NAME¶

std::multimap::multimap - std::multimap::multimap

Synopsis¶

multimap(); (1)
explicit multimap( const Compare& comp, (2)
const Allocator& alloc = Allocator() );
explicit multimap( const Allocator& alloc ); (3) (since C++11)
template< class InputIt >

multimap( InputIt first, InputIt last, (4)
const Compare& comp = Compare(),

const Allocator& alloc = Allocator() );
template< class InputIt >

multimap( InputIt first, InputIt last, (5) (since C++14)

const Allocator& alloc );
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(), (10) (since C++11)

const Allocator& alloc = Allocator() );
multimap( std::initializer_list<value_type> init, (11) (since C++14)
const Allocator& );

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 (since
std::allocator_traits<allocator_type>::select_on_container_copy_construction( C++11)
other.get_allocator()).
The template parameter Allocator is only deduced from the first argument (since
while used in class template argument deduction. 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 (since C++23)
argument while used in class template argument deduction.

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¶

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