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

template< class C2 > void merge( std::map<Key, T, C2, Allocator>& source );	(1)	(since C++17)
template< class C2 > void merge( std::map<Key, T, C2, Allocator>&& source );	(2)	(since C++17)
template< class C2 > void merge( std::multimap<Key, T, C2, Allocator>& source );	(3)	(since C++17)
template< class C2 > void merge( std::multimap<Key, T, C2, Allocator>&& source );	(4)	(since C++17)

Attempts to extract ("splice") each element in source and insert it into *this using the comparison object of *this.

No elements are copied or moved, only the internal pointers of the container nodes are repointed. All pointers and references to the transferred elements remain valid, but now refer into *this, not into source.

The behavior is undefined if get_allocator() != source.get_allocator().

Parameters

source

compatible container to transfer the nodes from

Return value

(none)

Exceptions

Does not throw unless comparison throws.

Complexity

N * log(size() + N)), where N is source.size().

Example

#include <iostream>
#include <map>
#include <string>
 
int main()
{
    std::multimap<int, std::string> ma{{1, "apple"}, {5, "pear"}, {10, "banana"}};
    std::multimap<int, std::string> mb{{2, "zorro"}, {4, "batman"}, {5, "X"}, {8, "alpaca"}};
    std::multimap<int, std::string> u;
    u.merge(ma);
    std::cout << "ma.size(): " << ma.size() << '\n';
    u.merge(mb);
    std::cout << "mb.size(): " << mb.size() << '\n';
 
    for (auto const& kv : u)
        std::cout << kv.first << ", " << kv.second << '\n';
}

Output:

ma.size(): 0
mb.size(): 0
1, apple
2, zorro
4, batman
5, pear
5, X
8, alpaca
10, banana