std::flat_map<Key,T,Compare,KeyContainer,MappedContainer>::try_emplace - cppreference.com (original) (raw)
| template< class... Args > std::pair<iterator, bool> try_emplace( const key_type& k, Args&&... args ); | (1) | (since C++23) |
|---|---|---|
| template< class... Args > std::pair<iterator, bool> try_emplace( key_type&& k, Args&&... args ); | (2) | (since C++23) |
| template< class K, class... Args > std::pair<iterator, bool> try_emplace( K&& k, Args&&... args ); | (3) | (since C++23) |
| template< class... Args >iterator try_emplace( const_iterator hint, const key_type& k, Args&&... args ); | (4) | (since C++23) |
| template< class... Args >iterator try_emplace( const_iterator hint, key_type&& k, Args&&... args ); | (5) | (since C++23) |
| template< class K, class... Args >iterator try_emplace( const_iterator hint, K&& k, Args&&... args ); | (6) | (since C++23) |
If a key equivalent to k already exists in the container, does nothing. Otherwise, inserts a new element into the underlying containers c with key k and value constructed with args.
The conversion from k into key_type must construct an object u, for which find(k) == find(u) is true. Otherwise, the behavior is undefined.
[edit] Parameters
| k | - | the key used both to look up and to insert if not found |
|---|---|---|
| hint | - | iterator to the position before which the new element will be inserted |
| args | - | arguments to forward to the constructor of the element |
[edit] Return value
[edit] Complexity
[edit] Notes
Unlike insert or emplace, these functions do not move from rvalue arguments if the insertion does not happen, which makes it easy to manipulate maps whose values are move-only types, such as std::flat_map<std::string, std::unique_ptr<foo>>. In addition, try_emplace treats the key and the arguments to the mapped_type separately, unlike emplace, which requires the arguments to construct a value_type (that is, a std::pair).
Overloads (3,6) can be called without constructing an object of type key_type.
[edit] Example
#include #include #include #include void print_node(const auto& node) { std::cout << '[' << node.first << "] = " << node.second << '\n'; } void print_result(auto const& pair) { std::cout << (pair.second ? "inserted: " : "ignored: "); print_node(*pair.first); } int main() { using namespace std::literals; std::map<std::string, std::string> m; print_result(m.try_emplace( "a", "a"s)); print_result(m.try_emplace( "b", "abcd")); print_result(m.try_emplace( "c", 10, 'c')); print_result(m.try_emplace( "c", "Won't be inserted")); for (const auto& p : m) print_node(p); }
Output:
inserted: [a] = a inserted: [b] = abcd inserted: [c] = cccccccccc ignored: [c] = cccccccccc [a] = a [b] = abcd [c] = cccccccccc
[edit] See also
| | constructs element in-place (public member function) [edit] | | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | constructs elements in-place using a hint (public member function) [edit] | | | inserts elements (public member function) [edit] | | | inserts an element or assigns to the current element if the key already exists (public member function) [edit] |