std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::merge
void merge( std::unordered_map <Key, T, H2, P2, Allocator>& source );
(constexpr since C++26)
void merge( std::unordered_map <Key, T, H2, P2, Allocator>&& source );
(constexpr since C++26)
void merge( std::unordered_multimap <Key, T, H2, P2, Allocator>& source );
(constexpr since C++26)
void merge( std::unordered_multimap <Key, T, H2, P2, Allocator>&& source );
(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.
Contents
[edit] Parameters
[edit] Complexity
Given \(\scriptsize S\)S as size() and \(\scriptsize N\)N as source.size():
[edit] Example
#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}}