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

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

Return value

(none)

Complexity

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

Example

#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;
}

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