Namespaces
Variants
Actions

std::optional<T>::optional

From cppreference.com
< cpp‎ | utility‎ | optional
 
 
Utilities library
Language support
Type support (basic types, RTTI)
Library feature-test macros (C++20)
Program utilities
Variadic functions
Coroutine support (C++20)
Contract support (C++26)
Three-way comparison
(C++20)
(C++20)
(C++20)(C++20)(C++20)  
(C++20)(C++20)(C++20)

General utilities
Relational operators (deprecated in C++20)
Integer comparison functions
(C++20)(C++20)(C++20)  
(C++20)
Swap and type operations
(C++20)
(C++14)
(C++11)
(C++11)
(C++23)
(C++11)
(C++17)
Common vocabulary types
(C++11)
(C++17)
(C++17)
(C++17)
(C++11)
(C++17)
(C++23)


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

constexpr explicit optional( std::in_place_t,
                             std::initializer_list <U> ilist,

                             Args&&... args );
(8) (since C++17)
template< class U = std::remove_cv_t <T> >
constexpr optional( U&& value );
(9) (since C++17)
(conditionally explicit)

Constructs a new optional object.

Contents

[edit] 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

[edit] Effects

 Overload  Initialization method Initializer for the contained value has_value() after construction
(1) N/A - false
(2)
(3) Direct (non-list) *other other.has_value()
  • If false, the contained value is not initialized.
(4) std::move(*other)
(5) *other
(6) std::move(*other)
(7) std::forward <Args>(args)... true
(8) ilist, std::forward <Args>(args)...
(9) std::forward <U>(value)

[edit] Constraints and supplement information

3) If std::is_copy_constructible_v <T> is false, the constructor is defined as deleted.
If std::is_trivially_copy_constructible_v <T> is true, the constructor is trivial.
4) This overload participates in overload resolution only if std::is_move_constructible_v <T> is true.
If std::is_trivially_move_constructible_v <T> is true, the constructor is trivial.
5) This overload participates in overload resolution only if all following conditions are satisfied:
This overload is declared as if with explicit(!std::is_convertible_v <const U&, T>).
6) This overload participates in overload resolution only if all following conditions are satisfied:
This overload is declared as if with explicit(!std::is_convertible_v <U, T>).
7) This overload participates in overload resolution only if std::is_constructible_v <T, Args...> is true.
If T’s constructor selected for the initialization is a constexpr constructor, this constructor is also a constexpr constructor.
8) This overload participates in overload resolution only if std::is_constructible_v <T, std::initializer_list <U>&, Args...> is true.
If T’s constructor selected for the initialization is a constexpr constructor, this constructor is also a constexpr constructor.
9) This overload participates in overload resolution only if all following conditions are satisfied:
This overload is declared as if with explicit(!std::is_convertible_v <U, T>).
If T’s constructor selected for the initialization is a constexpr constructor, this constructor is also a constexpr constructor.
  1. ↑ 1.0 1.1 In other words, T is neither constructible nor convertible from any expression of type (possibly const-qualified) std::optional <U>

[edit] Exceptions

3) Throws any exception thrown by the constructor of T.
4) Throws any exception thrown by the constructor of T. Has the following
noexcept specification:  
5-9) Throws any exception thrown by the constructor of T.

[edit] Deduction guides

[edit] Notes

Before the resolution of LWG issue 3836, constructing an std::optional <bool> from std::optional <U> would select overload (9) instead of overloads (5,6) if U is not bool. This is because overloads (5,6) did not participate in overload resolution if T (bool in this case) can be constructed or converted from std::optional <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_zero); // DEFECT (LWG 3836): contains true because
 // op_zero contains a value, even if initializing
 // bool from that value gives false
Feature-test macro Value Std Feature
__cpp_lib_optional 202106L (C++20)
(DR20)		 Fully constexpr (5,6)

[edit] 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

[edit] 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 (9) if U is not bool 		always selects the
converting copy/move
constructor in this case
LWG 3886 C++17 the default template argument of overload (9) was T changed to std::remove_cv_t <T>
P0602R4 C++17 copy/move constructors might not be trivial
even if underlying constructor is trivial 		required to
propagate triviality
P2231R1 C++20 overloads (5,6) from another std::optional was not constexpr made constexpr

[edit] See also

creates an optional object
(function template) [edit]

AltStyle によって変換されたページ (->オリジナル) /