precompiled header file

Hello everyone,


Here is my precompiled header settings,

--------------------
Create/Use PCH Through File stdafx.h
PreCompiled Header File .\Debug/client.pch
--------------------

1.

It means all the content before stdafx.h will be precompiled into client.pch? So that each time compiler "assumes" the content before stfafx.h will not be changed. Right?

2.

How will compiler solve this situation?

Code:

---

// in file foo.c
#include "foo.h"
#include "stdafx.h"

// in file goo.c
#include "goo.h"
#include "stdafx.h"

---

Compiler will pre-compile foo.h, goo.h and stdafx.h into one client.pch file? Or pre-compile foo.h + stdafx.h into one file, and pre-compile goo.h and stdafx.h into another file?


thanks in advance,
George

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

Thanks IfYouSaySo,


Great link, cool!!

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?

Here is the link from MSDN, and I quote related information from this link. :-)

http://msdn2.microsoft.com/en-us/library/7zc28563.aspx

--------------------
If you use /Ycfilename, the compiler compiles all code up to and including the specified file for subsequent use with the /Yu option.
--------------------

Quote:

---

Originally Posted by IfYouSaySo

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

---

regards,
George

Quote:

---

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.

Thanks Elysia,

Quote:

---

Originally Posted by Elysia

No, that is incorrect. Only the contents of stdafx.h is precompiled.

---

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


Hi CornedBee,

Quote:

---

Originally Posted by CornedBee

Code:

---

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

---

Compile again with
cl.exe source.cpp

---

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?


regards,
George

Quote:

---

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.

It is simple. In any source file in a project that uses precompiled header, you must include the precompiled header "stdafx.h" in VC++. If you do not, then your program will not compile however hard you try.

Hi Elysia,


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

This is what you said,

--------------------
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.
--------------------

Quote:

---

Originally Posted by Elysia

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.

---

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?

Quote:

---

Originally Posted by Raigne

It is simple. In any source file in a project that uses precompiled header, you must include the precompiled header "stdafx.h" in VC++. If you do not, then your program will not compile however hard you try.

---

regards,
George

Quote:

---

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.)

Quote:

---

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.

Quote:

---

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.

Hi Mats,


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

Quote:

---

Originally Posted by matsp

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

---

Thanks CornedBee,


I have checked you are correct. The content of stdafx.h is only one line, #pragma once, and the content of stdafx.cpp is only one line, #include "stdafx.h".

My questions,

1. Why do we need to include an empty include file stdafx.h? For what purpose?

2. Do we always need to include it (stdafx.h)?

3. I do not explicitly add stdafx.cpp into my project, do you mean it is added to each project implicitly?

4. How is stdafx.pch generated? If it is generated from an empty stdafx.h, does it have any practical usage?

Quote:

---

Originally Posted by CornedBee

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.

---

regards,
George

Quote:

---

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

Hi Mats,


To me stdafx.h and stdafx.cpp are both empty, can you post yours please?

Quote:

---

Originally Posted by matsp

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

---

regards,
George

Stdafx.h:

Code:

---

// stdafx.h : include file for standard system include files,
// or project specific include files that are used frequently, but
// are changed infrequently
//

#pragma once

// Modify the following defines if you have to target a platform prior to the ones specified below.
// Refer to MSDN for the latest info on corresponding values for different platforms.
#ifndef WINVER                                // Allow use of features specific to Windows XP or later.
#define WINVER 0x0501                // Change this to the appropriate value to target other versions of Windows.
#endif

#ifndef _WIN32_WINNT                // Allow use of features specific to Windows XP or later.                 
#define _WIN32_WINNT 0x0501        // Change this to the appropriate value to target other versions of Windows.
#endif                                               

#ifndef _WIN32_WINDOWS                // Allow use of features specific to Windows 98 or later.
#define _WIN32_WINDOWS 0x0410 // Change this to the appropriate value to target Windows Me or later.
#endif

#ifndef _WIN32_IE                        // Allow use of features specific to IE 6.0 or later.
#define _WIN32_IE 0x0600        // Change this to the appropriate value to target other versions of IE.
#endif

