Question

I was looking at the doom3 code on github and I notice something unusual. Several files have only one include for a file called idlib/precompiled.h and this file have includes for several other headers like

...
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#include <string.h>
#include <assert.h>
#include <time.h>
#include <ctype.h>
#include <typeinfo>
#include <errno.h>
#include <math.h>
...

and to program headers

#include "../framework/BuildVersion.h"
#include "../framework/BuildDefines.h"
#include "../framework/Licensee.h"
#include "../framework/CmdSystem.h"
#include "../framework/CVarSystem.h"

I wonder if there’s any good reason for that, because that’s the first time I see such thing

Answer 1

This is called precompiled headers. The main benefit is dramatically increased compilation speed.

All the headers in precompiled.h are compiled only once per project. Without precompiled headers, each header content would be compiled multiple times: for each .cpp file it is included in.