A.18.3 The Generic Package Containers.Doubly_Linked_Lists
{
AI95-00302-03}
The language-defined generic package Containers.Doubly_Linked_Lists provides
private types List and Cursor, and a set of operations for each type.
A list container is optimized for insertion and deletion at any position.
{
AI95-00302-03}
A doubly-linked list container object manages a linked
list of internal
nodes, each of which contains an element and
pointers to the next (successor) and previous (predecessor) internal
nodes. A cursor designates a particular node within a list (and by extension
the element contained in that node). A cursor keeps designating the same
node (and element) as long as the node is part of the container, even
if the node is moved in the container.
{
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The
length of a list is the number of elements it contains.
Static Semantics
{
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The generic library package Containers.Doubly_Linked_Lists has the following
declaration:
{
AI05-0084-1}
{
AI05-0212-1}
with Ada.Iterator_Interfaces;
generic
type Element_Type
is private;
with function "=" (Left, Right : Element_Type)
return Boolean
is <>;
package Ada.Containers.Doubly_Linked_Lists
is
pragma Preelaborate(Doubly_Linked_Lists);
pragma Remote_Types(Doubly_Linked_Lists);
{
AI05-0212-1}
type List
is tagged private
with Constant_Indexing => Constant_Reference,
Variable_Indexing => Reference,
Default_Iterator => Iterate,
Iterator_Element => Element_Type;
pragma Preelaborable_Initialization(List);
type Cursor
is private;
pragma Preelaborable_Initialization(Cursor);
Empty_List :
constant List;
No_Element :
constant Cursor;
{
AI05-0212-1}
function Has_Element (Position : Cursor)
return Boolean;
{
AI05-0212-1}
package List_Iterator_Interfaces
is new
Ada.Iterator_Interfaces (Cursor, Has_Element);
function "=" (Left, Right : List) return Boolean;
function Length (Container : List)
return Count_Type;
function Is_Empty (Container : List)
return Boolean;
procedure Clear (Container :
in out List);
function Element (Position : Cursor)
return Element_Type;
procedure Replace_Element (Container :
in out List;
Position :
in Cursor;
New_Item :
in Element_Type);
procedure Query_Element
(Position :
in Cursor;
Process :
not null access procedure (Element :
in Element_Type));
procedure Update_Element
(Container :
in out List;
Position :
in Cursor;
Process :
not null access procedure
(Element :
in out Element_Type));
{
AI05-0212-1}
type Constant_Reference_Type
(Element :
not null access constant Element_Type)
is private
with Implicit_Dereference => Element;
{
AI05-0212-1}
type Reference_Type (Element :
not null access Element_Type)
is private
with Implicit_Dereference => Element;
{
AI05-0212-1}
function Constant_Reference (Container :
aliased in List;
Position :
in Cursor)
return Constant_Reference_Type;
{
AI05-0212-1}
function Reference (Container :
aliased in out List;
Position :
in Cursor)
return Reference_Type;
{
AI05-0001-1}
procedure Assign (Target :
in out List; Source :
in List);
{
AI05-0001-1}
function Copy (Source : List)
return List;
procedure Move (Target :
in out List;
Source :
in out List);
procedure Insert (Container :
in out List;
Before :
in Cursor;
New_Item :
in Element_Type;
Count :
in Count_Type := 1);
procedure Insert (Container :
in out List;
Before :
in Cursor;
New_Item :
in Element_Type;
Position :
out Cursor;
Count :
in Count_Type := 1);
procedure Insert (Container :
in out List;
Before :
in Cursor;
Position :
out Cursor;
Count :
in Count_Type := 1);
procedure Prepend (Container :
in out List;
New_Item :
in Element_Type;
Count :
in Count_Type := 1);
procedure Append (Container :
in out List;
New_Item :
in Element_Type;
Count :
in Count_Type := 1);
procedure Delete (Container :
in out List;
Position :
in out Cursor;
Count :
in Count_Type := 1);
procedure Delete_First (Container :
in out List;
Count :
in Count_Type := 1);
procedure Delete_Last (Container :
in out List;
Count :
in Count_Type := 1);
procedure Reverse_Elements (Container :
in out List);
procedure Swap (Container :
in out List;
I, J :
in Cursor);
procedure Swap_Links (Container :
in out List;
I, J :
in Cursor);
procedure Splice (Target :
in out List;
Before :
in Cursor;
Source :
in out List);
procedure Splice (Target :
in out List;
Before :
in Cursor;
Source :
in out List;
Position :
in out Cursor);
procedure Splice (Container:
in out List;
Before :
in Cursor;
Position :
in Cursor);
function First (Container : List)
return Cursor;
function First_Element (Container : List)
return Element_Type;
function Last (Container : List)
return Cursor;
function Last_Element (Container : List)
return Element_Type;
function Next (Position : Cursor)
return Cursor;
function Previous (Position : Cursor)
return Cursor;
procedure Next (Position :
in out Cursor);
procedure Previous (Position :
in out Cursor);
function Find (Container : List;
Item : Element_Type;
Position : Cursor := No_Element)
return Cursor;
function Reverse_Find (Container : List;
Item : Element_Type;
Position : Cursor := No_Element)
return Cursor;
function Contains (Container : List;
Item : Element_Type)
return Boolean;
procedure Iterate
(Container :
in List;
Process :
not null access procedure (Position :
in Cursor));
procedure Reverse_Iterate
(Container :
in List;
Process :
not null access procedure (Position :
