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std::optional::optional(3) C++ Standard Libary std::optional::optional(3) NAME std::optional::optional - std::optional::optional Synopsis constexpr optional() noexcept; (since constexpr optional( std::nullopt_t ) (1) C++17) noexcept; constexpr optional( const optional& other (2) (since ); C++17) constexpr optional( optional&& other ) (3) (since noexcept(/* see below */); C++17) (since C++17) template < class U > (until C++20) optional( const optional<U>& other ); (condition- ally explicit) template < class U > (since C++20) constexpr optional( const optional<U>& (condition- ally other ); explicit) (since C++17) template < class U > (until C++20) optional( optional<U>&& other ); (conditionally ex- plicit) template < class U > (since C++20) constexpr optional( optional<U>&& other ); (conditionally (4) explicit) template< class... Args > constexpr explicit optional( (6) (since C++17) std::in_place_t, Args&&... args ); (5) template< class U, class... Args > constexpr explicit optional( std::in_place_t, (7) (since C++17) std::initializer_list<U> ilist, Args&&... args ); template < class U = T > (since C++17) constexpr optional( U&& value ); (8) (conditionally ex- plicit) Constructs a new optional object. 1) Constructs an object that does not contain a value. 2) Copy constructor: If other contains a value, initializes the con- tained value as if direct-initializing (but not direct-list-initializing) an object of type T with the expression *other. If other does not contain a value, constructs an object that does not contain a value. * This constructor is defined as deleted if std::is_copy_con- structible_v<T> is false. * It is a trivial constructor if std::is_trivially_copy_con- structible_v<T> is true. 3) Move constructor: If other contains a value, initializes the con- tained value as if direct-initializing (but not direct-list-initializing) an object of type T with the expression std::move(*other) and does not make other empty: a moved-from optional still contains a value, but the value itself is moved from. If other does not contain a value, constructs an object that does not contain a value. * This constructor does not participate in overload resolution un- less std::is_move_constructible_v<T> is true. * It is a trivial constructor if std::is_trivially_move_con- structible_v<T> is true. 4) Converting copy constructor: If other doesn't contain a value, constructs an optional object that does not contain a value. Otherwise, constructs an optional object that contains a value, initialized as if direct-initializing (but not direct-list-initializing) an object of type T with the expression *other. * This constructor does not participate in overload resolution un- less the following conditions are met: * std::is_constructible_v<T, const U&> is true. * T is not constructible or convertible from any expression of type (possibly const) std::optional<U>, i.e., the following 8 type traits are all false: * std::is_constructible_v<T, std::optional<U>&> * std::is_constructible_v<T, const std::optional<U>&> * std::is_constructible_v<T, std::optional<U>&&> * std::is_constructible_v<T, const std::optional<U>&&> * std::is_convertible_v<std::optional<U>&, T> * std::is_convertible_v<const std::optional<U>&, T> * std::is_convertible_v<std::optional<U>&&, T> * std::is_convertible_v<const std::optional<U>&&, T> * This constructor is explicit if and only if std::is_convert- ible_v<const U&, T> is false. 5) Converting move constructor: If other doesn't contain a value, constructs an optional object that does not contain a value. Otherwise, constructs an optional object that contains a value, initialized as if direct-initializing (but not direct-list-initializing) an object of type T with the expression std::move(*other). * This constructor does not participate in overload resolution un- less the following conditions are met: * std::is_constructible_v<T, U&&> is true. * T is not constructible or convertible from any expression of type (possibly const) std::optional<U>, i.e., the following 8 type traits are all false: * std::is_constructible_v<T, std::optional<U>&> * std::is_constructible_v<T, const std::optional<U>&> * std::is_constructible_v<T, std::optional<U>&&> * std::is_constructible_v<T, const std::optional<U>&&> * std::is_convertible_v<std::optional<U>&, T> * std::is_convertible_v<const std::optional<U>&, T> * std::is_convertible_v<std::optional<U>&&, T> * std::is_convertible_v<const std::optional<U>&&, T> * This constructor is explicit if and only if std::is_convert- ible_v<U&&, T> is false. 6) Constructs an optional object that contains a value, initialized as if direct-initializing (but not direct-list-initializing) an object of type T from the arguments std::forward<Args>(args).... * If the selected constructor of T is a constexpr constructor, this constructor is a constexpr constructor. * The function does not participate in the overload resolution un- less std::is_constructible_v<T, Args...> is true 7) Constructs an optional object that contains a value, initialized as if direct-initializing (but not direct-list-initializing) an object of type T from the arguments ilist, std::forward<Args>(args).... * If the selected constructor of T is a constexpr constructor, this constructor is a constexpr constructor. * The function does not participate in the overload resolution un- less std::is_constructible_v<T, std::initializer_list<U>&, Args&&...> is true 8) Constructs an optional object that contains a value, initialized as if direct-initializing (but not direct-list-initializing) an object of type T with the expression std::forward<U>(value). * If the selected constructor of T is a constexpr constructor, this constructor is a constexpr constructor. * This constructor does not participate in overload resolution un- less std::is_constructible_v<T, U&&> is true and std::decay_t<U> (until C++20) std::remove_cvref_t<U> (since C++20) is neither std::in_place_t nor std::optional<T>. * This constructor is explicit if and only if std::is_convert- ible_v<U&&, T> is false. Parameters other - another optional object whose contained value is copied value - value with which to initialize the contained value args... - arguments with which to initialize the contained value ilist - initializer list with which to initialize the contained value Exceptions 2) Throws any exception thrown by the constructor of T. 3) Throws any exception thrown by the constructor of T. Has the fol- lowing noexcept specification: noexcept(std::is_nothrow_move_constructible<T>::value) . 4-8) Throws any exception thrown by the constructor of T. Deduction guides Example // Run this code #include <optional> #include <iostream> #include <string> int main() { std::optional<int> o1, // empty o2 = 1, // init from rvalue o3 = o2; // copy-constructor // calls std::string( initializer_list<CharT> ) constructor std::optional<std::string> o4(std::in_place, {'a', 'b', 'c'}); // calls std::string( size_type count, CharT ch ) constructor std::optional<std::string> o5(std::in_place, 3, 'A'); // Move-constructed from std::string using deduction guide to pick the type std::optional o6(std::string{"deduction"}); std::cout << *o2 << ' ' << *o3 << ' ' << *o4 << ' ' << *o5 << ' ' << *o6 << '\n'; } Output: 1 1 abc AAA deduction 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 P0602R4 C++17 copy/move constructors may not be trivial re- quired to propagate even if underlying constructor is trivial triv- iality converting constructors from another P2231R1 C++20 optional was not constexpr made constexpr while the required operations can be in C++20 See also make_optional creates an optional object (C++17) (function template) http://cppreference.com 2022.07.31 std::optional::optional(3)
NAME | Synopsis | Parameters | Exceptions | Example | Output: | See also
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