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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 <map>
#include <iostream>
#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