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 ); (conditionally explicit) template < class U > (since C++20) constexpr optional( const optional<U>& (conditionally 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 ex- plicit) template< class... Args > (4) constexpr explicit optional( (6) (since C++17) std::in_place_t, Args&&... args ); template< class U, class... Args > (5) 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 std::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 unless 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 does not contain a value, con- structs 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 unless the following conditions are met: * std::is_constructible_v<T, const U&> is true. * If T is not (possibly cv-qualified) bool, T is not con- structible or convertible from any expression of type (possibly const) std::optional<U>, i.e., the following 8 values 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 does not contain a value, con- structs 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 unless the following conditions are met: * std::is_constructible_v<T, U&&> is true. * If T is not (possibly cv-qualified) bool, T is not con- structible or convertible from any expression of type (possibly const) std::optional<U>, i.e., the following 8 values 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 unless 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 unless 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 unless the following conditions are met: * std::is_constructible_v<T, U&&> is true. * 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>. * If T is (possibly cv-qualified) bool, std::decay_t<U> (until C++20) std::remove_cvref_t<U> (since C++20) is not a specialization of std::optional. * 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 follow- ing noexcept specification: noexcept(std::is_nothrow_move_constructible<T>::value) 4-8) Throws any exception thrown by the constructor of T. Deduction guides Notes Before the resolution of LWG issue 3836, constructing an std::op- tional<bool> from std::optional<U> would select overload (8) instead of overloads (4,5) if U is not bool. This is because overloads (4,5) did not participate in overload resolution if T (bool in this case) can be constructed or converted from std::op- tional<U>, but std::optional::operator bool makes the conversion possible for any U. As a result, the constructed std::optional<bool> always contains a value. That value is determined by whether the provided std::optional<U> object contains a value, rather than the bool value direct-initialized from the contained value: std::optional<bool> op_false(false); std::optional<int> op_zero(0); std::optional<int> from_bool(op_false); // OK: contains 0 (initialized from false) std::optional<bool> from_int(op_0); // DEFECT (LWG 3836): contains true // because op_0 contains a value, even if // initializing bool from that value gives false Example // Run this code #include <iostream> #include <optional> #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 when constructing an std::optional<bool> always selects the LWG 3836 C++17 from std::optional<U>, the overload convert- ing copy/move resolution construc- tor in this would select overload (8) if U is not bool case copy/move constructors might not be required to P0602R4 C++17 trivial propagate triviality even if underlying constructor is trivial converting constructors from another P2231R1 C++20 std::optional was made con- stexpr not constexpr while the required operations can be in C++20 See also make_optional creates an optional object (C++17) (function template) Category: * conditionally noexcept http://cppreference.com 2024.06.10 std::optional::optional(3)
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