std::make_optional

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std::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)...) ; .
This overload participates in overload resolution only if std::is_constructible_v<T, Args...> is true
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)...) ; .
This overload participates in overload resolution only if std::is_constructible_v <T, std::initializer_list <U> &, Args...> is true

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 <iomanip>
#include <iostream>
#include <optional>
#include <string>
#include <vector>
 
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: " << std::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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