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.
Static Semantics
The declaration of
the generic library package Containers.Bounded_Hashed_Maps has the same
contents and semantics as Containers.Hashed_Maps except:
The 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 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.
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).
The 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.
Erroneous Execution
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.
Implementation Requirements
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
parameters); and
the preceding two conditions also hold for the
Key_Type parameters of the instances.
Implementation Advice
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.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
