Template magic.

Concise Result Extraction in Modern C++

by David Gorski

From the article:

A popular idiom in functional programming is the use of sum types to express results or optional values. When a function returns, it either succeeded and we get the result, or it failed and we have an error on our hands. This is a pattern in Modern C++ as well, enabled by standard library types such as std::variant and std::optional. In this article we will explore how to improve the ergonomics of handling multiple results and potential error values...