_Generic, Tagged Unions, and Macro Boundaries

_Generic: compile-time selection

#define type_name(x) _Generic((x), int: "int", double: "double", char *: "char *", default: "other")

The expression's type (not value) picks an arm at compile time. This gives C its only real compile-time overload: type-safe wrappers over type-erased machinery — for example, dispatching to hash_int vs hash_str while callers never touch the wrong one.

Tagged unions: runtime generics

typedef struct { unsigned char tag; union { long i; double d; char *s; } as; } value_t;

The tag says which arm is live; readers switch on it. This is how interpreters, JSON libraries, and every dynamic value in C works. The union itself stores representations, not types — the discipline lives in the tag.

Macros: know the boundary

Token pasting and stringizing build small DSLs (register tables, X-macro enum/string lists). But macros do not respect scope, cannot be taken as pointers, and double-evaluate arguments. The professional rule: macro for boilerplate elimination, functions (even tiny static ones) for logic.