std::optional<T>:: optional

Constructs a new optional object.

Parameters

Effects

Constraints and supplement information

1. ↑ ^{1.0 1.1} In other words, T is neither constructible nor convertible from any expression of type (possibly const-qualified) std:: optional < U >

Exceptions

Deduction guides

Notes

Before the resolution of LWG issue 3836 , constructing an std:: optional < bool > from std:: optional < U > would select overload ( 9 ) instead of overloads ( 5,6 ) if U is not bool . This is because overloads ( 5,6 ) did not participate in overload resolution if T ( bool in this case) can be constructed or converted from std:: optional < U > , but std::optional::operator bool makes the conversion possible for any U .

As a result, the constructed std:: optional < bool > always contains a value. That value is determined by whether the provided std:: optional < U > object contains a value, rather than the bool value direct-initialized from the contained value:

std::optional<bool> op_false(false);
std::optional<int> op_zero(0);
std::optional<int> from_bool(op_false); // OK: contains 0 (initialized from false)
std::optional<bool> from_int(op_zero);  // DEFECT (LWG 3836): contains true because
                                        // op_zero contains a value, even if initializing
                                        // bool from that value gives false

Example

#include <iostream>
#include <optional>
#include <string>
int main()
{
    std::optional<int> o1, // empty
                       o2 = 1, // init from rvalue
                       o3 = o2; // copy-constructor
    // calls std::string( initializer_list<CharT> ) constructor
    std::optional<std::string> o4(std::in_place, {'a', 'b', 'c'});
    // calls std::string( size_type count, CharT ch ) constructor
    std::optional<std::string> o5(std::in_place, 3, 'A');
    // Move-constructed from std::string using deduction guide to pick the type
    std::optional o6(std::string{"deduction"});
    std::cout << *o2 << ' ' << *o3 << ' ' << *o4 << ' ' << *o5  << ' ' << *o6 << '\n';
}

1 1 abc AAA deduction

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also