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std::variant::emplace(3) C++ Standard Libary std::variant::emplace(3) NAME std::variant::emplace - std::variant::emplace Synopsis (since template <class T, class... Args> C++17) T& emplace(Args&&... args); (until C++20) template <class T, class... Args> (since constexpr T& emplace(Args&&... args); C++20) template <class T, class U, class... Args> (since T& emplace( std::initializer_list<U> il, C++17) Args&&... args ); (until C++20) template <class T, class U, class... Args> constexpr T& emplace( (since std::initializer_list<U> il, Args&&... args C++20) ); template <std::size_t I, class... Args> (since std::variant_alternative_t<I, variant>& C++17) emplace( Args&&... args ); (until (1) C++20) template <std::size_t I, class... Args> (since constexpr std::variant_alternative_t<I, C++20) variant>& emplace( Args&&... args ); (2) template <std::size_t I, class U, class... Args> (since C++17) std::variant_alternative_t<I, variant>& (3) (until C++20) emplace( std::initializer_list<U> il, Args&&... args ); template <std::size_t I, class U, class... (4) Args> constexpr std::variant_alternative_t<I, (since variant>& C++20) emplace( std::initializer_list<U> il, Args&&... args ); Creates a new value in-place, in an existing variant object 1) Equivalent to emplace<I>(std::forward<Args>(args)...), where I is the zero-based index of T in Types.... * This overload participates in overload resolution only if std::is_constructible_v<T, Args...> is true, and T occurs ex- actly once in Types... 2) Equivalent to emplace<I>(il, std::forward<Args>(args)...), where I is the zero-based index of T in Types.... * This overload participates in overload resolution only if std::is_constructible_v<T, std::initializer_list<U>&, Args...> is true, and T occurs exactly once in Types... 3) First, destroys the currently contained value (if any). Then di- rect-initializes the contained value as if constructing a value of type T_I with the arguments std::forward<Args>(args).... If an exception is thrown, *this may become valueless_by_exception. * This overload participates in overload resolution only if std::is_constructible_v<T_I, Args...> is true. * It is a compile-time error if I is not less than sizeof...(Types). 4) First, destroys the currently contained value (if any). Then di- rect-initializes the contained value as if constructing a value of type T_I with the arguments il, std::forward<Args>(args).... If an exception is thrown, *this may become valueless_by_exception. * This overload participates in overload resolution only if std::is_constructible_v<T_I, std::initializer_list<U>&, Args...> is true. * It is a compile-time error if I is not less than sizeof...(Types). Parameters args - constructor arguments to use when constructing the new value il - initializer_list argument to use when constructing the new value Return value A reference to the new contained value. Exceptions 1-4) Any exception thrown during the initialization of the contained value. Example // Run this code #include <iostream> #include <string> #include <variant> int main() { std::variant<std::string> v1; v1.emplace<0>("abc"); // OK std::cout << std::get<0>(v1) << '\n'; v1.emplace<std::string>("def"); // OK std::cout << std::get<0>(v1) << '\n'; std::variant<std::string, std::string> v2; v2.emplace<1>("ghi"); // OK std::cout << std::get<1>(v2) << '\n'; // v2.emplace<std::string>("abc"); -> Error } Output: abc def ghi Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct behavior P2231R1 C++20 emplace was not constexpr while the required made constexpr operations can be constexpr in C++20 See also operator= assigns a variant (public member function) http://cppreference.com 2022.07.31 std::variant::emplace(3)
NAME | Synopsis | Parameters | Return value | Exceptions | Example | Output: | See also
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