Precompiled Headers in Header Files

14
votes

I ran into precompiled headers today for the first time..forever changing my life. I can't believe compiling my C++ code could be that fast. It makes total sense now..

Anyway, one thing that is confusing me is that from what I've read so far, pre-compiled headers only should be added to source files( cpp? ).

In Visual Studio, there is an option under Project Properties->C/C++->Advanced to "Force Include File". I set that compiler option to stdafx.h.

After doing this..I no longer require to include the headers I have added to my stdafx.h, even inside my header files ( source files are supposed to automatically include stdafx.h ). Is this expected behaviour?

I can't find a place that's clear in the distinction between header/source files.

If it does..great but I'm afraid it's another one of those things VC++ lets you get away with but will break in GCC. And yes..it needs to be portable; at least between GCC and VC++.

3
c++visual-studiovisual-c++gccprecompiled-headers
Well, if you want your project to work on gcc as well then you'd better stay away from the "Force Include File" option. Boilerplate is to make the first #include in your source code file the precompiled header file. – Hans Passant
@Hans, the equivalent gcc feature would be to use the -include command-line option to include stdafx.h at the start of every file. – Rob Kennedy

3 Answers

19
votes

StdAfx.h really should only be included in source files, not headers. I would suggest you #include "StdAfx.h" first in every cpp and not use the "Force Include File" option. Thats how I do it with my cross-platform projects. For the record, I don't actually use precompiled headers in GCC I just build it normally and it works well.

For some background. The compiler only looks at source files (ie, *.cpp, *.c, etc) and so when it compiles them it has to include every header and compile any code found in the headers as well. The precompiled headers option allows for compiling all of that code (ie, the globally include'd code in StdAfx.h) once so that you don't have to do it all of the time. Thats what StdAfx.cpp is for. The compiler compiles StdAfx.cpp with all of the code included in StdAfx.h once instead of having to do it every time you build.

So, since you include StdAfx.h in every source file as the first item, it doesn't make sense to include it in any of the headers since they will be included AFTER StdAfx.h and thus will have access to all of the code in StdAfx.h. Plus you can then use those headers in other projects without having to worry about having a StdAfx.h around or including the wrong one.

5
votes

Yes, it is expected behaviour. The Project Properties->C/C++->Advanced to "Force Include File" setting controls Visual C++ compiler option /FI:

This option has the same effect as specifying the file with double quotation marks in an #include directive on the first line of every source file

So, it frees you from including the stdafx.h manually.

Although, you can use precompiled headers with GCC and other compilers The Visual C++'s shortcut behaviour is not portable across other compilers. So, check How to handle stdafx.h in cross-platform code? where ideas for portable solutions are discussed.

Long story short, include stdafx.h manually in your .cpp source files and you should be fine also with GCC (assuming, you will configure your build for GCC to use precompiled headers).

2
votes

Do not use the "Force Include File" setting (/FI) as it breaks Edit & Continue ! (and MS doesn't seem to want to fix this issue)

See https://connect.microsoft.com/VisualStudio/feedback/details/668339/vs-2010-sp1-c-edit-and-continue-fails-with-fi

and https://connect.microsoft.com/VisualStudio/feedback/details/342441/visual-studio-2005-force-includes-breaks-edit-and-continue-with-pre-compiled-headers

#include "stdafx.h" should only be found as the first non-comment line in your source files, not in header files.