulus
with the map capacity as its argument; otherwise, the map modulus is
the value of the Modulus parameter.
Bounded (Run-Time) Errors
It is a bounded error to assign
from a bounded map 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 map object M is finalized,
if tampering with cursors is prohibited for M other than due to
an assignment from another map, 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_Maps and
each instance of Containers.Bounded_Hashed_Maps, if the two instances
meet the following conditions, then the output generated by the Map'Output
or Map'Write subprograms of either instance shall be readable by the
Map'Input or Map'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
the preceding two conditions also hold for the
Key_Type parameters of the instances.
Bounded hashed map objects should be implemented
without implicit pointers or dynamic allocation.
Bounded hashed map 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 maps.
Extensions to Ada 2005
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe