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std::unorde...imap::merge(3)   C++ Standard Libary  std::unorde...imap::merge(3)

NAME
     std::unordered_multimap::merge - std::unordered_multimap::merge

Synopsis
	template< class H2, class P2 >
	void  merge(  std::unordered_map<Key, T, H2, P2, Allocator>& source  (1)
     (since C++17)
	);
	template< class H2, class P2 >
	void merge( std::unordered_map<Key, T, H2, P2, Allocator>&&  source  (2)
     (since C++17)
	);
	template< class H2, class P2 >
	void  merge(  std::unordered_multimap<Key, T, H2, P2, Allocator>&    (3)
     (since C++17)
	source );
	template< class H2, class P2 >
	void merge( std::unordered_multimap<Key, T, H2, P2,  Allocator>&&    (4)
     (since C++17)
	source );

	Attempts to extract ("splice") each element in source and insert it into
     *this using
	the hash function and key equality predicate of *this.

	No  elements are copied or moved, only the internal pointers of the con-
     tainer nodes
	are repointed. All pointers and references to the  transferred	elements
     remain valid,
	but  now  refer  into *this, not into source. Iterators referring to the
     transferred
	elements and all iterators referring to *this are invalidated.

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

Parameters
	source - compatible container to transfer the nodes from

Return value
	(none)

Complexity
	Average  case  O(N),  worst  case  O(N	*  size()  +  N),  where  N   is
     source.size().

Example
     // Run this code

      #include <iostream>
      #include <string>
      #include <unordered_map>
      #include <utility>

      // print out a std::pair
      template<class Os, class U, class V>
      Os& operator<<(Os& os, const std::pair<U,V>& p)
      {
	  return os << '{' << p.first << ", " << p.second << '}';
      }

      // print out an associative container
      template<class Os, class K, class V>
      Os& operator<<(Os& os, const std::unordered_multimap<K, V>& v)
      {
	  os << '[' << v.size() << "] {";
	  bool o{};
	  for (const auto& e : v)
	      os << (o ? ", " : (o = 1, "")) << e;
	  return os << "}\n";
      }

      int main()
      {
	  std::unordered_multimap<std::string, int>
	      p{{"C", 3}, {"B", 2}, {"A", 1}, {"A", 0}},
	      q{{"E", 6}, {"E", 7}, {"D", 5}, {"A", 4}};

	  std::cout << "p: " << p << "q: " << q;

	  p.merge(q);

	  std::cout << "p.merge(q);\n" << "p: " << p << "q: " << q;
      }

Possible output:
      p: [4] {{A, 1}, {A, 0}, {B, 2}, {C, 3}}
      q: [4] {{A, 4}, {D, 5}, {E, 6}, {E, 7}}
      p.merge(q);
      p: [8] {{E, 6}, {E, 7}, {C, 3}, {A, 1}, {A, 0}, {A, 4}, {D, 5}, {B, 2}}
      q: [0] {}

See also
	extract extracts nodes from the container
	(C++17) (public member function)
		inserts elements
	insert	or nodes
		(since C++17)
		(public member function)

http://cppreference.com 	   2024.06.10	    std::unorde...imap::merge(3)
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