The include file mechanism is a hack that was acceptable at the time when machines were extremely underpowered, so only the simplest solutions had a chance to be implemented within a reasonable time frame.
By now, of course, precompiled headers exist, but their interplay with #define allows for fun inconsistencies.
And, of course, they leak implementation as much as the author wants, all the way to .h-only single-file libraries.
If you want an example of a sane approach to separate interface and implementation files from last century, take a look e.g. at Modula-2 with its .int and .mod files.
Precompiled headers are a terrible misfeature. I ban them in any code base I am responsible for.
They encourage the use of large header files that group unrelated concerns. In turn that makes small changes in header files produce massive, unnecessary rebuilds of zillions of object files.
The clean practice is to push down #includes into .c files, and to ruthlessly eliminate them if at all possible. That speeds up partial rebuilds enormously. And once you adopt that clean practice, pre-compiled headers yield no benefit anyway.
By now, of course, precompiled headers exist, but their interplay with #define allows for fun inconsistencies.
And, of course, they leak implementation as much as the author wants, all the way to .h-only single-file libraries.
If you want an example of a sane approach to separate interface and implementation files from last century, take a look e.g. at Modula-2 with its .int and .mod files.