#define _HAS_TR1 1

#include <stdio.h>
#include <tchar.h>
#define _ATL_CSTRING_EXPLICIT_CONSTRUCTORS        // some CString constructors will be explicit

#ifndef VC_EXTRALEAN
#define VC_EXTRALEAN                // Exclude rarely-used stuff from Windows headers
#endif

#include <afx.h>
#include <afxwin.h>        // MFC core and standard components
#include <afxext.h>        // MFC extensions
#ifndef _AFX_NO_OLE_SUPPORT
#include <afxdtctl.h>                // MFC support for Internet Explorer 4 Common Controls
#endif
#ifndef _AFX_NO_AFXCMN_SUPPORT
#include <afxcmn.h>                        // MFC support for Windows Common Controls
#endif // _AFX_NO_AFXCMN_SUPPORT

#include <afxmt.h>
#include <vector>
#include <map>
#include <fstream>
#include <iostream>
#include <cstdlib>
#include <iomanip>
#include <ctime>
#include <cmath>
#include <string>
#include <list>
#include <string>
#include <new>
//#include <stdint.h>
#include <sstream>
#include <Stuff\Requirements.h>

// TODO: reference additional headers your program requires here

---

Stdafx.cpp:

Code:

---

// stdafx.cpp : source file that includes just the standard includes
// Help.pch will be the pre-compiled header
// stdafx.obj will contain the pre-compiled type information

#include "stdafx.h"

// TODO: reference any additional headers you need in STDAFX.H
// and not in this file

---

Far from empty if you ask me.

Quote:

---

Originally Posted by George2

Hi Mats,


To me stdafx.h and stdafx.cpp are both empty, can you post yours please?




regards,
George

---

Considering I work with an embedded compiler that is NOT Microsoft compatible, the answer is no.

In my hobby projects, I don't use precompiled headers for the very reason that I want to have all the includes needed for the project clearly visible in the C(++)-file, and not hidden inside some other file.

If both stdafx.cpp and stdafx.h are empty, then precompiled headers make absolutely no sense at all.

--
Mats

Thanks Mats,

Quote:

---

Originally Posted by matsp

In my hobby projects, I don't use precompiled headers for the very reason that I want to have all the includes needed for the project clearly visible in the C(++)-file, and not hidden inside some other file.
--
Mats

---

Why using pre-compiled header file will make code not convenient to read? I think even if we use /Yu in compile option, when reading code, the inlcude files are still in the source file, using /Yu or not only impact compiler's optimization, there is no code change in source file, why do you think using pre-compiled header file will make code not clear to read?


Thanks Mats,


"turn off" you mean turn off pre-compiled header creation (/Yc) or turn off pre-compiled header using (/Yu)?

Quote:

---

Originally Posted by matsp

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

---

Thanks Elysia,


This conflicts with CornedBes's point in post #13, where he said the stdafx.h is empty. Any comments?

(BTW: I did search for stdafx.h and stdafx.cpp in my VC folder, and some of them are empty and some of them are not.)

Quote:

---

Originally Posted by Elysia

Stdafx.h:

Code:

---

// stdafx.h : include file for standard system include files,
// or project specific include files that are used frequently, but
// are changed infrequently
//

#pragma once

...

---

Far from empty if you ask me.

---

Thanks CornedBee,


I think whether or not pre-compiled header is used is decided by the compiler option /Yu, why do you think it is decided by whether the related header file is empty or not -- "the compiler sees the include of the empty stdafx.h as a directive to use stdafx.pch"

Quote:

---

Originally Posted by CornedBee

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.

---

regards,
George

Quote:

---

Originally Posted by George2

Thanks Elysia,

This conflicts with CornedBes's point in post #13, where he said the stdafx.h is empty. Any comments?

(BTW: I did search for stdafx.h and stdafx.cpp in my VC folder, and some of them are empty and some of them are not.)

---

I'm a little fuzzy on the full details on how PCH works, so I don't know much about that,

Thanks all the same, Elysia!


Let us wait and learn from CornedBee and Mats. PCH is really fuzzy sometimes, and even you guru guys are sometimes having conflicting points. :-)

Quote:

---

Originally Posted by Elysia

I'm a little fuzzy on the full details on how PCH works, so I don't know much about that,

---

regards,
George