As another commenter pointed out, this is obsolete C, however: this is just not how modern compilers work. at all. Compilation is performed in phases, you have syntax analysis, semantic analysis, and then optimization and codegen. It’s trivial for a compiler to handle this code, heck C++ compilers (which your C compiler is under the hood) already do this for struct/classes (allowing member functions to be called before being declared). Pretty much every other language supports this too, but universally and not just in classes. We’ve come a long way since the 60’s.