std::optional

std::optional is quite comfortable for calculations such as database queries that may have a result.

🔑 Don’t use no-results
Before C++17 it was common practice to use a special value such as a null pointer, an empty string, or a unique integer to denote the absence of a result. These special values or no-results are very error-prone because you have to misuse the type system to check the return value. This means that for the type system that you have to use a regular value such as an empty string to define an irregular value.

The various constructors and the convenience function std::make_optional let you define an optional object opt with or without a value. opt.emplace will construct the contained value in-place and opt.reset will destroy the container value. You can explicitly ask a std::optional container if it has a value or you can check it in a logical expression. opt.value returns the value and opt.value_or returns the value or a default value. If opt has no contained value, the call opt.value will throw a std::bad_optional_access exception.

Here is a short example of using std::optional.