std::map merge() method

since C++17

// (1) Non const version only
template<class C2>
void merge( std::map<Key, T, C2, Allocator>& source );

// (2) Non const version only
template<class C2>
void merge( std::map<Key, T, C2, Allocator>&& source );

// (3) Non const version only
template<class C2>
void merge( std::multimap<Key, T, C2, Allocator>& source );

// (4) Non const version only
template<class C2>
void merge( std::multimap<Key, T, C2, Allocator>&& source );

Attempts to extract ("splice") each element in source and insert it into *this using the comparison object of *this. If there is an element in *this with key equivalent to the key of an element from source, then that element is not extracted from source.

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.

Undefined behaviour

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

Parameters

• source - compatible container to transfer the nodes from

Return value

(none)

Complexity

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

Exceptions

Does not throw unless comparison throws.

Example

Main.cpp

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

Output

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