std::optional::optional
std::optional::optional
| constexpr optional( constexpr optional( std::nullopt_t | (1) | (since C++17) |
|---|---|---|
| constexpr optional( const optional& other | (2) | (since C++17) |
| constexpr optional( optional&& other | (3) | (since C++17) |
| template < class U > /* EXPLICIT */ optional( const optional<U>& other | (4) | (since C++17) |
| template < class U > /* EXPLICIT */ optional( optional<U>&& other | (5) | (since C++17) |
| 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 = value_type > /* EXPLICIT */ constexpr optional( U&& value | (8) | (since C++17) |
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. This constructor is defined as deleted if std::is_copy_constructible_v<T> is false. It is a constexpr 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 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 constexpr constructor if std::is_trivially_move_constructible_v<T> is true.
4) Converting copy constructor: If other doesn't 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.
This constructor is explicit if and only if std::is_convertible_v<const U&, T> is false.
5) Converting move constructor: If other doesn't 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.
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<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 (where T = value_type) 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 std::is_constructible_v<T, U&&> is true and std::decay_t<U> is neither std::in_place_t nor std::optional<T>. This constructor is explicit if and only if std::is_convertible_v<U&&, T> is false.
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 |
Exceptions
1)
noexcept specification:
noexcept
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)
.
4-8) Throws any exception thrown by the constructor of T.
Example
#include <optional>
#include <iostream>
#include <string>
int main()
{
std::optional<int> o1, // empty
o2 = 1, // init from rvalue
o3 = o2; // copy-constructor
// calls string( initializer_list<CharT> ) constructor
std::optional<std::string> o4(std::in_place, {'a', 'b', 'c'}
// calls string( size_type count, CharT ch ) constructor
std::optional<std::string> o5(std::in_place, 3, 'A'
std::cout << *o2 << ' ' << *o3 << ' ' << *o4 << ' ' << *o5 << '\n';
}
Output:
1 1 abc AAA
See also
| make_optional (C++17) | creates an optional object (function template) |
|---|
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