NAME

std::unordered_multimap::merge - std::unordered_multimap::merge

Synopsis

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

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 <utility>
#include <unordered_map>

// 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] { }