in Cursor));
{
AI05-0212-1}
function Iterate (Container :
in List)
return List_Iterator_Interfaces.Reversible_Iterator'Class;
{
AI05-0212-1}
function Iterate (Container :
in List; Start :
in Cursor)
return List_Iterator_Interfaces.Reversible_Iterator'Class;
generic
with function "<" (Left, Right : Element_Type)
return Boolean is <>;
package Generic_Sorting
is
function Is_Sorted (Container : List)
return Boolean;
procedure Sort (Container :
in out List);
procedure Merge (Target :
in out List;
Source :
in out List);
end Generic_Sorting;
private
... -- not specified by the language
end Ada.Containers.Doubly_Linked_Lists;
{
AI95-00302-03}
The actual function for the generic formal function "=" on
Element_Type values is expected to define a reflexive and symmetric relationship
and return the same result value each time it is called with a particular
pair of values. If it behaves in some other manner, the functions Find,
Reverse_Find, and "=" on list values return an unspecified
value. The exact arguments and number of calls of this generic formal
function by the functions Find, Reverse_Find, and "=" on list
values are unspecified.
Ramification: If the actual function
for "=" is not symmetric and consistent, the result returned
by the listed functions cannot be predicted. The implementation is not
required to protect against "=" raising an exception, or returning
random results, or any other “bad” behavior. And it can call
"=" in whatever manner makes sense. But note that only the
results of Find, Reverse_Find, and List "=" are unspecified;
other subprograms are not allowed to break if "=" is bad (they
aren't expected to use "=").
{
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The type List is used to represent lists. The type List needs finalization
(see
7.6).
{
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Empty_List represents the empty List object. It has a length of 0. If
an object of type List is not otherwise initialized, it is initialized
to the same value as Empty_List.
{
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No_Element represents a cursor that designates no element. If an object
of type Cursor is not otherwise initialized, it is initialized to the
same value as No_Element.
{
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The predefined "=" operator for type Cursor returns True if
both cursors are No_Element, or designate the same element in the same
container.
{
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Execution of the default implementation of the Input, Output, Read, or
Write attribute of type Cursor raises Program_Error.
Reason: A cursor will probably be implemented
in terms of one or more access values, and the effects of streaming access
values is unspecified. Rather than letting the user stream junk by accident,
we mandate that streaming of cursors raise Program_Error by default.
The attributes can always be specified if there is a need to support
streaming.
{
AI05-0001-1}
{
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List'Write for a List object
L writes Length(
L) elements
of the list to the stream. It also may write additional information about
the list.
{
AI05-0001-1}
{
AI05-0262-1}
List'Read reads the representation of a list from the stream, and assigns
to
Item a list with the same length and elements as was written
by List'Write.
Ramification: Streaming more elements
than the container length is wrong. For implementation implications of
this rule, see the Implementation Note in
A.18.2.
{
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[Some operations of this generic package have access-to-subprogram parameters.
To ensure such operations are well-defined, they guard against certain
actions by the designated subprogram. In particular, some operations
check for “tampering with cursors” of a container because
they depend on the set of elements of the container remaining constant,
and others check for “tampering with elements” of a container
because they depend on elements of the container not being replaced.]
{
AI95-00302-03}
A subprogram is said to
tamper with cursors
of a list object
L if:
it inserts or deletes elements of L, that
is, it calls the Insert, Clear, Delete, or Delete_Last procedures with
L as a parameter; or
To be honest: Operations which are defined
to be equivalent to a call on one of these operations also are included.
Similarly, operations which call one of these as part of their definition
are included.
it reorders the elements of L, that is,
it calls the Splice, Swap_Links, or Reverse_Elements procedures or the
Sort or Merge procedures of an instance of Generic_Sorting with L
as a parameter; or
it finalizes L; or
{
AI05-0001-1}
it calls the Assign procedure with
L as the Target parameter;
or
Ramification: We don't need to explicitly
mention
assignment_statement,
because that finalizes the target object as part of the operation, and
finalization of an object is already defined as tampering with cursors.
it calls the Move procedure with L as a
parameter.
Reason: Swap copies elements rather than
reordering them, so it doesn't tamper with cursors.
{
AI95-00302-03}
A subprogram is said to
tamper with elements
of a list object
L if:
it tampers with cursors of L; or
it replaces one or more elements of L, that
is, it calls the Replace_Element or Swap procedures with L as
a parameter.
