std::optional<T>::value (3) - Linux Manuals

std::optional<T>::value: std::optional<T>::value

NAME

std::optional<T>::value - std::optional<T>::value

Synopsis

constexpr T& value() &; (1) (since C++17)
constexpr const T & value() const &;
constexpr T&& value() &&; (2) (since C++17)
constexpr const T&& value() const &&;

If *this contains a value, returns a reference to the contained value.
Otherwise, throws a std::bad_optional_access exception.

Parameters

(none)

Return value

A reference to the contained value.

Exceptions

std::bad_optional_access if *this does not contain a value.

Notes

The dereference operator operator*() does not check if this optional contains a value, which may be more efficient than value().

Example

// Run this code

  #include <optional>
  #include <iostream>
  int main()
  {
      std::optional<int> opt = {};

      try {
          int n = opt.value();
      } catch(const std::bad_optional_access& e) {
          std::cout << e.what() << '\n';
      }
  }

Possible output:

  bad optional access