Keep headers “composition-safe” and minimal. Apply these rules: - **Do not define non-inline function implementations in headers.** Put the implementation in a `.cpp` file to avoid multiple-definition/linker problems. - **Avoid leaking preprocessor macros from headers.** If a constant/behavior is needed, prefer `constexpr`, `enum`, or an internal-scoped construct rather than a global `#define`. - **Use correct include discipline.** Remove duplicated includes and avoid unnecessary headers. Ensure all declarations are protected by appropriate header/platform guards. Example (bad → good): ```cpp // bad: in header // foo.h int gettimeofday_impl(); // ... int gettimeofday_impl() { return 0; } // implementation in header // good: in header // foo.h int gettimeofday_impl(); // foo.cpp int gettimeofday_impl() { return 0; } ``` Checklist for changes to `.h` files: - Are there any **function bodies** (non-`inline`) in the header? - Are there any **global `#define`** entries? - Are there **duplicate/unneeded includes**? - Are declarations properly **guarded** (header guard / platform guard)?