Std::experimental::optional::value_or (3) Linux Manual Page

std::experimental::optional<T>::value_or – std::experimental::optional<T>::value_or

Synopsis

template <class U>
(library fundamentals TS)

    constexpr T value_or(U &&default_value) const &;

template <class U>
(library fundamentals TS)

    constexpr T value_or(U &&default_value) &&;

Returns the contained value if *this has a value, otherwise returns default_value.
1) Equivalent to bool(*this) ? **this : static_cast<T>(std::forward<U>(default_value))
2) Equivalent to bool(*this) ? std::move(**this) : static_cast<T>(std::forward<U>(default_value))

Parameters

default_value – the value to use in case *this is empty

Type requirements

–
T must meet the requirements of CopyConstructible in order to use overload (1).
–
T must meet the requirements of MoveConstructible in order to use overload (2).
–
U&& must be convertible to T

Return value

The current value if *this has a value, or default_value otherwise.

Exceptions

Any exception thrown by the selected constructor of the return value T.

Example

// Run this code

#include <experimental/optional>

#include <iostream>

#include <cstdlib>
std::experimental::optional<const char *> maybe_getenv(const char *n)

{
    if (const char *x = std::getenv(n))
        return x;
    else
        return {};
}

int main()

{
    std::cout << maybe_getenv("MYPWD").value_or("(none)") << '\n';
}

Possible output:

(none)

std::experimental::optional::value_or (3) Linux Manual Page