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

std::optional<T>::value_or: std::optional<T>::value_or

NAME

std::optional<T>::value_or - std::optional<T>::value_or

Synopsis

template< class U > (since C++17)
constexpr T value_or( U&& default_value ) const&;
template< class U > (since C++17)
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 <optional>
  #include <iostream>
  #include <cstdlib>

  std::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)