Reason: Complete replacement of an element
can cause its memory to be deallocated while another operation is holding
onto a reference to it. That can't be allowed. However, a simple modification
of (part of) an element is not a problem, so Update_Element does not
cause a problem.
{
AI05-0265-1}
{
AI12-0110-1}
When tampering with cursors is
prohibited for a particular list object
L, Program_Error
is propagated by a call of any language-defined subprogram that is defined
to tamper with the cursors of
L, leaving
L unmodified.
Similarly, when tampering with elements is
prohibited for a particular
list object
L, Program_Error is propagated by a call of any language-defined
subprogram that is defined to tamper with the elements of
L [(or
tamper with the cursors of
L)], leaving
L unmodified. These
checks are made before any other defined behavior of the body of the
language-defined subprogram.
Proof: Tampering with elements includes
tampering with cursors, so we mention it only from completeness in the
second sentence.
function Has_Element (Position : Cursor) return Boolean;
{
AI05-0212-1}
Returns True if Position designates an element, and returns False otherwise.
To be honest: {
AI05-0005-1}
{
AI05-0212-1}
This function might not detect cursors that designate deleted elements;
such cursors are invalid (see below) and the result of calling Has_Element
with an invalid cursor is unspecified (but not erroneous).
function "=" (Left, Right : List) return Boolean;
{
AI95-00302-03}
{
AI05-0264-1}
If Left and Right denote the same list object, then the function returns
True. If Left and Right have different lengths, then the function returns
False. Otherwise, it compares each element in Left to the corresponding
element in Right using the generic formal equality operator. If any such
comparison returns False, the function returns False; otherwise, it returns
True. Any exception raised during evaluation of element equality is propagated.
Implementation Note: This wording describes
the canonical semantics. However, the order and number of calls on the
formal equality function is unspecified for all of the operations that
use it in this package, so an implementation can call it as many or as
few times as it needs to get the correct answer. Specifically, there
is no requirement to call the formal equality additional times once the
answer has been determined.
function Length (Container : List) return Count_Type;
function Is_Empty (Container : List) return Boolean;
procedure Clear (Container : in out List);
function Element (Position : Cursor) return Element_Type;
{
AI95-00302-03}
If Position equals No_Element, then Constraint_Error is propagated. Otherwise,
Element returns the element designated by Position.
procedure Replace_Element (Container : in out List;
Position : in Cursor;
New_Item : in Element_Type);
{
AI95-00302-03}
{
AI05-0264-1}
If Position equals No_Element, then Constraint_Error is propagated; if
Position does not designate an element in Container, then Program_Error
is propagated. Otherwise, Replace_Element assigns the value New_Item
to the element designated by Position.
procedure Query_Element
(Position : in Cursor;
Process : not null access procedure (Element : in Element_Type));
{
AI95-00302-03}
{
AI05-0021-1}
{
AI05-0265-1}
If Position equals No_Element, then Constraint_Error is propagated. Otherwise,
Query_Element calls Process.
all with the element designated by
Position as the argument. Tampering with the elements of the list that
contains the element designated by Position is prohibited during the
execution of the call on Process.
all. Any exception raised by
Process.
all is propagated.
procedure Update_Element
(Container : in out List;
Position : in Cursor;
Process : not null access procedure (Element : in out Element_Type));
{
AI95-00302-03}
{
AI05-0264-1}
{
AI05-0265-1}
If Position equals No_Element, then Constraint_Error is propagated; if
Position does not designate an element in Container, then Program_Error
is propagated. Otherwise, Update_Element calls Process.
all with
the element designated by Position as the argument. Tampering with the
elements of Container is prohibited during the execution of the call
on Process.
all. Any exception raised by Process.
all is
propagated.
If Element_Type
is unconstrained and definite, then the actual Element parameter of Process.all
shall be unconstrained.
Ramification: This means that the elements
cannot be directly allocated from the heap; it must be possible to change
the discriminants of the element in place.
type Constant_Reference_Type
(Element : not null access constant Element_Type) is private
with Implicit_Dereference => Element;
type Reference_Type (Element : not null access Element_Type) is private
with Implicit_Dereference => Element;
{
AI05-0212-1}
The types Constant_Reference_Type and Reference_Type need finalization.
The default initialization of an object of type
Constant_Reference_Type or Reference_Type propagates Program_Error.
Reason: It is expected that Reference_Type
(and Constant_Reference_Type) will be a controlled type, for which finalization
will have some action to terminate the tampering check for the associated
container. If the object is created by default, however, there is no
associated container. Since this is useless, and supporting this case
would take extra work, we define it to raise an exception.
function Constant_Reference (Container : aliased in List;
Position : in Cursor)
return Constant_Reference_Type;
{
AI05-0212-1}
{
AI05-0269-1}
This function (combined with the Constant_Indexing and Implicit_Dereference
aspects) provides a convenient way to gain read access to an individual
element of a list given a cursor.
