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):

// 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)?