W3cubDocs

std::optional<T>::optional

constexpr optional() noexcept; constexpr optional( std::nullopt_t ) noexcept;	(1)	(since C++17)
constexpr optional( const optional& other );	(2)	(since C++17)
constexpr optional( optional&& other ) noexcept(/* see below */);	(3)	(since C++17)
	(4)
template < class U > optional( const optional<U>& other );		(since C++17) (until C++20) (conditionally explicit)
template < class U > constexpr optional( const optional<U>& other );		(since C++20) (conditionally explicit)
	(5)
template < class U > optional( optional<U>&& other );		(since C++17) (until C++20) (conditionally explicit)
template < class U > constexpr optional( optional<U>&& other );		(since C++20) (conditionally explicit)
template< class... Args > constexpr explicit optional( std::in_place_t, Args&&... args );	(6)	(since C++17)
template< class U, class... Args > constexpr explicit optional( std::in_place_t, std::initializer_list<U> ilist, Args&&... args );	(7)	(since C++17)
template < class U = T > constexpr optional( U&& value );	(8)	(since C++17) (conditionally explicit)

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 contained 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_constructible_v<T> is false.
It is a trivial constructor if std::is_trivially_copy_constructible_v<T> is true.

3) Move constructor: If other contains a value, initializes the contained 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_constructible_v<T> is true.

4) Converting copy constructor: If other does not 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 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 constructible or convertible from any expression of type (possibly const) std::optional, i.e., the following 8 values are all false:
 - std::is_constructible_v<T, std::optional&>
 - std::is_constructible_v<T, const std::optional&>
 - std::is_constructible_v<T, std::optional&&>
 - std::is_constructible_v<T, const std::optional&&>
 - std::is_convertible_v<std::optional&, T>
 - std::is_convertible_v<const std::optional&, T>
 - std::is_convertible_v<std::optional&&, T>
 - std::is_convertible_v<const std::optional&&, T>
This constructor is explicit if and only if std::is_convertible_v<const U&, T> is false.

5) Converting move constructor: If other does not 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 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 constructible or convertible from any expression of type (possibly const) std::optional, i.e., the following 8 values are all false:
 - std::is_constructible_v<T, std::optional&>
 - std::is_constructible_v<T, const std::optional&>
 - std::is_constructible_v<T, std::optional&&>
 - std::is_constructible_v<T, const std::optional&&>
 - std::is_convertible_v<std::optional&, T>
 - std::is_convertible_v<const std::optional&, T>
 - std::is_convertible_v<std::optional&&, T>
 - std::is_convertible_v<const std::optional&&, T>
This constructor is explicit if and only if std::is_convertible_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&, 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(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 (until C++20)std::remove_cvref_t (since C++20) is neither std::in_place_t nor std::optional<T>.
- If T is (possibly cv-qualified) bool, std::decay_t (until C++20)std::remove_cvref_t (since C++20) is not a specialization of std::optional.
This constructor is explicit if and only if std::is_convertible_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 following

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::optional<bool> from std::optional 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::optional, 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 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

#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 retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3836	C++17	when constructing an std::optional<bool> from std::optional<U>, the overload resolution would select overload (8) if `U` is not bool	always selects the converting copy/move constructor in this case
P0602R4	C++17	copy/move constructors might not be trivial even if underlying constructor is trivial	required to propagate triviality
P2231R1	C++20	converting constructors from another `std::optional` was not constexpr while the required operations can be in C++20	made constexpr