std::make_optional
Defined in header <optional> |
||
---|---|---|
template< class T > constexpr std::optional<std::decay_t<T>> make_optional( T&& value ); |
(1) | (since C++17) |
template< class T, class... Args > constexpr std::optional<T> make_optional( Args&&... args ); |
(2) | (since C++17) |
template< class T, class U, class... Args > constexpr std::optional<T> make_optional( std::initializer_list<U> il, Args&&... args ); |
(3) | (since C++17) |
1) Creates an optional object from
value
. Effectively calls
std::optional<std::decay_t<T>>(std::forward<T>(value))
2) Creates an optional object constructed in-place from
args...
. Equivalent to
return std::optional<T>(std::in_place, std::forward<Args>(args)...);
.
3) Creates an optional object constructed in-place from
il
and
args...
. Equivalent to
return std::optional<T>(std::in_place, il, std::forward<Args>(args)...);
.
Parameters
value | - | the value to construct optional object with |
il, args | - | arguments to be passed to the constructor of T . |
Return value
The constructed optional object.
Exceptions
Throws any exception thrown by the constructor of T
.
Notes
T
need not be movable for overloads (2-3) due to guaranteed copy elision.
Example
#include <optional> #include <iostream> #include <iomanip> #include <vector> #include <string> int main() { auto op1 = std::make_optional<std::vector<char>>({'a','b','c'}); std::cout << "op1: "; for (char c: op1.value()){ std::cout << c << ","; } auto op2 = std::make_optional<std::vector<int>>(5, 2); std::cout << "\nop2: "; for (int i: *op2){ std::cout << i << ","; } std::string str{"hello world"}; auto op3 = std::make_optional<std::string>(std::move(str)); std::cout << "\nop3: " << quoted(op3.value_or("empty value")) << '\n'; std::cout << "str: " << std::quoted(str) << '\n'; }
Possible output:
op1: a,b,c, op2: 2,2,2,2,2, op3: "hello world" str: ""
See also
constructs the optional object (public member function) |
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https://en.cppreference.com/w/cpp/utility/optional/make_optional