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std::map::try_emplace(3) C++ Standard Libary std::map::try_emplace(3) NAME std::map::try_emplace - std::map::try_emplace Synopsis template< class... Args > (1) (since C++17) pair<iterator, bool> try_emplace( const Key& k, Args&&... args ); template< class... Args > (2) (since C++17) pair<iterator, bool> try_emplace( Key&& k, Args&&... args ); template< class... Args > iterator try_emplace( const_iterator hint, const Key& k, Args&&... (3) (since C++17) args ); template< class... Args > iterator try_emplace( const_iterator hint, Key&& k, Args&&... args (4) (since C++17) ); Inserts a new element into the container with key k and value con- structed with args, if there is no element with the key in the container. 1) If a key equivalent to k already exists in the container, does nothing. Otherwise, behaves like emplace except that the element is con- structed as value_type(std::piecewise_construct, std::forward_as_tuple(k), std::forward_as_tuple(std::forward<Args>(args)...)) 2) If a key equivalent to k already exists in the container, does nothing. Otherwise, behaves like emplace except that the element is con- structed as value_type(std::piecewise_construct, std::forward_as_tuple(std::move(k)), std::forward_as_tuple(std::forward<Args>(args)...)) 3) If a key equivalent to k already exists in the container, does nothing. Otherwise, behaves like emplace_hint except that the element is con- structed as value_type(std::piecewise_construct, std::forward_as_tuple(k), std::forward_as_tuple(std::forward<Args>(args)...)) 4) If a key equivalent to k already exists in the container, does nothing. Otherwise, behaves like emplace_hint except that the element is con- structed as value_type(std::piecewise_construct, std::forward_as_tuple(std::move(k)), std::forward_as_tuple(std::forward<Args>(args)...)) No iterators or references are invalidated. 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 Return value 1,2) Same as for emplace 3,4) Same as for emplace_hint Complexity 1,2) Same as for emplace 3,4) Same as for emplace_hint 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::map<std::string, std::unique_ptr<foo>>. In addition, try_emplace treats the key and the arguments to the mapped_type sep- arately, unlike emplace, which requires the arguments to construct a value_type (that is, a std::pair). Feature-test macro: __cpp_lib_map_try_emplace Example // Run this code #include <iostream> #include <utility> #include <string> #include <map> auto print_node = [](const auto &node) { std::cout << "[" << node.first << "] = " << node.second << '\n'; }; auto 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 See also emplace constructs element in-place (C++11) (public member function) emplace_hint constructs elements in-place using a hint (C++11) (public member function) inserts elements insert or nodes (since C++17) (public member function) http://cppreference.com 2022.07.31 std::map::try_emplace(3)
NAME | Synopsis | Parameters | Return value | Complexity | Notes | Example | Output: | See also
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