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Книга: Standard Template Library Programmer

pair

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Description
Example
Definition
Template parameters
Model of
Type requirements
Public base classes
Members
New members
See also

pair<T1, T2>

Category: utilities

Component type: type

Description

Pair<T1,T2> is a heterogeneous pair: it holds one object of type T1 and one of type T2. A pair is much like a Container, in that it "owns" its elements. It is not actually a model of Container, though, because it does not support the standard methods (such as iterators) for accessing the elements of a Container.

Functions that need to return two values often return a pair.

Example

pair<bool, double> result = do_a_calculation(); if (result.first) do_something_more(result.second); else report_error();

Definition

Defined in the standard header utility, and in the nonstandard backward-compatibility header pair.h.

Template parameters

Parameter	Description
`T1`	The type of the first element stored in the pair
`T2`	The type of the second element stored in the pair

Model of

Assignable

Type requirements

T1 and T2 must both be models of Assignable. Additional operations have additional requirements. Pair's default constructor may only be used if both T1 and T2 are DefaultConstructible, operator== may only be used if both T1 and T2 are EqualityComparable, and operator< may only be used if both T1 and T2 are LessThanComparable.

Public base classes

None.

Members

Member	Where defined	Description
`first_type`	`pair`	See below.
`second`	`type pair`	See below.
`pair()`	`pair`	The default constructor. See below.
`pair(const first_type&, const second_type&)`	`pair`	The pair constructor. See below.
`pair(const pair&)`	Assignable	The copy constructor
`pair& operator=(const pair&)`	Assignable	The assignment operator
`first`	`pair`	See below.
`second`	`pair`	See below.
`bool operator==(const pair&, const pair&)`	`pair`	See below.
`bool operator<(const pair&, const pair&)`	`pair`	See below.
`template <class T1, class T2> pair<T1, T2> make_pair(const T1&, const T2&)`	`pair`	See below.

New members

These members are not defined in the Assignable requirements, but are specific to pair.

Member	Description
`first_type`	The type of the pair's first component. This is a typedef for the template parameter T1
second_type	The type of the pair's second component. This is a typedef for the template parameter T2
`pair()`	The default constructor. It uses constructs objects of types T1 and T2 using their default constructors. This constructor may only be used if both T1 and T2 are DefaultConstructible.
`pair(const first_type& x, const second_type& y)`	The pair constructor. Constructs a pair such that first is constructed from x and second is constructed from y.
`first`	Public member variable of type first_type: the first object stored in the pair.
`second`	Public member variable of type second_type: The second object stored in the pair.
`template <class T1, class T2> bool operator==(const pair<T1,T2>& x, const pair<T1,T2>& y);`	The equality operator. The return value is true if and only the first elements of x and y are equal, and the second elements of x and y are equal. This operator may only be used if both T1 and T2 are EqualityComparable . This is a global function, not a member function.
`template <class T1, class T2> bool operator<(const pair<T1,T2>& x, const pair<T1,T2>& y);`	The comparison operator. It uses lexicographic comparison: the return value is true if the first element of x is less than the first element of y, and false if the first element of y is less than the first element of x. If neither of these is the case, then operator< returns the result of comparing the second elements of x and y. This operator may only be used if both T1 and T2 are LessThanComparable . This is a global function, not a member function.
`template <class T1, class T2> pair<T1, T2> make_pair(const T1& x, const T2& x)`	Equivalent to pair<T1, T2>(x, y). This is a global function, not a member function. It exists only for the sake of convenience.