{
AI05-0212-1}
{
AI05-0265-1}
If Position equals No_Element, then Constraint_Error is propagated; if
Position does not designate an element in Container, then Program_Error
is propagated. Otherwise, Constant_Reference returns an object whose
discriminant is an access value that designates the element designated
by Position. Tampering with the elements of Container is prohibited while
the object returned by Constant_Reference exists and has not been finalized.
function Reference (Container : aliased in out List;
Position : in Cursor)
return Reference_Type;
{
AI05-0212-1}
{
AI05-0269-1}
This function (combined with the Variable_Indexing and Implicit_Dereference
aspects) provides a convenient way to gain read and write access to an
individual element of a list given a cursor.
{
AI05-0212-1}
{
AI05-0265-1}
If Position equals No_Element, then Constraint_Error is propagated; if
Position does not designate an element in Container, then Program_Error
is propagated. Otherwise, Reference returns an object whose discriminant
is an access value that designates the element designated by Position.
Tampering with the elements of Container is prohibited while the object
returned by Reference exists and has not been finalized.
procedure Assign (Target : in out List; Source : in List);
{
AI05-0001-1}
{
AI05-0248-1}
If Target denotes the same object as Source, the operation has no effect.
Otherwise, the elements of Source are copied to Target as for an
assignment_statement
assigning Source to Target.
Discussion: {
AI05-0005-1}
This routine exists for compatibility with the bounded list container.
For an unbounded list,
Assign(A, B) and
A := B behave
identically. For a bounded list, := will raise an exception if the container
capacities are different, while Assign will not raise an exception if
there is enough room in the target.
function Copy (Source : List) return List;
{
AI05-0001-1}
Returns a list whose elements match the elements of Source.
procedure Move (Target : in out List;
Source : in out List);
{
AI95-00302-03}
{
AI05-0001-1}
{
AI05-0248-1}
{
AI05-0262-1}
If Target denotes the same object as Source, then the operation has no
effect. Otherwise, the operation is equivalent to Assign (Target, Source)
followed by Clear (Source).
procedure Insert (Container : in out List;
Before : in Cursor;
New_Item : in Element_Type;
Count : in Count_Type := 1);
{
AI95-00302-03}
If Before is not No_Element, and does not designate an element in Container,
then Program_Error is propagated. Otherwise, Insert inserts Count copies
of New_Item prior to the element designated by Before. If Before equals
No_Element, the new elements are inserted after the last node (if any).
Any exception raised during allocation of internal storage is propagated,
and Container is not modified.
Ramification: The check on Before checks
that the cursor does not belong to some other Container. This check implies
that a reference to the container is included in the cursor value. This
wording is not meant to require detection of dangling cursors; such cursors
are defined to be invalid, which means that execution is erroneous, and
any result is allowed (including not raising an exception).
procedure Insert (Container : in out List;
Before : in Cursor;
New_Item : in Element_Type;
Position : out Cursor;
Count : in Count_Type := 1);
{
AI95-00302-03}
{
AI05-0257-1}
If Before is not No_Element, and does not designate an element in Container,
then Program_Error is propagated. Otherwise, Insert allocates Count copies
of New_Item, and inserts them prior to the element designated by Before.
If Before equals No_Element, the new elements are inserted after the
last element (if any). Position designates the first newly-inserted element,
or if Count equals 0, then Position is assigned the value of Before.
Any exception raised during allocation of internal storage is propagated,
and Container is not modified.
procedure Insert (Container : in out List;
Before : in Cursor;
Position : out Cursor;
Count : in Count_Type := 1);
{
AI95-00302-03}
{
AI05-0257-1}
If Before is not No_Element, and does not designate an element in Container,
then Program_Error is propagated. Otherwise, Insert inserts Count new
elements prior to the element designated by Before. If Before equals
No_Element, the new elements are inserted after the last node (if any).
The new elements are initialized by default (see
3.3.1).
Position designates the first newly-inserted element, or if Count equals
0, then Position is assigned the value of Before. Any exception raised
during allocation of internal storage is propagated, and Container is
not modified.
procedure Prepend (Container : in out List;
New_Item : in Element_Type;
Count : in Count_Type := 1);
{
AI95-00302-03}
Equivalent to Insert (Container, First (Container), New_Item, Count).
procedure Append (Container : in out List;
New_Item : in Element_Type;
Count : in Count_Type := 1);
{
AI95-00302-03}
Equivalent to Insert (Container, No_Element, New_Item, Count).
procedure Delete (Container : in out List;
Position : in out Cursor;
Count : in Count_Type := 1);
{
AI95-00302-03}
{
AI05-0264-1}
If Position equals No_Element, then Constraint_Error is propagated. If
Position does not designate an element in Container, then Program_Error
is propagated. Otherwise, Delete removes (from Container) Count elements
starting at the element designated by Position (or all of the elements
starting at Position if there are fewer than Count elements starting
at Position). Finally, Position is set to No_Element.
procedure Delete_First (Container : in out List;
Count : in Count_Type := 1);
{
AI95-00302-03}
{
AI05-0021-1}
If Length (Container) <= Count, then Delete_First is equivalent to
Clear (Container). Otherwise, it removes the first Count nodes from Container.
procedure Delete_Last (Container : in out List;
Count : in Count_Type := 1);
{
AI95-00302-03}
{
AI05-0264-1}
If Length (Container) <= Count, then Delete_Last is equivalent to
Clear (Container). Otherwise, it removes the last Count nodes from Container.
procedure Reverse_Elements (Container : in out List);
{
AI95-00302-03}
Reorders the elements of Container in reverse order.
