A.18.6 The Generic Package Containers.Ordered_Maps
Static Semantics
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The generic library package Containers.Ordered_Maps has the following
declaration:
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with Ada.Iterator_Interfaces;
generic
type Key_Type
is private;
type Element_Type
is private;
with function "<" (Left, Right : Key_Type)
return Boolean
is <>;
with function "=" (Left, Right : Element_Type)
return Boolean
is <>;
package Ada.Containers.Ordered_Maps
is
pragma Preelaborate(Ordered_Maps);
pragma Remote_Types(Ordered_Maps);
function Equivalent_Keys (Left, Right : Key_Type)
return Boolean;
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type Map
is tagged private
with Constant_Indexing => Constant_Reference,
Variable_Indexing => Reference,
Default_Iterator => Iterate,
Iterator_Element => Element_Type;
pragma Preelaborable_Initialization(Map);
type Cursor
is private;
pragma Preelaborable_Initialization(Cursor);
Empty_Map :
constant Map;
No_Element :
constant Cursor;
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function Has_Element (Position : Cursor)
return Boolean;
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package Map_Iterator_Interfaces
is new
Ada.Iterator_Interfaces (Cursor, Has_Element);
function "=" (Left, Right : Map) return Boolean;
function Length (Container : Map)
return Count_Type;
function Is_Empty (Container : Map)
return Boolean;
procedure Clear (Container :
in out Map);
function Key (Position : Cursor)
return Key_Type;
function Element (Position : Cursor)
return Element_Type;
procedure Replace_Element (Container :
in out Map;
Position :
in Cursor;
New_Item :
in Element_Type);
procedure Query_Element
(Position :
in Cursor;
Process :
not null access procedure (Key :
in Key_Type;
Element :
in Element_Type));
procedure Update_Element
(Container :
in out Map;
Position :
in Cursor;
Process :
not null access procedure
(Key :
in Key_Type;
Element :
in out Element_Type));
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type Constant_Reference_Type
(Element :
not null access constant Element_Type)
is private
with Implicit_Dereference => Element;
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type Reference_Type (Element :
not null access Element_Type)
is private
with Implicit_Dereference => Element;
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function Constant_Reference (Container :
aliased in Map;
Position :
in Cursor)
return Constant_Reference_Type;
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function Reference (Container :
aliased in out Map;
Position :
in Cursor)
return Reference_Type;
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function Constant_Reference (Container :
aliased in Map;
Key :
in Key_Type)
return Constant_Reference_Type;
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function Reference (Container :
aliased in out Map;
Key :
in Key_Type)
return Reference_Type;
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procedure Assign (Target :
in out Map; Source :
in Map);
procedure Move (Target :
in out Map;
Source :
in out Map);
procedure Insert (Container :
in out Map;
Key :
in Key_Type;
New_Item :
in Element_Type;
Position :
out Cursor;
Inserted :
out Boolean);
procedure Insert (Container :
in out Map;
Key :
in Key_Type;
Position :
out Cursor;
Inserted :
out Boolean);
procedure Insert (Container :
in out Map;
Key :
in Key_Type;
New_Item :
in Element_Type);
procedure Include (Container :
in out Map;
Key :
in Key_Type;
New_Item :
in Element_Type);
procedure Replace (Container :
in out Map;
Key :
in Key_Type;
New_Item :
in Element_Type);
procedure Exclude (Container :
in out Map;
Key :
in Key_Type);
procedure Delete (Container :
in out Map;
Key :
in Key_Type);
procedure Delete (Container :
in out Map;
Position :
in out Cursor);
procedure Delete_First (Container :
in out Map);
procedure Delete_Last (Container :
in out Map);
function First (Container : Map)
return Cursor;
function First_Element (Container : Map)
return Element_Type;
function First_Key (Container : Map)
return Key_Type;
function Last (Container : Map)
return Cursor;
function Last_Element (Container : Map)
