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Showing content from https://cplusplus.com/reference/map/map/swap/ below:

public member function

<map>

std::map::swap

Swap content

Exchanges the content of the container by the content of x, which is another map of the same type. Sizes may differ.

After the call to this member function, the elements in this container are those which were in x before the call, and the elements of x are those which were in this. All iterators, references and pointers remain valid for the swapped objects.

Notice that a non-member function exists with the same name, swap, overloading that algorithm with an optimization that behaves like this member function.


Parameters
x
Another map container of the same type as this (i.e., with the same template parameters, Key, T, Compare and Alloc) whose content is swapped with that of this container.

Return value none

Example 
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// swap maps
#include <iostream>
#include <map>

int main ()
{
  std::map<char,int> foo,bar;

  foo['x']=100;
  foo['y']=200;

  bar['a']=11;
  bar['b']=22;
  bar['c']=33;

  foo.swap(bar);

  std::cout << "foo contains:\n";
  for (std::map<char,int>::iterator it=foo.begin(); it!=foo.end(); ++it)
    std::cout << it->first << " => " << it->second << '\n';

  std::cout << "bar contains:\n";
  for (std::map<char,int>::iterator it=bar.begin(); it!=bar.end(); ++it)
    std::cout << it->first << " => " << it->second << '\n';

  return 0;
}

Output: 
foo contains:
a => 11
b => 22
c => 33
bar contains:
x => 100
y => 200


Complexity Constant.

Iterator validity All iterators, pointers and references referring to elements in both containers remain valid, but now are referring to elements in the other container, and iterate in it.
Note that the end iterators do not refer to elements and may be invalidated.

Data races Both the container and x are modified.
No contained elements are accessed by the call (although see iterator validity above).

Exception safety If the allocators in both containers compare equal, or if their allocator traits indicate that the allocators shall propagate, the function never throws exceptions (no-throw guarantee).
Otherwise, it causes undefined behavior.

See also
swap (map)
Exchanges the contents of two maps (function template)
swap_ranges
Exchange values of two ranges (function template)

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