#if #include

Question

#if #include

Flaviu_ 1,031

Which one is the correct way to do:

#if _MSC_VER > 1900 // VS2017 or above
    #include "pch.h"
#else // <-- Error
    #include "stdafx.h"
#endif

I got: fatal error C1019: unexpected #else

How can I write condition to compile a file for VS2017 or above, and below 2017 ?

David Lowndes 4,726 Reputation points

2021-07-06T18:02:57.06+00:00

It would be wrong to check this based on the compiler version - you can compile an old project with a newer compiler.
RLWA32 49,536 Reputation points

2021-07-06T18:08:48.78+00:00

Agreed. And even though the VS2017 and above wizards might use pch.h the name of the header can always be set by the user in the project's property pages to something else.

5 answers

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David Lowndes 4,726 Reputation points

2021-07-06T18:02:57.06+00:00

It would be wrong to check this based on the compiler version - you can compile an old project with a newer compiler.
RLWA32 49,536 Reputation points

2021-07-06T18:08:48.78+00:00

Agreed. And even though the VS2017 and above wizards might use pch.h the name of the header can always be set by the user in the project's property pages to something else.

Answer 1

This is valid irrelevant of VS version. C++ preprocessor symbols haven't changed in this regard.

I'm guessing at what could be wrong here. Note that I'm also assuming this code is in your .cpp file and not a .h file which would make this even harder to diagnose.

The first thing that comes to mind is "pch.h". The "pch.h" file has a bad preprocessor directive causing the compiler to fail. You can test this by temporarily replacing this #include with a #include of windows.h or something. If it then gets past this error then the issue is inside pch.h.

The second thing is precompiled headers. You are using precompiled headers in project. PCHs work funny in VS. It has been a long time since I've used them but IIRC when you enable PCHs then you specify the PCH file to look for (default is "stdafx.h". The preprocessor has to look for the PCH first so it literally skips all code until it finds the #include "stdafx.h" line. Everything before that is effectively skipped. This causes errors. The workaround historically has been to ensure that the PCH include is the very first line in the .cpp file.

Sam of Simple Samples 5,546 Reputation points

2021-07-06T18:21:13.16+00:00

Yes the PCH include must be the very first line in the .cpp file other than comments. It seems to not like the #if at least. The problem is that beginning with VS 2017 the PCH file is pch.h instead of stdafx.h. As far as I know there is nothing special about those names; I think Microsoft finally decided to be logical. I assume that the name stdafx.h is related to MFC. The name pch.h is more logical. That creates an incompatibility as described in the question.

Answer 2

RLWA32 49,536

Seems to me it has something to do with the way VC++ handles precompiled headers. VS2017 threw up the following warning before compilation failed with C1019.

Answer 3

For VS2017 or above rename the pch.h file to stdafx.h. You will need to change all the .cpp files to use the stdafx.h name when relevant but you will not need any preprocessor conditions. You will need to make the change in the Project Properties under Configuration Properties -> C/C++ -> Precompiled Headers. I tested that in a simple console project using VS2017 and it worked for me.

Depending on the version of VS being used, you will have problems with VS generating new files with the wrong name for the PCH but that can be fixed in a couple of seconds.

Answer 4

Flaviu_ 1,031

I have a MFC control, spread in .h and .cpp file, and have been generated with

#include "pch.h"

Of course, I don't have stdafx.h in my project, but when I used this control in older project, this cpp file request stdafx.h at the first line.

I thought I can write a conditional code to use it without any modification in new projects and old projects as well, but I guess I have to moditfy it accordingly.

David Lowndes 4,726 Reputation points

2021-07-07T08:58:53.25+00:00

When I share code between projects, I usually don't use precompiled headers for the shared files, so one solution would be to alter the project settings for the shared source files and not #include the precompiled header.
ridilculous 1 Reputation point

2022-12-15T13:18:27.353+00:00
You also have to provide a dummy header for this solution which is what i'm usually doing but it still requires manual steps which need to be done for every shared project. Of course one could create libraries to work around even that (or just use stdafx.h everywhere) but the quick and dirty solution would be the preprocessor solution from the OP, just with some other define, f.e.

#ifdef PROJECTA #include "projecta.pch" #else // whatever #endif

and it's a pity that this isn't supported.

Answer 5

ridilculous 1

This isn't possible.

The problem is this:

The compiler treats all code occurring before the .h file as precompiled. It skips to just beyond the #include directive associated with the .h file, uses the code contained in the .pch file, and then compiles all code after filename.

This means that the following is valid (msv)c++:

this isn't valid c++ but valid msvcpp  
#include "stdafx.h"

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#if #include

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