Discussion: Unlike the similar routine
for a vector, elements should not be copied; rather, the nodes should
be exchanged. Cursors are expected to reference the same elements afterwards.
procedure Swap (Container : in out List;
I, J : in Cursor);
{
AI95-00302-03}
If either I or J is No_Element, then Constraint_Error is propagated.
If either I or J do not designate an element in Container, then Program_Error
is propagated. Otherwise, Swap exchanges the values of the elements designated
by I and J.
Ramification: After a call to Swap, I
designates the element value previously designated by J, and J designates
the element value previously designated by I. The cursors do not become
ambiguous from this operation.
To be honest: The implementation is not
required to actually copy the elements if it can do the swap some other
way. But it is allowed to copy the elements if needed.
procedure Swap_Links (Container : in out List;
I, J : in Cursor);
{
AI95-00302-03}
If either I or J is No_Element, then Constraint_Error is propagated.
If either I or J do not designate an element in Container, then Program_Error
is propagated. Otherwise, Swap_Links exchanges the nodes designated by
I and J.
Ramification: Unlike Swap, this exchanges
the nodes, not the elements. No copying is performed. I and J designate
the same elements after this call as they did before it. This operation
can provide better performance than Swap if the element size is large.
procedure Splice (Target : in out List;
Before : in Cursor;
Source : in out List);
{
AI95-00302-03}
If Before is not No_Element, and does not designate an element in Target,
then Program_Error is propagated. Otherwise, if Source denotes the same
object as Target, the operation has no effect. Otherwise, Splice reorders
elements such that they are removed from Source and moved to Target,
immediately prior to Before. If Before equals No_Element, the nodes of
Source are spliced after the last node of Target. The length of Target
is incremented by the number of nodes in Source, and the length of Source
is set to 0.
procedure Splice (Target : in out List;
Before : in Cursor;
Source : in out List;
Position : in out Cursor);
{
AI95-00302-03}
{
AI05-0264-1}
If Position is No_Element, then Constraint_Error is propagated. If Before
does not equal No_Element, and does not designate an element in Target,
then Program_Error is propagated. If Position does not equal No_Element,
and does not designate a node in Source, then Program_Error is propagated.
If Source denotes the same object as Target, then there is no effect
if Position equals Before, else the element designated by Position is
moved immediately prior to Before, or, if Before equals No_Element, after
the last element. In both cases, Position and the length of Target are
unchanged. Otherwise, the element designated by Position is removed from
Source and moved to Target, immediately prior to Before, or, if Before
equals No_Element, after the last element of Target. The length of Target
is incremented, the length of Source is decremented, and Position is
updated to represent an element in Target.
Ramification: If Source is the same as
Target, and Position = Before, or Next(Position) = Before, Splice has
no effect, as the element does not have to move to meet the postcondition.
procedure Splice (Container: in out List;
Before : in Cursor;
Position : in Cursor);
{
AI95-00302-03}
{
AI05-0264-1}
If Position is No_Element, then Constraint_Error is propagated. If Before
does not equal No_Element, and does not designate an element in Container,
then Program_Error is propagated. If Position does not equal No_Element,
and does not designate a node in Container, then Program_Error is propagated.
If Position equals Before there is no effect. Otherwise, the element
designated by Position is moved immediately prior to Before, or, if Before
equals No_Element, after the last element. The length of Container is
unchanged.
function First (Container : List) return Cursor;
{
AI95-00302-03}
{
AI05-0264-1}
If Container is empty, First returns the value No_Element. Otherwise,
it returns a cursor that designates the first node in Container.
function First_Element (Container : List) return Element_Type;
function Last (Container : List) return Cursor;
{
AI95-00302-03}
{
AI05-0264-1}
If Container is empty, Last returns the value No_Element. Otherwise,
it returns a cursor that designates the last node in Container.
function Last_Element (Container : List) return Element_Type;
function Next (Position : Cursor) return Cursor;
{
AI95-00302-03}
If Position equals No_Element or designates the last element of the container,
then Next returns the value No_Element. Otherwise, it returns a cursor
that designates the successor of the element designated by Position.
function Previous (Position : Cursor) return Cursor;
{
AI95-00302-03}
If Position equals No_Element or designates the first element of the
container, then Previous returns the value No_Element. Otherwise, it
returns a cursor that designates the predecessor of the element designated
by Position.
procedure Next (Position : in out Cursor);
procedure Previous (Position : in out Cursor);
function Find (Container : List;
Item : Element_Type;
Position : Cursor := No_Element)
return Cursor;
{
AI95-00302-03}
If Position is not No_Element, and does not designate an element in Container,
then Program_Error is propagated. Find searches the elements of Container
for an element equal to Item (using the generic formal equality operator).
