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| Deduction guides(C++17) | ||||||
template<class...Args> std::pair<iterator,bool> emplace( Args&&...args); | (since C++11) (constexpr since C++26) | |
Inserts a new element into the container constructed in-place with the givenargs, if there is no element with the key in the container.
The constructor of the new element (i.e.std::pair<const Key, T>) is called with exactly the same arguments as supplied toemplace, forwarded viastd::forward<Args>(args)....The element may be constructed even if there already is an element with the key in the container, in which case the newly constructed element will be destroyed immediately (seetry_emplace() if this behavior is undesirable).
Ifvalue_type is notEmplaceConstructible intounordered_map fromargs, the behavior is undefined.
If after the operation the new number of elements is greater than oldmax_load_factor() * bucket_count() a rehashing takes place.
If rehashing occurs (due to the insertion), all iterators are invalidated. Otherwise (no rehashing), iterators are not invalidated.
Contents |
| args | - | arguments to forward to the constructor of the element |
A pair consisting of an iterator to the inserted element (or to the element that prevented the insertion) and abool value set totrue if and only if the insertion took place.
If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).
Amortized constant on average, worst case linear in the size of the container.
Careful use ofemplace allows the new element to be constructed while avoiding unnecessary copy or move operations.
#include <iostream>#include <string>#include <utility>#include <unordered_map> int main(){std::unordered_map<std::string,std::string> m; // uses pair's move constructor m.emplace(std::make_pair(std::string("a"),std::string("a"))); // uses pair's converting move constructor m.emplace(std::make_pair("b","abcd")); // uses pair's template constructor m.emplace("d","ddd"); // emplace with duplicate key has no effect m.emplace("d","DDD"); // uses pair's piecewise constructor m.emplace(std::piecewise_construct,std::forward_as_tuple("c"),std::forward_as_tuple(10,'c'));// an alternative is: m.try_emplace("c", 10, 'c'); for(constauto& p: m)std::cout<< p.first<<" => "<< p.second<<'\n';}
Possible output:
a => ab => abcdc => ccccccccccd => ddd
| constructs elements in-place using a hint (public member function)[edit] | |
(C++17) | inserts in-place if the key does not exist, does nothing if the key exists (public member function)[edit] |
| inserts elementsor nodes(since C++17) (public member function)[edit] |
