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

From cppreference.net

C++
Containers library
std::unordered_set
Member types
Member functions
Non-member functions
Deduction guides (C++17)
template < class H2, class P2 >
void merge ( std:: unordered_set < Key, H2, P2, Allocator > & source ) ;
(1) (since C++17)
(constexpr since C++26)
template < class H2, class P2 >
void merge ( std:: unordered_set < Key, H2, P2, Allocator > && source ) ;
(2) (since C++17)
(constexpr since C++26)
template < class H2, class P2 >
void merge ( std:: unordered_multiset < Key, H2, P2, Allocator > & source ) ;
(3) (since C++17)
(constexpr since C++26)
template < class H2, class P2 >
void merge ( std:: unordered_multiset < Key, H2, P2, Allocator > && source ) ;
(4) (since C++17)
(constexpr since C++26)

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 . If get_allocator ( ) == source. get_allocator ( ) is false , the behavior is undefined.

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.

Contents

Parameters

source - compatible container to transfer the nodes from

Complexity

Given S as size ( ) and N as source. size ( ) :

1-4) Average case O(N) , worst case O(N·S+N) .

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

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