Maps
Author: JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor

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# A.18.4 Maps

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The language-defined generic packages Containers.Hashed_Maps and Containers.Ordered_Maps provide private types Map and Cursor, and a set of operations for each type. A map container allows an arbitrary type to be used as a key to find the element associated with that key. A hashed map uses a hash function to organize the keys, while an ordered map orders the keys per a specified relation.
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This section describes the declarations that are common to both kinds of maps. See A.18.5 for a description of the semantics specific to Containers.Hashed_Maps and A.18.6 for a description of the semantics specific to Containers.Ordered_Maps.

#### Static Semantics

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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 function "=" on map values returns an unspecified value. The exact arguments and number of calls of this generic formal function by the function "=" on map values are unspecified.
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The type Map is used to represent maps. The type Map needs finalization (see 7.6).
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A map contains pairs of keys and elements, called nodes. Map cursors designate nodes, but also can be thought of as designating an element (the element contained in the node) for consistency with the other containers. There exists an equivalence relation on keys, whose definition is different for hashed maps and ordered maps. A map never contains two or more nodes with equivalent keys. The length of a map is the number of nodes it contains.
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Each nonempty map has two particular nodes called the first node and the last node (which may be the same). Each node except for the last node has a successor node. If there are no other intervening operations, starting with the first node and repeatedly going to the successor node will visit each node in the map exactly once until the last node is reached. The exact definition of these terms is different for hashed maps and ordered maps.
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Some operations of these generic packages 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.
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A subprogram is said to tamper with cursors of a map object M if:
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- it inserts or deletes elements of M, that is, it calls the Insert, Include, Clear, Delete, or Exclude procedures with M as a parameter; or

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- it finalizes M; or

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- it calls the Move procedure with M as a parameter; or

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- it calls one of the operations defined to tamper with the cursors of M.

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A subprogram is said to tamper with elements of a map object M if:
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- it tampers with cursors of M; or

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- it replaces one or more elements of M, that is, it calls the Replace or Replace_Element procedures with M as a parameter.

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Empty_Map represents the empty Map object. It has a length of 0. If an object of type Map is not otherwise initialized, it is initialized to the same value as Empty_Map.
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No_Element represents a cursor that designates no node. 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.
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function "=" (Left, Right : Map) return Boolean;
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If Left and Right denote the same map object, then the function returns True. If Left and Right have different lengths, then the function returns False. Otherwise, for each key K in Left, the function returns False if:
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- a key equivalent to K is not present in Right; or

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- the element associated with K in Left is not equal to the element associated with K in Right (using the generic formal equality operator for elements).