return Element_Type;
function Last_Key (Container : Map)
return Key_Type;
function Next (Position : Cursor)
return Cursor;
procedure Next (Position :
in out Cursor);
function Previous (Position : Cursor)
return Cursor;
procedure Previous (Position :
in out Cursor);
function Find (Container : Map;
Key : Key_Type)
return Cursor;
function Element (Container : Map;
Key : Key_Type)
return Element_Type;
function Floor (Container : Map;
Key : Key_Type)
return Cursor;
function Ceiling (Container : Map;
Key : Key_Type)
return Cursor;
function Contains (Container : Map;
Key : Key_Type)
return Boolean;
function "<" (Left, Right : Cursor) return Boolean;
function ">" (Left, Right : Cursor) return Boolean;
function "<" (Left : Cursor; Right : Key_Type) return Boolean;
function ">" (Left : Cursor; Right : Key_Type) return Boolean;
function "<" (Left : Key_Type; Right : Cursor) return Boolean;
function ">" (Left : Key_Type; Right : Cursor) return Boolean;
procedure Iterate
(Container :
in Map;
Process :
not null access procedure (Position :
in Cursor));
procedure Reverse_Iterate
(Container :
in Map;
Process :
not null access procedure (Position :
in Cursor));
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function Iterate (Container :
in Map)
return Map_Iterator_Interfaces.Reversible_Iterator'Class;
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function Iterate (Container :
in Map; Start :
in Cursor)
return Map_Iterator_Interfaces.Reversible_Iterator'Class;
private
... -- not specified by the language
end Ada.Containers.Ordered_Maps;
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Two keys
K1 and
K2 are
equivalent
if both
K1 <
K2 and
K2 <
K1 return
False, using the generic formal "<" operator for keys. Function
Equivalent_Keys returns True if Left and Right are equivalent, and False
otherwise.
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The actual function for the generic formal function "<"
on Key_Type values is expected to return the same value each time it
is called with a particular pair of key values. It should define a strict
weak ordering relationship (see
A.18). If
the actual for "<" behaves in some other manner, the behavior
of this package is unspecified. Which subprograms of this package call
"<" and how many times they call it, is unspecified.
Implementation Note: The implementation
is not required to protect against "<" raising an exception,
or returning random results, or any other “bad” behavior.
It's not practical to do so, and a broken "<" function makes
the container unusable.
The implementation can call "<"
whenever it is needed; we don't want to specify how often that happens.
The result must remain the same (this is a logically pure function),
or the behavior is unspecified.
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If the value of a key stored in a map is changed other than by an operation
in this package such that at least one of "<" or "="
give different results, the behavior of this package is unspecified.
Implementation Note: The implementation
is not required to protect against changes to key values other than via
the operations declared in the Ordered_Maps package.
To see how this
could happen, imagine an instance of Ordered_Maps package where the key
type is an access-to-variable type and "<" returns a value
derived from comparing the components of the designated objects. Then,
any operation that has a key value (even if the key value is constant)
could modify those components and change the result of "<":
Key (Map).Some_Component := New_Value;
This is really a design error on the part of
the user of the map; it shouldn't be possible to modify keys stored in
a map such that "<" changes. But we can't prevent this error
anymore than we can prevent someone passing as "<" a routine
that produces random answers.
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The
first node of a nonempty map is the one whose key is less than
the key of all the other nodes in the map. The
last node of a
nonempty map is the one whose key is greater than the key of all the
other elements in the map. The
successor of a node is the node
with the smallest key that is larger than the key of the given node.
