# std::unordered_set<Key,Hash,KeyEqual,Allocator>::merge

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< cpp‎ | container‎ | unordered set

template<class H2, class P2> void merge( std::unordered_set<Key, H2, P2, Allocator>& source ); | (1) | (since C++17)
template<class H2, class P2> void merge( std::unordered_set<Key, H2, P2, Allocator>&& source ); | (2) | (since C++17)
template<class H2, class P2> void merge( std::unordered_multiset<Key, H2, P2, Allocator>& source ); | (3) | (since C++17)
template<class H2, class P2> void merge( std::unordered_multiset<Key, 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. 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`. Iterators referring to the transferred elements and all iterators referring to *this are invalidated. Iterators to elements remaining in `source` remain valid.

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 <unordered_set>

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

    int main()
    {
        std::unordered_set<char>
            p{ 'C', 'B', 'B', 'A' },
            q{ 'E', 'D', 'E', 'C' };

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

        p.merge(q);

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

Possible output:

    p: [3] { A, B, C }
    q: [3] { C, D, E }
    p.merge(q);
    p: [5] { E, D, A, B, C }
    q: [1] { C }

### See also

extract (C++17) | extracts nodes from the container (public member function)
insert (C++11) | inserts elements or nodes (since C++17) (public member function)

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