std::unordered_multimap<Key,T,Hash,KeyEqual,Allocator>::merge

 

C++

Compiler support

Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Containers library

Sequence

array (C++11)

vector

deque

forward_list (C++11)

list

Associative

set

multiset

map

multimap

Unordered associative

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

Adaptors

stack

queue

priority_queue

Views

span (C++20)

 

std::unordered_multimap

Member functions

unordered_multimap::unordered_multimap

unordered_multimap::~unordered_multimap

unordered_multimap::operator=

unordered_multimap::get_allocator

Iterators

unordered_multimap::beginunordered_multimap::cbegin

unordered_multimap::endunordered_multimap::cend

Capacity

unordered_multimap::empty

unordered_multimap::size

unordered_multimap::max_size

Modifiers

unordered_multimap::clear

unordered_multimap::insert

unordered_multimap::emplace

unordered_multimap::emplace_hint

unordered_multimap::erase

unordered_multimap::swap

unordered_multimap::extract (C++17)

unordered_multimap::merge (C++17)

Lookup

unordered_multimap::count

unordered_multimap::find

unordered_multimap::contains (C++20)

unordered_multimap::equal_range

Bucket interface

unordered_multimap::begin(size_type)unordered_multimap::cbegin(size_type)

unordered_multimap::end(size_type)unordered_multimap::cend(size_type)

unordered_multimap::bucket_count

unordered_multimap::max_bucket_count

unordered_multimap::bucket_size

unordered_multimap::bucket

Hash policy

unordered_multimap::load_factor

unordered_multimap::max_load_factor

unordered_multimap::rehash

unordered_multimap::reserve

Observers

unordered_multimap::hash_function

unordered_multimap::key_eq

Non-member functions

operator==operator!= (until C++20)

std::swap

erase_if (C++20)

Deduction guides(C++17)

 

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

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] { }

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)