(C++17) | ||||
| Sequence | ||||
(C++11) | ||||
(C++26) | ||||
(C++26) | ||||
(C++11) | ||||
| Associative | ||||
| Unordered associative | ||||
(C++11) | ||||
(C++11) | ||||
(C++11) | ||||
(C++11) | ||||
| Adaptors | ||||
(C++23) | ||||
(C++23) | ||||
(C++23) | ||||
(C++23) | ||||
| Views | ||||
(C++20) | ||||
(C++23) | ||||
| Tables | ||||
| Iterator invalidation | ||||
| Member function table | ||||
| Non-member function table |
template<class...Args> std::pair<iterator,bool> emplace( Args&&...args); | (since C++23) (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.
Initializes an objectt of typestd::pair<key_type, mapped_type> withstd::forward<Args>(args)...; if the map already contains an element whose key is equivalent tot.first,*this is unchanged. Otherwise, equivalent to:
auto key_it=ranges::upper_bound(c.keys, t.first, compare);auto value_it= c.values.begin()+std::distance(c.keys.begin(), key_it);c.keys.insert(key_it, std::move(t.first));c.values.insert(value_it, std::move(t.second));
This overload participates in overload resolution only ifstd::is_constructible_v<std::pair<key_type, mapped_type>, Args...> istrue.
Ifvalue_type is notEmplaceConstructible intoflat_map fromargs, the behavior is undefined.
| Information on iterator invalidation is copied fromhere |
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).
If insertion occurs, linear in the size of the container, otherwise logarithmic 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 <flat_map> int main(){std::flat_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';}
Output:
a => ab => abcdc => ccccccccccd => ddd
| constructs elements in-place using a hint (public member function)[edit] | |
| inserts in-place if the key does not exist, does nothing if the key exists (public member function)[edit] | |
| inserts elements (public member function)[edit] |