The search starts at the element designated by Position, or at the first
element if Position equals No_Element. It proceeds towards Last (Container).
If no equal element is found, then Find returns No_Element. Otherwise,
it returns a cursor designating the first equal element encountered.
function Reverse_Find (Container : List;
Item : Element_Type;
Position : Cursor := No_Element)
return Cursor;
{
AI95-00302-03}
If Position is not No_Element, and does not designate an element in Container,
then Program_Error is propagated. Find searches the elements of Container
for an element equal to Item (using the generic formal equality operator).
The search starts at the element designated by Position, or at the last
element if Position equals No_Element. It proceeds towards First (Container).
If no equal element is found, then Reverse_Find returns No_Element. Otherwise,
it returns a cursor designating the first equal element encountered.
function Contains (Container : List;
Item : Element_Type) return Boolean;
{
AI95-00302-03}
Equivalent to Find (Container, Item) /= No_Element.
Paragraphs 139
and 140 were moved above.
procedure Iterate
(Container : in List;
Process : not null access procedure (Position : in Cursor));
{
AI95-00302-03}
{
AI05-0265-1}
Iterate calls Process.
all with a cursor that designates each node
in Container, starting with the first node and moving the cursor as per
the Next function. Tampering with the cursors of Container is prohibited
during the execution of a call on Process.
all. Any exception raised
by Process.
all is propagated.
Implementation Note: The purpose of the
tamper with cursors check is to prevent erroneous execution from the
Position parameter of Process.all becoming invalid. This check
takes place when the operations that tamper with the cursors of the container
are called. The check cannot be made later (say in the body of Iterate),
because that could cause the Position cursor to be invalid and potentially
cause execution to become erroneous -- defeating the purpose of the check.
See Iterate for vectors (
A.18.2)
for a suggested implementation of the check.
procedure Reverse_Iterate
(Container : in List;
Process : not null access procedure (Position : in Cursor));
{
AI95-00302-03}
{
AI05-0212-1}
Iterates over the nodes in Container as per procedure Iterate, except
that elements are traversed in reverse order, starting with the last
node and moving the cursor as per the Previous function.
function Iterate (Container : in List)
return List_Iterator_Interfaces.Reversible_Iterator'Class;
{
AI05-0212-1}
{
AI05-0265-1}
{
AI05-0269-1}
Iterate returns a reversible iterator object (see
5.5.1)
that will generate a value for a loop parameter (see
5.5.2)
designating each node in Container, starting with the first node and
moving the cursor as per the Next function when used as a forward iterator,
and starting with the last node and moving the cursor as per the Previous
function when used as a reverse iterator. Tampering with the cursors
of Container is prohibited while the iterator object exists (in particular,
in the
sequence_of_statements
of the
loop_statement
whose
iterator_specification
denotes this object). The iterator object needs finalization.
function Iterate (Container : in List; Start : in Cursor)
return List_Iterator_Interfaces.Reversible_Iterator'Class;
{
AI05-0212-1}
{
AI05-0262-1}
{
AI05-0265-1}
{
AI05-0269-1}
If Start is not No_Element and does not designate an item in Container,
then Program_Error is propagated. If Start is No_Element, then Constraint_Error
is propagated. Otherwise, Iterate returns a reversible iterator object
(see
5.5.1) that will generate a value for
a loop parameter (see
5.5.2) designating
each node in Container, starting with the node designated by Start and
moving the cursor as per the Next function when used as a forward iterator,
or moving the cursor as per the Previous function when used as a reverse
iterator. Tampering with the cursors of Container is prohibited while
the iterator object exists (in particular, in the
sequence_of_statements
of the
loop_statement
whose
iterator_specification
denotes this object). The iterator object needs finalization.
Discussion:
Exits are allowed from the loops created using the iterator objects.
In particular, to stop the iteration at a particular cursor, just add
exit when Cur = Stop;
in the body of
the loop (assuming that Cur is the loop parameter and Stop
is the cursor that you want to stop at).
{
AI05-0044-1}
{
AI05-0262-1}
The actual function for the generic formal function "<"
of Generic_Sorting is expected to return the same value each time it
is called with a particular pair of element values. It should define
a strict weak ordering relationship (see
A.18);
it should not modify Container. If the actual for "<" behaves
in some other manner, the behavior of the subprograms of Generic_Sorting
are unspecified. The number of times the subprograms of Generic_Sorting
call "<" is unspecified.
function Is_Sorted (Container : List) return Boolean;
{
AI95-00302-03}
Returns True if the elements are sorted smallest first as determined
by the generic formal "<" operator; otherwise, Is_Sorted
returns False. Any exception raised during evaluation of "<"
is propagated.
procedure Sort (Container : in out List);
{
AI95-00302-03}
Reorders the nodes of Container such that the elements are sorted smallest
first as determined by the generic formal "<" operator provided.
