std::make_optional(3)

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 <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
   constructor constructs the optional object
                 (public member function)