#include cpp file - how it works?

[Vaclav_]

I am using DirectShow "common" dshowutil.cpp file and I just found this, to me new , usage of #include directive. It only works if included in a source file where its function(s) are used. Makes sense. Elsewhere the linker generates multiple definitions, so it must be included somewhere else, I did not search for it. Here is the #include: #pragma message("Common DirectShow support ") #include "D:\00\0 SDK\DIrectShow 8.1 SDK\samples\Multimedia\DirectShow\Common\dshowutil.cpp" And here is the usage sample: TRACE("\nFind input and output pins and connect them"); IPin * pCapOut = GetOutPin( pDocument->pC_Graph->gcap.pVCap, 0 ); In my view it prepends the working source file. I would like some other explanations from the group. Is such usage of including cpp file common? Thanks for your time. CHeers Vaclav

[Jochen Arndt]

The #include statement is a preprocessor directive. The preprocessor is reading input files, processes his commands, and writes the result to an output file (source file name with extension 'i'). The #include statement just inserts the processed content of another file. When invoking the compiler, the preprocessor is started first for each source file and the created preprocessed file is the input for the compiler. After compilation the file is deleted (the deletion can be omitted by passing a compiler option flag on the command line or setting it in the VS project setting). It is not very common to include other source files but allowed.

[Lost User]

Vaclav_Sal wrote:

In my view it prepends the working source file

But that is perfectly acceptable as long as all required include files are added within that cpp file. It is just the same as adding some extra functions at the top of a source file before the #include statements required by the remainder of the source.

[Vaclav_]

So are the include cpp functions global only to the attched cpp which is basically an object / class? Is that the reason I was getting multiple definitions when I included it in StdAfx header? I did not try to access the preprocessed functions outside the class. It should not work. Actually found that I was telling the DirectShow to add another filter and had no intermediate filter / pins to connect to. Fixed that so I do not need to do the connections manually.

[Lost User]

Without seeing all the code in the included file it is impossible to answer that.

[leon de boer]

You are spot on Val you will get multiple inclusions if the file is multiple included, unless the file contains standard one time load #ifdef protection around the top and bottom If the programmer knew what they were doing they would have something that looks like this top/bottom of include file // PROTECTION TO STOP MULTIPLE UNIT LOADING #ifndef _FILENAME_ID_ #define _FILENAME_ID_ #endif It is still quite common in a very specific circumstance .. can you think what it might be :) I will give you a hint: I know the old version of windows.h from Microsoft did it but they changed it all a few years ago when they broke that unit up. You will probably also find it on the android source unless they worked there way around it.