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std::optional<T>::optional

From cppreference.com
< cpp‎ | utility‎ | optional
 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
std::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)
template < class U >
optional( const optional<U>& other );
 (4) (since C++17)
(constexpr since C++20)
(conditionally explicit)
template < class U >
optional( optional<U>&& other );
 (5) (since C++17)
(constexpr 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.
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.
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.
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).
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).

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] 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.

[edit] Deduction guides

[edit] Notes

Before the resolution of LWG issue 3836, constructing an std::optional<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::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 (4,5)

[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 (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 (4,5) from another std::optional was
not constexpr while the required operations can be in C++20 		made constexpr

[edit] See also

(C++17)
 creates an optional object
(function template) [edit]
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