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

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< cpp‎ | container‎ | multimap

| (1) |
multimap(); |  | (until C++11)
multimap() : multimap(Compare()) { } |  | (since C++11)
explicit multimap( const Compare& comp, const Allocator& alloc = Allocator() ); | (2) |
explicit multimap( const Allocator& alloc ); | (3) | (since C++11)
template< class InputIt > multimap( InputIt first, InputIt last, const Compare& comp = Compare(), const Allocator& alloc = Allocator() ); | (4) |
template< class InputIt > multimap( InputIt first, InputIt last, const Allocator& alloc ) : multimap(first, last, Compare(), alloc) { } | (5) | (since C++14)
multimap( const multimap& other ); | (6) |
multimap( const multimap& other, const Allocator& alloc ); | (7) | (since C++11)
multimap( multimap&& other ); | (8) | (since C++11)
multimap( multimap&& other, const Allocator& alloc ); | (9) | (since C++11)
multimap( std::initializer_list<value_type> init, const Compare& comp = Compare(), const Allocator& alloc = Allocator() ); | (10) | (since C++11)
multimap( std::initializer_list<value_type> init, const Allocator& alloc ) : multimap(init, Compare(), alloc) { } | (11) | (since C++14)
template< container-compatible-range<value_type> R > multimap( std::from_range_t, R&& rg, const Compare& comp = Compare(), const Allocator& alloc = Allocator() ); | (12) | (since C++23)
template< container-compatible-range<value_type> R > multimap( std::from_range_t, R&& rg, const Allocator& alloc ) : multimap(std::from_range, std::forward<R>(rg), Compare(), alloc) { } | (13) | (since C++23)

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

1-3) Constructs an empty container.
4-5) Constructs the container with the contents of the range `[`first`,` last`)`.
6-7) 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()). | (since C++11)
During class template argument deduction, only the first argument contributes to the deduction of the container's `Allocator` template parameter. | (since C++23)

8-9) 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.

During class template argument deduction, only the first argument contributes to the deduction of the container's `Allocator` template parameter. | (since C++23)

10-11) Initializer-list constructor. Constructs the container with the contents of the initializer list init.
12-13) Constructs the container with the contents of rg.

### 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 `[`first`,` last`)` to copy the elements from
rg | - | a container compatible range, that is, an `input_range` whose elements are convertible to `value_type`
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

1-3) Constant.
4-5) N·log(N) where N = std::distance(first, last) in general, linear in `N` if the range is already sorted by value_comp().
6-7) Linear in size of other.
8-9) Constant. If alloc is given and alloc != other.get_allocator(), then linear.
10-11) N·log(N) where N = init.size() in general, linear in `N` if init is already sorted by value_comp().
12-13) N·log(N) where N = ranges::distance(rg) in general, linear in `N` if the range is already sorted by value_comp().

### Exceptions

Calls to `Allocator::allocate` may throw.

### Notes

After container move construction (overload (8-9)), 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.rev.reqmts]/17, and a more direct guarantee is under consideration via LWG issue 2321.

Although not formally required until C++23, some implementations has already put the template parameter `Allocator` into non-deduced contexts in earlier modes.

Feature-test macro | Value | Std | Comment
--- | --- | --- | ---
`__cpp_lib_containers_ranges` | 202202L | (C++23) | Ranges-aware construction and insertion; overloads (12,13)

### 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)

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