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If the function has not returned a result after checking all of the keys, it returns True. Any exception raised during evaluation of key equivalence or element equality is propagated.
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function Length (Container : Map) return Count_Type;
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Returns the number of nodes in Container.
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function Is_Empty (Container : Map) return Boolean;
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Equivalent to Length (Container) = 0.
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procedure Clear (Container : in out Map);
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Removes all the nodes from Container.
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function Key (Position : Cursor) return Key_Type;
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If Position equals No_Element, then Constraint_Error is propagated. Otherwise, Key returns the key component of the node designated by Position.
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function Element (Position : Cursor) return Element_Type;
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If Position equals No_Element, then Constraint_Error is propagated. Otherwise, Element returns the element component of the node designated by Position.
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procedure Replace_Element (Container : in out Map;
Position : in Cursor;
New_Item : in Element_Type);
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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 New_Item to the element of the node designated by Position.
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procedure Query_Element
(Position : in Cursor;
Process : not null access procedure (Key : in Key_Type;
Element : in Element_Type));
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If Position equals No_Element, then Constraint_Error is propagated. Otherwise, Query_Element calls Process.all with the key and element from the node designated by Position as the arguments. Program_Error is propagated if Process.all tampers with the elements of Container. Any exception raised by Process.all is propagated.
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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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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 key and element from the node designated by Position as the arguments. Program_Error is propagated if Process.all tampers with the elements of Container. Any exception raised by Process.all is propagated.
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If Element_Type is unconstrained and definite, then the actual Element parameter of Process.all shall be unconstrained.
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procedure Move (Target : in out Map;
Source : in out Map);
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If Target denotes the same object as Source, then Move has no effect. Otherwise, Move first calls Clear (Target). Then, each node from Source is removed from Source and inserted into Target. The length of Source is 0 after a successful call to Move.
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procedure Insert (Container : in out Map;
Key : in Key_Type;
New_Item : in Element_Type;
Position : out Cursor;
Inserted : out Boolean);
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Insert checks if a node with a key equivalent to Key is already present in Container. If a match is found, Inserted is set to False and Position designates the element with the matching key. Otherwise, Insert allocates a new node, initializes it to Key and New_Item, and adds it to Container; Inserted is set to True and Position designates the newly-inserted node. Any exception raised during allocation is propagated and Container is not modified.
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procedure Insert (Container : in out Map;
Key : in Key_Type;
Position : out Cursor;
Inserted : out Boolean);
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Insert inserts Key into Container as per the five-parameter Insert, with the difference that an element initialized by default (see 3.3.1) is inserted.
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procedure Insert (Container : in out Map;
Key : in Key_Type;
New_Item : in Element_Type);
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Insert inserts Key and New_Item into Container as per the five-parameter Insert, with the difference that if a node with a key equivalent to Key is already in the map, then Constraint_Error is propagated.
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procedure Include (Container : in out Map;
Key : in Key_Type;
New_Item : in Element_Type);
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Include inserts Key and New_Item into Container as per the five-parameter Insert, with the difference that if a node with a key equivalent to Key is already in the map, then this operation assigns Key and New_Item to the matching node. Any exception raised during assignment is propagated.
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procedure Replace (Container : in out Map;
Key : in Key_Type;
New_Item : in Element_Type);
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Replace checks if a node with a key equivalent to Key is present in Container. If a match is found, Replace assigns Key and New_Item to the matching node; otherwise, Constraint_Error is propagated.
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procedure Exclude (Container : in out Map;
Key : in Key_Type);
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Exclude checks if a node with a key equivalent to Key is present in Container. If a match is found, Exclude removes the node from the map.
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procedure Delete (Container : in out Map;
Key : in Key_Type);
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Delete checks if a node with a key equivalent to Key is present in Container. If a match is found, Delete removes the node from the map; otherwise, Constraint_Error is propagated.
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procedure Delete (Container : in out Map;
Position : in out Cursor);
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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 the node designated by Position from the map. Position is set to No_Element on return.
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function First (Container : Map) return Cursor;
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If Length (Container) = 0, then First returns No_Element. Otherwise, First returns a cursor that designates the first node in Container.
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function Next (Position : Cursor) return Cursor;
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Returns a cursor that designates the successor of the node designated by Position. If Position designates the last node, then No_Element is returned. If Position equals No_Element, then No_Element is returned.
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procedure Next (Position : in out Cursor);
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Equivalent to Position := Next (Position).
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function Find (Container : Map;
Key : Key_Type) return Cursor;
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If Length (Container) equals 0, then Find returns No_Element. Otherwise, Find checks if a node with a key equivalent to Key is present in Container. If a match is found, a cursor designating the matching node is returned; otherwise, No_Element is returned.
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function Element (Container : Map;
Key : Key_Type) return Element_Type;
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Equivalent to Element (Find (Container, Key)).
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function Contains (Container : Map;
Key : Key_Type) return Boolean;
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Equivalent to Find (Container, Key) /= No_Element.
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function Has_Element (Position : Cursor) return Boolean;
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Returns True if Position designates a node, and returns False otherwise.
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procedure Iterate
(Container : in Map;
Process : not null access procedure (Position : in Cursor));
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Iterate calls Process.all with a cursor that designates each node in Container, starting with the first node and moving the cursor according to the successor relation. Program_Error is propagated if Process.all tampers with the cursors of Container. Any exception raised by Process.all is propagated.

#### Erroneous Execution

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A Cursor value is invalid if any of the following have occurred since it was created:
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- The map that contains the node it designates has been finalized;

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- The map that contains the node it designates has been used as the Source or Target of a call to Move; or

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- The node it designates has been deleted from the map.

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The result of "=" or Has_Element is unspecified if these functions are called with an invalid cursor parameter. Execution is erroneous if any other subprogram declared in Containers.Hashed_Maps or Containers.Ordered_Maps is called with an invalid cursor parameter.

#### Implementation Requirements

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No storage associated with a Map object shall be lost upon assignment or scope exit.
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The execution of an assignment_statement for a map shall have the effect of copying the elements from the source map object to the target map object.

#### Implementation Advice

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Move should not copy elements, and should minimize copying of internal data structures.
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If an exception is propagated from a map operation, no storage should be lost, nor any elements removed from a map unless specified by the operation.

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