http://www.cplusplus.com/reference/map/

std::multimap::operator=

http://www.cplusplus.com/reference/map/multimap/operator=/

#include <iostream>
#include <map>

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

  foo.insert(std::make_pair('x', 32));
  foo.insert(std::make_pair('y', 64));
  foo.insert(std::make_pair('y', 96));
  foo.insert(std::make_pair('z', 128));
  foo.insert({'b', 16});
  foo.insert({'a', 8});

  bar = foo;                  // bar now contains 6 ints
  foo.clear();                // and first is now empty

  std::cout << "Size of foo: " << foo.size() << '\n';
  std::cout << "Size of bar: " << bar.size() << '\n';

  // 遍历 
  for (auto x : bar) {
      std::cout << x.first << ' ' << x.second << std::endl;
  }

  return 0;
}

运行结果：

Size of foo: 0
Size of bar: 6
a 8
b 16
x 32
y 64
y 96
z 128