A.18.20 The Generic Package Containers.Bounded_Doubly_Linked_Lists
The language-defined generic package Containers.Bounded_Doubly_Linked_Lists
provides a private type List and a set of operations. It provides the
same operations as the package Containers.Doubly_Linked_Lists (see A.18.3),
with the difference that the maximum storage is bounded.
The declaration of the generic library package
Containers.Bounded_Doubly_Linked_Lists has the same contents and semantics
as Containers.Doubly_Linked_Lists except:
type List is declared with a discriminant that specifies the capacity
(maximum number of elements) as follows:
type List (Capacity : Count_Type) is tagged private;
The type List needs finalization
if and only if type Element_Type needs finalization.
The type List 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.
The allocation of internal
storage includes a check that the capacity is not exceeded, and Capacity_Error
is raised if this check fails.
In procedure Assign, if Source
length is greater than Target capacity, then Capacity_Error is propagated.
function Copy is replaced with:
function Copy (Source : List; Capacity : Count_Type := 0)
If Capacity is 0, then the
list capacity is the length of Source; if Capacity is equal to or greater
than the length of Source, the list capacity equals the value of the
Capacity parameter; otherwise, the operation propagates Capacity_Error.
In the three-parameter procedure
Splice whose Source has type List, if the sum of the length of Target
and the length of Source is greater than the capacity of Target, then
Splice propagates Capacity_Error.
In the four-parameter procedure
Splice, if the length of Target equals the capacity of Target, then Splice
Bounded (Run-Time) Errors
It is a bounded error to assign
from a bounded list 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 list object L is finalized,
if tampering with cursors is prohibited for L other than due to
an assignment from another list, 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.Doubly_Linked_Lists
and each instance of Containers.Bounded_Doubly_Linked_Lists, if the two
instances meet the following conditions, then the output generated by
the List'Output or List'Write subprograms of either instance shall be
readable by the List'Input or List'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 list objects should be implemented without
implicit pointers or dynamic allocation.
Bounded list 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 lists.
Extensions to Ada 2005
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe