std::set merge() method

od C++17

// (1) Non const version only
template<class C2>
void merge( std::set<Key, C2, Allocator>& source );

// (2) Non const version only
template<class C2>
void merge( std::set<Key, C2, Allocator>&& source );

// (3) Non const version only
template<class C2>
void merge( std::multiset<Key, C2, Allocator>& source );

// (4) Non const version only
template<class C2>
void merge( std::multiset<Key, C2, Allocator>&& source );

Attempts to extract ("splice") each element in source and insert it into *this using the comparison object 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.

important

All pointers and references to the transferred elements remain valid, but now refer into *this, not into source.

Undefined behaviour

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

Parameters

• source - compatible container to transfer the nodes from

Return value

(none)

Complexity

N * log(size() + N)), where N is source.size().

Exceptions

(none)

Example

Main.cpp

#include <iostream>
#include <set>
 
// print out a container
template <class Os, class K>
Os& operator<<(Os& os, const std::set<K>& v) {
    os << '[' << v.size() << "] {";
    bool o{};
    for (const auto& e : v)
        os << (o ? ", " : (o = 1, " ")) << e;
    return os << " }\n";
}
 
int main()
{
    std::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] { A, B, C, D, E }
q: [1] { C }