std::unordered_multimap<Key,T,Hash,KeyEqual,Allocator>::merge
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Tables
std::unordered_multimap template< class H2, class P2 >
void merge( std::unordered_map <Key, T, H2, P2, Allocator>& source );
(1)
(since C++17) void merge( std::unordered_map <Key, T, H2, P2, Allocator>& source );
(constexpr since C++26)
template< class H2, class P2 >
void merge( std::unordered_map <Key, T, H2, P2, Allocator>&& source );
(2)
(since C++17) void merge( std::unordered_map <Key, T, H2, P2, Allocator>&& source );
(constexpr since C++26)
template< class H2, class P2 >
void merge( std::unordered_multimap <Key, T, H2, P2, Allocator>& source );
(3)
(since C++17) void merge( std::unordered_multimap <Key, T, H2, P2, Allocator>& source );
(constexpr since C++26)
template< class H2, class P2 >
void merge( std::unordered_multimap <Key, T, H2, P2, Allocator>&& source );
(4)
(since C++17) 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 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.
Contents
[edit] Parameters
source
-
compatible container to transfer the nodes from
[edit] 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).
[edit] 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_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] {}