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

 

C++

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std::unordered_map

Member types
Member functions
unordered_map::unordered_map unordered_map::~unordered_map unordered_map::operator= unordered_map::get_allocator Iterators unordered_map::beginunordered_map::cbegin unordered_map::endunordered_map::cend Capacity unordered_map::size unordered_map::max_size unordered_map::empty Modifiers unordered_map::clear unordered_map::erase unordered_map::swap unordered_map::extract (C++17) unordered_map::merge (C++17) unordered_map::insert unordered_map::insert_range (C++23) unordered_map::insert_or_assign (C++17)   unordered_map::emplace unordered_map::emplace_hint unordered_map::try_emplace (C++17)	Lookup unordered_map::at unordered_map::operator[] unordered_map::count unordered_map::find unordered_map::contains (C++20) unordered_map::equal_range Bucket interface unordered_map::begin(size_type)unordered_map::cbegin(size_type) unordered_map::end(size_type)unordered_map::cend(size_type) unordered_map::bucket_count unordered_map::max_bucket_count unordered_map::bucket_size unordered_map::bucket Hash policy unordered_map::load_factor unordered_map::max_load_factor unordered_map::rehash unordered_map::reserve Observers unordered_map::hash_function unordered_map::key_eq
Non-member functions
operator==operator!= (until C++20)	std::swap(std::unordered_map) erase_if(std::unordered_map) (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. 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

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 <unordered_map>
#include <utility>
 
// 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_map<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_map<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: [3] {{A, 1}, {B, 2}, {C, 3}}
q: [3] {{A, 4}, {D, 5}, {E, 6}}
p.merge(q);
p: [5] {{E, 6}, {D, 5}, {A, 1}, {B, 2}, {C, 3}}
q: [1] {{A, 4}}

See also

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