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

#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