The sort is stable. Any exception raised during evaluation of "<"
is propagated.
Ramification: Unlike array sorts, we
do require stable sorts here. That's because algorithms in the merge
sort family (as described by Knuth) can be both fast and stable. Such
sorts use the extra memory as offered by the links to provide better
performance.
Note that list sorts never copy elements; it
is the nodes, not the elements, that are reordered.
procedure Merge (Target : in out List;
Source : in out List);
{
AI95-00302-03}
{
AI05-0021-1}
If Source is empty, then Merge does nothing. If Source and Target are
the same nonempty container object, then Program_Error is propagated.
Otherwise, Merge removes elements from Source and inserts them into Target;
afterwards, Target contains the union of the elements that were initially
in Source and Target; Source is left empty. If Target and Source are
initially sorted smallest first, then Target is ordered smallest first
as determined by the generic formal "<" operator; otherwise,
the order of elements in Target is unspecified. Any exception raised
during evaluation of "<" is propagated.
Ramification: It is a bounded error if
either of the lists is unsorted, see below. The bounded error can be
recovered by sorting Target after the merge call, or the lists can be
pretested with Is_Sorted.
Bounded (Run-Time) Errors
{
AI95-00302-03}
Calling Merge in an instance of Generic_Sorting with
either Source or Target not ordered smallest first using the provided
generic formal "<" operator is a bounded error. Either Program_Error
is raised after Target is updated as described for Merge, or the operation
works as defined.
{
AI05-0022-1}
{
AI05-0248-1}
It is a bounded error for the actual function associated
with a generic formal subprogram, when called as part of an operation
of this package, to tamper with elements of any List parameter of the
operation. Either Program_Error is raised, or the operation works as
defined on the value of the List either prior to, or subsequent to, some
or all of the modifications to the List.
{
AI05-0027-1}
It is a bounded error to call any subprogram declared
in the visible part of Containers.Doubly_Linked_Lists when the associated
container has been finalized. If the operation takes Container as an
in out parameter, then it raises Constraint_Error or Program_Error.
Otherwise, the operation either proceeds as it would for an empty container,
or it raises Constraint_Error or Program_Error.
Erroneous Execution
{
AI95-00302-03}
A Cursor value is
invalid if any of the following have occurred
since it was created:
The list that contains the element it designates
has been finalized;
{
AI05-0160-1}
The list that contains the element it designates has been used as the
Target of a call to Assign, or as the target of an
assignment_statement;
[The list that contains the element it designates
has been used as the Source or Target of a call to Move;] or
Proof: {
AI05-0001-1}
Move has been reworded in terms of Assign and Clear, which are covered
by other bullets, so this text is redundant.
{
AI05-0160-1}
{
AI05-0262-1}
The element it designates has been removed from the list that previously
contained the element.
To be honest: {
AI05-0160-1}
The cursor modified by the four parameter Splice is not invalid, even
though the element it designates has been removed from the source list,
because that cursor has been modified to designate that element in the
target list – the cursor no longer designates an element in the
source list.
Ramification: {
AI05-0160-1}
This can happen directly via calls to Delete, Delete_Last, Clear, Splice
with a Source parameter, and Merge; and indirectly via calls to Delete_First,
Assign, and Move.
{
AI95-00302-03}
The result of "=" or Has_Element is unspecified if it is called
with an invalid cursor parameter. Execution is erroneous if any other
subprogram declared in Containers.Doubly_Linked_Lists is called with
an invalid cursor parameter.
Discussion: The list above is intended
to be exhaustive. In other cases, a cursor value continues to designate
its original element. For instance, cursor values survive the insertion
and deletion of other nodes.
While it is possible to check for these cases,
in many cases the overhead necessary to make the check is substantial
in time or space. Implementations are encouraged to check for as many
of these cases as possible and raise Program_Error if detected.
{
AI05-0212-1}
Execution is erroneous if the list associated with the result of a call
to Reference or Constant_Reference is finalized before the result object
returned by the call to Reference or Constant_Reference is finalized.
Reason: Each object of Reference_Type
and Constant_Reference_Type probably contains some reference to the originating
container. If that container is prematurely finalized (which is only
possible via Unchecked_Deallocation, as accessibility checks prevent
passing a container to Reference that will not live as long as the result),
the finalization of the object of Reference_Type will try to access a
nonexistent object. This is a normal case of a dangling pointer created
by Unchecked_Deallocation; we have to explicitly mention it here as the
pointer in question is not visible in the specification of the type.
(This is the same reason we have to say this for invalid cursors.)
Implementation Requirements
{
AI95-00302-03}
No storage associated with a doubly-linked List object shall be lost
upon assignment or scope exit.
{
AI95-00302-03}
{
AI05-0262-1}
The execution of an
assignment_statement
for a list shall have the effect of copying the elements from the source
list object to the target list object and changing the length of the
target object to that of the source object.
