#ifdef _cplusplus

**Chandana** · 02-09-2010

Hi,

I have the following code - FIRST.CPP, CALL.C and stdafx.h

FIRST.CPP:

Code:

   #include "stdafx.h"
  extern "C" void callmethod();

  int _tmain(int argc, _TCHAR* argv[])
 {
	callmethod();
	return 0;
 }
 void justcall(){
    #ifdef  __cplusplus
	    printf("This is C++");
    #else
        printf("This is C code");
    #endif;
 }

CALL.C

Code:

void callmethod(){
	justcall();
}

stdafx.h:

Code:

#include <stdio.h>
#include <tchar.h>

extern "C" void justcall();

The output when I run FIRST.CPP is "This is C++".
From FIRST.CPP, I call callmethod() which is defined in a .c file and from this callmethod() I call justcall() which is in a .cpp file.

My question is justcall() is called from a C program then why is the code inside #ifdef _cplusplus getting executed?

Thanks,
Chandana.

**laserlight** · 02-09-2010

Originally Posted by Chandana

My question is justcall() is called from a C program then why is the code inside #ifdef _cplusplus getting executed?

Because of the context of where that function was compiled, it was compiled as C++, even though it is called from a C function.

**Chandana** · 02-09-2010

Thanks for the reply.

what should I do if I want to get "This is C code" as the output.

**laserlight** · 02-09-2010

What are you actually trying to do?

After all, taken out of the context in which you asked it, your question is dumb, since the answer is: print "This is C code" unconditionally

**Elysia** · 02-09-2010

The #ifdef is to determine the capabilities of the compiler compiling the code, so to speak. Since in executable form, it doesn't matter if it's compiled using C++ or C.
The ifdef should be used as a means of knowing whether a C function or a C++ function calls the said function!

**brewbuck** · 02-09-2010

The use of _cplusplus in a .cpp file is redundant. This is not how it is normally used. It's normally used from within header files, which are typically named .h whether they are C or C++, and even then, its normal purpose is to turn off name mangling in order to allow C code to make calls to C++ code.

The reason it prints "This is C++" is because the code is C++. The fact that it's being called from a C function is irrelevant.

**grumpy** · 02-10-2010

Originally Posted by Chandana

what should I do if I want to get "This is C code" as the output.

Compile your code with a C compiler that is not a C++ compiler.

Some compilers have dual hats: depending on command line options or other settings, they compile code as C or as C++. With those compilers, you need to pick the right mode (eg via command line settings). As brewbuck noted, some compilers compile as C if the file has a .c extension, and as C++ if the file has a .cpp (or .cc) extension.

#ifdef _cplusplus is not checked at run time. It is checked as one of the phases during compilation (specifically, by the preprocessor).

**_Mike** · 02-10-2010

what should I do if I want to get "This is C code" as the output.

Since you have stdafx.h I'm assuming you are using visual c++ (If any other compiler also uses this header file then ignore this)
Right-click the file(s) you want to compile as C code and select properties.
Then go to "C/C++" -> "Advanced" -> "Compile As" and select "Compile as C Code"
(or just rename the file to something.c)

**Elysia** · 02-10-2010

Bad idea.
Compiling .cpp files as C for some reason will serve to confuse and break compatibility with other compilers. Do the right thing: name it with a .c extension instead. It will compile fine in VS and other compilers.

**CornedBee** · 02-10-2010

It won't, since it contains C++ constructs; specifically, the 'extern "C"'.

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