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

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multimap();
 (1)
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 );
 (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& );
 (11) (since C++14)

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

1-3) 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)

The template parameter Allocator is only deduced from the first argument while used in class template argument deduction.

(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.

The template parameter Allocator is only deduced from the first argument while used in class template argument deduction.

(since C++23)
10-11) 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

1-3) 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().

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.requirements.general]/12, and a more direct guarantee is under consideration via LWG 2321.

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

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

 assigns values to the container
(public member function)
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