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std::optional - cppreference.com

template< class T >
class optional;

The class template std::optional manages an optional contained value, i.e. a value that may or may not be present.

A common use case for optional is the return value of a function that may fail. As opposed to other approaches, such as std::pair<T, bool>, optional handles expensive-to-construct objects well and is more readable, as the intent is expressed explicitly.

Any instance of optional at any given point in time either contains a value or does not contain a value.

If an optional contains a value, the value is guaranteed to be nested within the optional object. Thus, an optional object models an object, not a pointer, even though operator*() and operator->() are defined.

When an object of type optional<T> is contextually converted to bool, the conversion returns true if the object contains a value and false if it does not contain a value.

The optional object contains a value in the following conditions:

• The object is initialized with/assigned from a value of type T or another optional that contains a value.

The object does not contain a value in the following conditions:

• The object is default-initialized.
• The object is initialized with/assigned from a value of type std::nullopt_t or an optional object that does not contain a value.
• The member function reset() is called.

The optional object is a view that contains either one element if it contains a value, or otherwise zero elements if it does not contain a value. The lifetime of the contained element is bound to the object.

There are no optional references, functions, arrays, or (possibly cv-qualified) void; a program is ill-formed if it instantiates an optional with such a type. In addition, a program is ill-formed if it instantiates an optional with the (possibly cv-qualified) tag types std::nullopt_t or std::in_place_t.

All requirements on the iterator types of a Container apply to the iterator type of optional as well.

This specialization of ranges::enable_view makes optional satisfy view.

template< class T >
constexpr auto format_kind<std::optional<T>> = range_format::disabled;

This specialization of format_kind disables the range formatting support of optional.

#include <iostream>
#include <optional>
#include <string>
 
// optional can be used as the return type of a factory that may fail
std::optional<std::string> create(bool b)
{
    if (b)
        return "Godzilla";
    return {};
}
 
// std::nullopt can be used to create any (empty) std::optional
auto create2(bool b)
{
    return b ? std::optional<std::string>{"Godzilla"} : std::nullopt;
}
 
int main()
{
    std::cout << "create(false) returned "
              << create(false).value_or("empty") << '\n';
 
    // optional-returning factory functions are usable as conditions of while and if
    if (auto str = create2(true))
        std::cout << "create2(true) returned " << *str << '\n';
}

Output:

create(false) returned empty
create2(true) returned Godzilla

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

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