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

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< cpp‎ | container‎ | multimap

 
C++
 
Containers library
 
std::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

 assigns values to the container
(public member function)
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