The
predecessor of a node is the node with the largest key that
is smaller than the key of the given node. All comparisons are done using
the generic formal "<" operator for keys.
function Copy (Source : Map) return Map;
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Returns a map whose keys and elements are initialized from the corresponding
keys and elements of Source.
procedure Delete_First (Container : in out Map);
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If Container is empty, Delete_First has no effect. Otherwise, the node
designated by First (Container) is removed from Container. Delete_First
tampers with the cursors of Container.
procedure Delete_Last (Container : in out Map);
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If Container is empty, Delete_Last has no effect. Otherwise, the node
designated by Last (Container) is removed from Container. Delete_Last
tampers with the cursors of Container.
function First_Element (Container : Map) return Element_Type;
function First_Key (Container : Map) return Key_Type;
function Last (Container : Map) return Cursor;
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Returns a cursor that designates the last node in Container. If Container
is empty, returns No_Element.
function Last_Element (Container : Map) return Element_Type;
function Last_Key (Container : Map) return Key_Type;
function Previous (Position : Cursor) return Cursor;
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If Position equals No_Element, then Previous returns No_Element. Otherwise,
Previous returns a cursor designating the predecessor node of the one
designated by Position. If Position designates the first element, then
Previous returns No_Element.
procedure Previous (Position : in out Cursor);
function Floor (Container : Map;
Key : Key_Type) return Cursor;
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Floor searches for the last node whose key is not greater than Key, using
the generic formal "<" operator for keys. If such a node
is found, a cursor that designates it is returned. Otherwise, No_Element
is returned.
function Ceiling (Container : Map;
Key : Key_Type) return Cursor;
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Ceiling searches for the first node whose key is not less than Key, using
the generic formal "<" operator for keys. If such a node
is found, a cursor that designates it is returned. Otherwise, No_Element
is returned.
function "<" (Left, Right : Cursor) return Boolean;
function ">" (Left, Right : Cursor) return Boolean;
function "<" (Left : Cursor; Right : Key_Type) return Boolean;
function ">" (Left : Cursor; Right : Key_Type) return Boolean;
function "<" (Left : Key_Type; Right : Cursor) return Boolean;
function ">" (Left : Key_Type; Right : Cursor) return Boolean;
procedure Reverse_Iterate
(Container : in Map;
Process : not null access procedure (Position : in Cursor));
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Iterates over the nodes in Container as per procedure Iterate, with the
difference that the nodes are traversed in predecessor order, starting
with the last node.
function Iterate (Container : in Map)
return Map_Iterator_Interfaces.Reversible_Iterator'Class;
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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 according to the successor relation when used as a
forward iterator, and starting with the last node and moving the cursor
according to the predecessor relation 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 Map; Start : in Cursor)
return Map_Iterator_Interfaces.Reversible_Iterator'Class;
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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 according to the successor relation when used as a
forward iterator, or moving the cursor according to the predecessor relation
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).
Implementation Advice
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If
N is the length of a map, then the worst-case time complexity
of the Element, Insert, Include, Replace, Delete, Exclude and Find operations
that take a key parameter should be
O((log
N)**2) or better.
The worst-case time complexity of the subprograms that take a cursor
parameter should be
O(1).
Implementation Advice: The worst-case
time complexity of Element, Insert, Include, Replace, Delete, Exclude
and Find operations that take a key parameter for Containers.Ordered_Maps
should be O((log N)**2) or better. The worst-case time
complexity of the subprograms of Containers.Ordered_Maps that take a
cursor parameter should be O(1).
Implementation Note: A balanced (red-black)
tree for keys has O(log N) worst-case performance. Note
that a O(N) worst-case implementation (like a list) would
be wrong.
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) to find elements, that program could be
unusable when the maps are large. We allow the extra log N factors
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.
Extensions to Ada 95
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The generic package Containers.Ordered_Maps is new.
Incompatibilities With Ada 2005
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Subprograms Assign and Copy are added to Containers.Ordered_Maps.
If an instance of Containers.Ordered_Maps is referenced in a
use_clause,
and an entity
E with the same
defining_identifier
as a new entity in Containers.Ordered_Maps 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
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Added iterator and indexing support to make ordered
map containers more convenient to use.
Wording Changes from Ada 2005
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Correction: Redefined "<" actuals to require a strict
weak ordering; the old definition allowed indeterminant comparisons that
would not have worked in a container.
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Correction: Added a pragma Remote_Types so that containers can
be used in distributed programs.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
