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Standard Template Library Programmer's Guide
(document number: 007-3426-004 / published: 1999-05-21) table of contents | additional info | download
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swap


Category: algorithms	Component type: function

Prototype

template <class Assignable> 
void swap(Assignable& a, Assignable& b);

Description

Assigns the contents of a to b and the contents of b to a. This is used as a primitive operation by many other algorithms.

Definition

Defined in the standard header algorithm, and in the nonstandard backward-compatibility header algo.h.

Requirements on types

Assignable is a model of Assignable.

Preconditions

None.

Complexity

Amortized constant time. [1] [2]

Example

int x = 1;
int y = 2;
assert(x == 1 && y == 2);
swap(x, y);
assert(x == 2 && y == 1);

Notes

[1] The time required to swap two objects of type T will obviously depend on the type; "constant time" does not mean that performance will be the same for an 8-bit char as for a 128-bit complex<double>.

[2] This implementation of swap makes one call to a copy constructor and two calls to an assignment operator; roughly, then, it should be expected to take about the same amount of time as three assignments. In many cases, however, it is possible to write a specialized version of swap that is far more efficient. Consider, for example, swapping two vector<double>s each of which has N elements. The unspecialized version requires 3*N assignments of double, but a specialized version requires only nine pointer assignments. This is important because swap is used as a primitive operation in many other STL algorithms, and because containers of containers (list<vector<char> >, for example) are very common. The STL includes specialized versions of swap for all container classes. User-defined types should also provide specialized versions of swap whenever it is possible to write one that is more efficient than the general version.