# std::unordered_multimap<Key,T,Hash,KeyEqual,Allocator>::merge

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< cpp‎ | container‎ | unordered multimap

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

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

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