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std::make_optional

From cppreference.com
< cpp‎ | utility‎ | optional
 
 
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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)...);.

Contents

[edit] Parameters

value - the value to construct optional object with
il, args - arguments to be passed to the constructor of T.

[edit] Return value

The constructed optional object.

[edit] Exceptions

Throws any exception thrown by the constructor of T.

[edit] Notes

T need not be movable for overloads (2-3) due to guaranteed copy elision.

[edit] 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: ""

[edit] See also

constructs the optional object
(public member function) [edit]