Here’s a neat trick to create an “overloaded macro” in C, such that
M(x) does one thing and M(x, y) does something else. For example,
we could make a macro ARCTAN such that ARCTAN(v) calls atan(v)
and ARCTAN(y,x) calls atan2(y,x).

#define GET_ARCTAN_MACRO(_1, _2, x, ...) x
#define ARCTAN(...) GET_ARCTAN_MACRO(__VA_ARGS__, atan2, atan)(__VA_ARGS__)

Authored by Arthur O'Dwyer, Quuxplusone offers  explorations of C++ programming language features, standard library design, and modern C++ development practices. Readers can delve into advanced topics such as template metaprogramming, design patterns, and effective use of language features. Additionally, they can learn about best practices for writing efficient and maintainable C++ code, optimizing performance, and leveraging the latest language standards.

Arthur O'Dwyer

A walkthrough of the 'macro overloading' idiom in C/C++, which lets a single macro dispatch to different implementations based on the number of arguments. Using variadic macros and a helper GET_*_MACRO pattern, you can make M(x) and M(x,y) call different functions. Examples include ARCTAN dispatching to atan vs atan2, and HYPOT handling 1–3 arguments. The post also covers edge cases (zero args, too many args), the C23/C++20 __VA_OPT__ improvement, and an MSVC workaround using an extra EXPAND indirection layer.

The “macro overloading” idiom