std::experimental::optional<T>::optional
From cppreference.com
< cpp | experimental | optional
|
constexpr optional(
)
noexcept
;
constexpr optional( std::experimental::nullopt_t ) noexcept ; |
(1) | (library fundamentals TS) |
optional( const optional& other ); |
(2) | (library fundamentals TS) |
|
optional( optional&& other )
noexcept
(
/* see below */
)
;
|
(3) | (library fundamentals TS) |
|
constexpr optional( const T& value );
|
(4) | (library fundamentals TS) |
constexpr optional( T&& value ); |
(5) | (library fundamentals TS) |
|
template
<
class... Args
>
constexpr explicit optional( std::experimental::in_place_t, Args&&... args ) ; |
(6) | (library fundamentals TS) |
|
template
<
class U, class... Args
>
constexpr
explicit optional(
std::experimental::in_place_t, |
(7) | (library fundamentals TS) |
Constructs a new optional object.
1) Constructs the 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 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) 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 value. This constructor is constexpr if the constructor of T selected by direct-initialization is constexpr
5) 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(value). This constructor is constexpr if the constructor of T selected by direct-initialization is constexpr
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)
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)
. The function does not participate in the overload resolution if
std::is_constructible
<T, std::initializer_list
<U>
&, Args&&...>
::
value
!
=
true
Parameters
| other | - | another optional object whose contained value to copy
|
| value | - | value to initialize the contained value with |
| args... | - | arguments to initialize the contained value with |
| ilist | - | initializer list to initialize the contained value with |
| Type requirements | ||
-
T must meet the requirements of CopyConstructible
|
||
-
T must meet the requirements of MoveConstructible
|
||
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 declaration:
noexcept specification:
noexcept
(
std::is_nothrow_move_constructible
<T>
::
value
)
4-7) Throws any exception thrown by the constructor of
T.Example
Run this code
#include <experimental/optional> #include <iostream> #include <string> int main() { std::experimental::optional<int> o1, // empty o2 = 1, // init from rvalue o3 = o2; // copy-constructor std::experimental::optional<std::string> o4(std::experimental::in_place, {'a', 'b', 'c'}); std::cout << *o2 << ' ' << *o3 << ' ' << *o4 << '\n'; }
Output:
1 1 abc
See also
creates an optional object (function template) |