# std::multimap<Key,T,Compare,Allocator>::multimap

From cppreference.com
< cpp‎ | container‎ | multimap

multimap(); explicit multimap( const Compare& comp, const Allocator& alloc = Allocator() ); | (1) |
explicit multimap( const Allocator& alloc ); | (1) | (since C++11)
| (2) |
template< class InputIt > multimap( InputIt first, InputIt last, const Compare& comp = Compare(), const Allocator& alloc = Allocator() ); |  |
template< class InputIt > multimap( InputIt first, InputIt last, const Allocator& alloc ); |  | (since C++14)
multimap( const multimap& other ); | (3) |
multimap( const multimap& other, const Allocator& alloc ); | (3) | (since C++11)
multimap( multimap&& other ); | (4) | (since C++11)
multimap( multimap&& other, const Allocator& alloc ); | (4) | (since C++11)
| (5) |
multimap( std::initializer_list <value_type> init, const Compare& comp = Compare(), const Allocator& alloc = Allocator() ); |  | (since C++11)
multimap( std::initializer_list <value_type> init, const Allocator& ); |  | (since C++14)

Constructs new container from a variety of data sources and optionally using user supplied allocator `alloc` or comparison function object `comp`.

1) Constructs an empty container.
2) Constructs the container with the contents of the range `[first, last)`.
3) Copy constructor. Constructs the container with the copy of the contents of `other`. If `alloc` is not provided, allocator is obtained by calling std::allocator_traits <allocator_type>::select_on_container_copy_construction(other.get_allocator()).
4) Move constructor. Constructs the container with the contents of `other` using move semantics. If `alloc` is not provided, allocator is obtained by move-construction from the allocator belonging to `other`.
5) Constructs the container with the contents of the initializer list `init`.

### Parameters

alloc | - | allocator to use for all memory allocations of this container
comp | - | comparison function object to use for all comparisons of keys
first, last | - | the range to copy the elements from
other | - | another container to be used as source to initialize the elements of the container with
init | - | initializer list to initialize the elements of the container with
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `Compare` must meet the requirements of Compare.
- `Allocator` must meet the requirements of Allocator.

### Complexity

1) Constant
2) N log(N) where N = std::distance (first, last) in general, linear in `N` if the range is already sorted by `value_comp()`.
3) Linear in size of `other`
4) Constant. If `alloc` is given and alloc != other.get_allocator(), then linear.
5) N log(N) where N = init.size()) in general, linear in `N` if `init` is already sorted by `value_comp()`.

### Exceptions

Calls to `Allocator::allocate` may throw.

### Notes

After container move construction (overload (4)), 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 §23.2.1[container.requirements.general]/12, and a more direct guarantee is under consideration via LWG 2321.

### Example

Run this code

    #include <iostream>
    #include <map>

    struct Point { double x, y; };
    struct PointCmp {
        bool operator()(const Point& lhs, const Point& rhs) const {
            return lhs.x < rhs.x; // NB. ignores y on purpose
        }
    };

    int main() {
        std::multimap<int, int> m = {{1,1},{2,2},{3,3},{4,4},{5,5},{4,4},{3,3},{2,2},{1,1}};
        for(auto& p: m) std::cout << p.first << ' ' << p.second << '\n';

        // custom comparison
        std::multimap<Point, double, PointCmp> mag{
            { {5, 12}, 13 },
            { {3, 4}, 5 },
            { {8, 15}, 17 },
            { {3, -3}, -1 },
        };

      for(auto p : mag)
          std::cout << "The magnitude of (" << p.first.x
                    << ", " << p.first.y << ") is "
                    << p.second << '\n';
    }

Output:

    1 1
    1 1
    2 2
    2 2
    3 3
    3 3
    4 4
    4 4
    5 5
    The magnitude of (3, 4) is 5
    The magnitude of (3, -3) is -1
    The magnitude of (5, 12) is 13
    The magnitude of (8, 15) is 17

### 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 2193 | C++11 | the default constructor is explicit | made non-explicit

### See also

operator= | assigns values to the container (public member function)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/container/multimap/multimap&oldid=50589"