template <class Assignable> void swap(Assignable& a, Assignable& b);
DescriptionAssigns the contents of a to b and the contents of b to a. This is used as a primitive operation by many other algorithms.
DefinitionDefined in the standard header algorithm, and in the nonstandard backward-compatibility header algo.h.
Requirements on types
ComplexityAmortized constant time.  
int x = 1; int y = 2; assert(x == 1 && y == 2); swap(x, y); assert(x == 2 && y == 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>.
 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.
See alsoiter_swap, swap_ranges
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