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

 

C++

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Feature test macros (C++20)

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Symbols index

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node-handle (C++17)

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vector

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unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

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std::unordered_set

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

Parameters

source

-

compatible container to transfer the nodes from

Complexity

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

1-4) Average case \(\scriptsize O(N)\)O(N), worst case \(\scriptsize O(N \cdot S + N)\)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

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