A `.cpp` file must include the header file(s) that defines its interface

Reason

This enables the compiler to do an early consistency check.

Example, bad

// foo.h:
void foo(int);
int bar(long);
int foobar(int);

// foo.cpp:
void foo(int) { /* ... */ }
int bar(double) { /* ... */ }
double foobar(int);

The errors will not be caught until link time for a program calling bar or foobar.

Example

// foo.h:
void foo(int);
int bar(long);
int foobar(int);

// foo.cpp:
#include "foo.h"

void foo(int) { /* ... */ }
int bar(double) { /* ... */ }
double foobar(int);   // error: wrong return type

The return-type error for foobar is now caught immediately when foo.cpp is compiled. The argument-type error for bar cannot be caught until link time because of the possibility of overloading, but systematic use of .h files increases the likelihood that it is caught earlier by the programmer.

Enforcement

???