Implementation Note: {
AI05-0298-1}
An assignment of a List is a “deep” copy; that is the elements
are copied as well as the data structures. We say “effect of”
in order to allow the implementation to avoid copying elements immediately
if it wishes. For instance, an implementation that avoided copying until
one of the containers is modified would be allowed. (Note that this implementation
would require care, see
A.18.2 for more.)
Implementation Advice
{
AI95-00302-03}
Containers.Doubly_Linked_Lists should be implemented similarly to a linked
list. In particular, if
N is the length of a list, then the worst-case
time complexity of Element, Insert with Count=1, and Delete with Count=1
should be
O(log
N).
Implementation Advice: The worst-case
time complexity of Element, Insert with Count=1, and Delete with Count=1
for Containers.Doubly_Linked_Lists should be O(log N).
Reason: We do not mean to overly constrain
implementation strategies here. However, it is important for portability
that the performance of large containers has roughly the same factors
on different implementations. If a program is moved to an implementation
that takes O(N) time to access elements, that program could
be unusable when the lists are large. We allow O(log N)
access because the proportionality constant and caching effects are likely
to be larger than the log factor, and we don't want to discourage innovative
implementations.
{
AI95-00302-03}
The worst-case time complexity of a call on procedure Sort of an instance
of Containers.Doubly_Linked_Lists.Generic_Sorting should be
O(
N**2),
and the average time complexity should be better than
O(
N**2).
Implementation Advice: A call on procedure
Sort of an instance of Containers.Doubly_Linked_Lists.Generic_Sorting
should have an average time complexity better than O(N**2)
and worst case no worse than O(N**2).
Ramification: In other words, we're requiring
the use of a better than O(N**2) sorting algorithm, such
as Quicksort. No bubble sorts allowed!
{
AI95-00302-03}
Move should not copy elements, and should minimize copying of internal
data structures.
Implementation Advice: Containers.Doubly_Linked_Lists.Move
should not copy elements, and should minimize copying of internal data
structures.
Implementation Note: Usually that can
be accomplished simply by moving the pointer(s) to the internal data
structures from the Source container to the Target container.
{
AI95-00302-03}
If an exception is propagated from a list operation, no storage should
be lost, nor any elements removed from a list unless specified by the
operation.
Implementation Advice: If an exception
is propagated from a list operation, no storage should be lost, nor any
elements removed from a list unless specified by the operation.
Reason: This is important so that programs
can recover from errors. But we don't want to require heroic efforts,
so we just require documentation of cases where this can't be accomplished.
50 {
AI95-00302-03}
Sorting a list never copies elements, and is a stable sort (equal elements
remain in the original order). This is different than sorting an array
or vector, which may need to copy elements, and is probably not a stable
sort.
Extensions to Ada 95
{
AI95-00302-03}
The generic package Containers.Doubly_Linked_Lists
is new.
Inconsistencies With Ada 2005
{
AI05-0248-1}
{
AI05-0257-1}
Correction: The Insert versions that return
a Position parameter are now defined to return Position = Before if Count
= 0. This was unspecified for Ada 2005; so this will only be inconsistent
if an implementation did something else and a program depended on that
something else — this should be very rare.
Incompatibilities With Ada 2005
{
AI05-0001-1}
Subprograms Assign and Copy are added to Containers.Doubly_Linked_Lists.
If an instance of Containers.Doubly_Linked_Lists is referenced in a
use_clause,
and an entity
E with the same
defining_identifier
as a new entity in Containers.Doubly_Linked_Lists is defined in a package
that is also referenced in a
use_clause,
the entity
E may no longer be use-visible, resulting in errors.
This should be rare and is easily fixed if it does occur.
Extensions to Ada 2005
{
AI05-0212-1}
Added iterator, reference, and indexing support to
make list containers more convenient to use.
Wording Changes from Ada 2005
{
AI05-0001-1}
Generalized the definition of Move. Specified which elements are read/written
by stream attributes.
{
AI05-0022-1}
Correction: Added a Bounded (Run-Time) Error to cover tampering
by generic actual subprograms.
{
AI05-0027-1}
Correction: Added a Bounded (Run-Time) Error to cover access to
finalized list containers.
{
AI05-0044-1}
Correction: Redefined "<" actuals to require a strict
weak ordering; the old definition allowed indeterminant comparisons that
would not have worked in a container.
{
AI05-0084-1}
Correction: Added a pragma Remote_Types so that containers can
be used in distributed programs.
{
AI05-0160-1}
Correction: Revised the definition of invalid cursors to cover
missing (and new) cases.
{
AI05-0257-1}
Correction: Added missing wording to describe the Position after
Inserting 0 elements.
{
AI05-0265-1}
Correction: Defined when a container prohibits tampering in order
to more clearly define where the check is made and the exception raised.
Wording Changes from Ada 2012
{
AI12-0110-1}
Corrigendum: Clarified that tampering checks precede all other
checks made by a subprogram (but come after those associated with the
call).
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
