std::multimap operator=

od C++17

// (1) Non const version only
multimap& operator=( const multimap& other );

// (2) Non const version only
multimap& operator=( multimap&& other ) noexcept(/* see below */);

// (3) Non const version only
multimap& operator=( std::initializer_list<value_type> ilist );

od C++11

// (1) Non const version only
multimap& operator=( const multimap& other );

// (2) Non const version only
multimap& operator=( multimap&& other );

// (3) Non const version only
multimap& operator=( std::initializer_list<value_type> ilist );

od C++98

// (1) Non const version only
multimap& operator=( const multimap& other );

Replaces the contents of the containter.

Parameters

• other - another container to use as data source
• ilist - initializer list to use as data source

Return value

A reference to the container, *this.

Exceptions

od C++17

• (1, 3) May throw implementation-defined exceptions.
• (2) noexcept specification:

  noexcept(std::allocator_traits<Allocator>::is_always_equal::value
  && std::is_nothrow_move_assignable<Compare>::value)

do C++17

May throw implementation-defined exceptions.

Complexity

• (1) Linear in the size of *this and other - O(size() + other.size()).
• (2)
  Linear in the size of *this - O(size()). If the allocators do not compare equal and do not propagate - linear in the size of *this and other - O(size() + other.
• (3)
  O(N log N) in general, where N is size() + ilist.size(). Linear if ilist is sorted with respect to value_comp() - O(size()).

Notes

After container move assignment (overload (2)), unless element-wise move assignment is forced by incompatible allocators, references, pointers, and iterators (other than the end iterator) to other remain valid, but refer to elements that are now in *this. The current standard makes this guarantee via the blanket statement in [container.requirements.general]/12, and a more direct guarantee is under consideration via LWG 2321.

Example

Main.cpp

#include <map>
#include <iterator>
#include <iostream>
#include <utility>
#include <initializer_list>
 
void print(auto const comment, auto const& container)
{
    auto size = std::size(container);
    std::cout << comment << "{ ";
    for (auto const& [key, value]: container)
        std::cout << '{' << key << ',' << value << (--size ? "}, " : "} ");
    std::cout << "}\n";
}
 
int main()
{
    std::multimap<int, int> x { {1,1}, {2,2}, {3,3} }, y, z;
    const auto w = { std::pair<const int, int>{4,4}, {5,5}, {6,6}, {7,7} };
 
    std::cout << "Initially:\n";
    print("x = ", x);
    print("y = ", y);
    print("z = ", z);
 
    std::cout << "Copy assignment copies data from x to y:\n";
    y = x;
    print("x = ", x);
    print("y = ", y);
 
    std::cout << "Move assignment moves data from x to z, modifying both x and z:\n";
    z = std::move(x);
    print("x = ", x);
    print("z = ", z);
 
    std::cout << "Assignment of initializer_list w to z:\n";
    z = w;
    print("w = ", w);
    print("z = ", z);
}

Output

Initially:
x = { {1,1}, {2,2}, {3,3} }
y = { }
z = { }
Copy assignment copies data from x to y:
x = { {1,1}, {2,2}, {3,3} }
y = { {1,1}, {2,2}, {3,3} }
Move assignment moves data from x to z, modifying both x and z:
x = { }
z = { {1,1}, {2,2}, {3,3} }
Assignment of initializer_list w to z:
w = { {4,4}, {5,5}, {6,6}, {7,7} }
z = { {4,4}, {5,5}, {6,6}, {7,7} }