Question

I’ve been using precompiled header for a while and been told (and saw) how they can reduce compile time. But I would really like to know what is going on (under the hood) so it can make my compilation faster.

Because from what I know, adding unused include in a .cpp can slower your compile time, and a header file can contain a lot of unused header to a .cpp.

So how does a precompiled header make my compilation faster?

Answer 1

From http://gamesfromwithin.com/the-care-and-feeding-of-pre-compiled-headers Thank you (@Pablo)

A C++ compiler operates on one compilation unit (cpp file) at the
time. For each file, it applies the pre-preprocessor (which takes care
of doing all the includes and “baking” them into the cpp file itself),
and then it compiles the module itself. Move on to the next cpp file,
rinse and repeat. Clearly, if several files include the same set of
expensive header files (large and/or including many other header files
in turn), the compiler will be doing a lot of duplicated effort.

The simplest way to think of pre-compiled headers is as a cache for
header files. The compiler can analyze a set of headers once, compile
them, and then have the results ready for any module that needs them.