Thread: precompiled header file

I am not 100% sure that I'm correct, but my understanding was that only stdafx.h (or whatever header file you call out in the Visual Studio settings) will be precompiled, and it must be the first header #included in the file.

Here is more info:
http://www.cygnus-software.com/paper...edheaders.html

Originally Posted by George2

But I do not agree with you only the file "stdafx.h" is pre-compiled, I think all the files before and including the specified file parameter (e.g. stdafx.h) will be pre-compiled into one pch file.

Do you have any comments?

No, that is incorrect. Only the contents of stdafx.h is precompiled. You must include stdafx.h as the first header in each source file or you will get compile errors.
The compiler (or IDE?) can also typically detect when a header included in the PCH has changed, thus forcing a recompile of the header. Sometimes it doesn't work and requires a rebuild.
The best bet is to include headers into the PCH that does not change.

You only use /Yc on a source file that contains nothing but the #include directive for the PCH source file. Every other source file uses /Yu.

In a standard VS project, there's stdafx.h, stdafx.cpp, and various other files. The stdafx.cpp file looks like this:

Code:

#include "stdafx.h"

and is compiled with a command line like this:
cl.exe /Ycstdafx.h stdafx.cpp
yielding stdafx.pch.
All other sources are compiled with
cl.exe /Yustdafx.h source.cpp


OK, that much for the sane part. Now, the options really are quite insane. Consider that stdafx.h is empty and stdafx.cpp looks like this:

Code:

#include <iostream>
#include <string>
#include <vector>
#include <windows.h>
#include "stdafx.h"

Then the compiler will create stdafx.pch containing the information from all these headers.

While this is possible, it would be an extremely bad idea to rely on it. Let's say that source.cpp looks like this:

Code:

#include "stdafx.h"

int APIENTRY WinMain(HINSTANCE inst, HINSTANCE, LPSTR cmdline, int showcmd)
{
  std::vector<std::string> args;
  parse_cmdline(cmdline, args);
  // ...
}

Now compile with
cl.exe /Yustdafx.h source.cpp
This will work.

Compile again with
cl.exe source.cpp
This will fail! Basically, you have different behaviour depending on whether you use PCH or not. That's totally unacceptable, which is why you should ignore this peculiarity of PCH support and simply do the sane thing: put the include directives in stdafx.h, where they belong.

Originally Posted by George2

No you are wrong, please read CornedBee's reply below. All the contents before stdafx.h and including stdafx.h will be pre-compiled.

No. You are confused.
Stdafx.cpp is a special file with special rules. Try to include any header before a precompiled header in any other file and you'll get a compile error.

Originally Posted by George2

You are talking about stdafx.cpp? But we are talking about stdafx.h in your case. Can you read your post #4 again?

Well, as I mentioned... in stdafx.cpp, this may be valid:

#include <iostream>
#include "stdafx.h"

And in my_file.cpp, this is not valid:

#include <iostream>
#include "stdafx.h"

You will get a compile error.
At the same time, if my_file.cpp uses precompiled headers, the following won't work either:

#include <iostream>

(Note the missing "stdafx.h include.)

Originally Posted by George2

Hi Raigne,


I am totally confused. Why must I inlcude "stdafx.h" even if I do not need it? Suppose I have a project which needs to include foo.h in each source file, and foo.h is not prone to change, so I pre-compile foo.h into foo.pch and using /Yufoo.h to compile source files.

Why do I need to add "stdafx.h" inside all source files in the scenario I described?




regards,
George

Because of the way that the compiler handles precompiled headers is that way - I presume what it actually does is load the precompiled header(s) into the internal tables first, then start parsing the rest of the code.

It is an error to attempt to compile one module with precompiled headers and another module without. So don't do that.

Edit: Note Elysia expands on this - it is possible, as long as you use the correct settings for each file, to have some files include the precompiled headers, and other files to NOT do that. However, we now come to a contentious issue of "what use is precompiled headers if they are not used in all [or at least most] source files?

--
Mats

Speaking of which, you can tell Visual Studio that you don't want some source files to use precompiled headers via source-specific settings. It will compile fine if you do.

Originally Posted by George2

I do not know why above compile will fail. If you do not use /Yu, no optimization which will use pre-compiled header file will be used. So, it should re-compile all the headers files into source.cpp. Why do you think there is an error?

stdafx.h is empty. If PCH is disabled (no /Yu option), source.cpp will include the empty file and then attempt to use symbols from the headers included by stdafx.cpp. This will lead to compile errors.
If PCH is enabled, the compiler sees the include of the empty stdafx.h as a directive to use stdafx.pch, which does include all those system headers.

Originally Posted by George2

Hi Mats,


stdafx.h is empty, do we have any practical benefits to always put it as the 1st include file into each source?

Not that I can see - but stdafx.pch is created when you compile stdafx.cpp, so if stdafx.cpp contains other include files, those will be part of stdafx.pch as I understand it - but that also means that if you turn off PCH, then the project won't compile, which is bad.

--
Mats