std::basic_string<CharT,Traits,Allocator>::swap

void swap( basic_string& other );		(until C++17)
void swap( basic_string& other ) noexcept(/* see below */);			(since C++17) (constexpr since C++20)

Exchanges the contents of the string with those of other. All iterators and references may be invalidated.

If std::allocator_traits<allocator_type>::propagate_on_container_swap::value is true, then the allocators are exchanged using an unqualified call to non-member swap. Otherwise, they are not swapped (and if get_allocator() != other.get_allocator(), the behavior is undefined).

(since C++11)

Parameters

other

string to exchange the contents with

Complexity

Constant.

Exceptions

No exception is thrown.

(until C++11)

No exception is thrown, unless the behavior is undefined.

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).

(since C++11)

noexcept specification:

noexcept(std::allocator_traits<Allocator>::propagate_on_container_swap::value ||
         std::allocator_traits<Allocator>::is_always_equal::value)

(since C++17)

Example

#include <iostream>
#include <string>

int main() 
{
    std::string a = "AAA";
    std::string b = "BBBB";
    
    std::cout << "Before swap:\n"
                 "a = " << a << "\n"
                 "b = " << b << "\n\n";
    
    a.swap(b);
    
    std::cout << "After swap:\n"
                 "a = " << a << "\n"
                 "b = " << b << '\n';
}

Output:

Before swap:
a = AAA
b = BBBB

After swap:
a = BBBB
b = AAA

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 403	C++98	`swap()` might throw an exception	no exception is thrown
LWG 535	C++98	swapping strings did not preserve the character orders	orders are also preserved
LWG 2151 (P1148R0)	C++11	no exception was thrown in the case of unequal non-propagating allocators	the behavior is undefined in this case