std::unordered_map merge() method
- od C++17
// (1) Non const version only
template<class H2, class P2>
void merge( std::unordered_map<Key, T, H2, P2, Allocator>& source );
// (2) Non const version only
template<class H2, class P2>
void merge( std::unordered_map<Key, T, H2, P2, Allocator>&& source );
// (3) Non const version only
template<class H2, class P2>
void merge( std::unordered_multimap<Key, T, H2, P2, Allocator>& source );
// (4) Non const version only
template<class H2, class P2>
void merge( std::unordered_multimap<Key, T, H2, P2, Allocator>&& source );
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, linear in the size of source - O(source.size()).
Worst case - O(N * size() + N), where N is source.size()
.
Exceptions
(none)
Example
#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_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;
}
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} }
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