NAME¶

std::make_optional - std::make_optional

Synopsis¶

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 > (2) (since C++17)
constexpr std::optional<T> make_optional( Args&&... args );
template< class T, class U, class... Args >
constexpr std::optional<T> make_optional( std::initializer_list<U> (3) (since C++17)
il, Args&&... args );

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¶

// Run this code

#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¶

constructor constructs the optional object
(public member function)