Why mandatory to include stdafx.h at the start?

Question

I have this extremely simple main function

#include "stdafx.h"
#include "abc.h"

int _tmain(int argc, _TCHAR* argv[])
{
    abc obj;
    obj.show();
    return 0;
}

Everything is compiling normally...but when i am writing

#include "abc.h" 
#include "stdafx.h"


    int _tmain(int argc, _TCHAR* argv[])
    {
        abc obj;
        obj.show();
        return 0;
    }

The compiler is going haywire..

error C2065: 'abc' : undeclared identifier

error C2146: syntax error : missing ';' before identifier 'obj'

error C2065: 'obj' : undeclared identifier

error C2228: left of '.show' must have class/struct/union

type is ''unknown-type''

Why is it mandatory to include

stdafx.h

at the start? I am new in C++ ...maybe I am making a silly mistake. Please help :(

(Using: VS2005 with C++ 98)

You should include stdafx.h directy in your abc.h if you requires declarations from it. At least, if the include order matters, it's a sign there is something wrong with the content of the .h files themselves. — Bregalad
Its mandatory to include stdafx.h in every .cpp file but it has to be mentioned on the top of all the other .h includes......why is that?? thats my question... :( — DeepN
@Bregalad No, no, no. stdafx.h is related to precompiled headers mechanism in VS. It has to be 1st ... because it has (it uses precompiler magic) =). It's not a dependency. One read, second read. — luk32

display101 display101 · Accepted Answer · 2015-03-27T10:09:36

The issue that you're seeing is the fact that MS Visual C++ uses a feature called precompiled headers by default (other compilers on other platforms have a similar feature, I recall GCC for example having this feature). This ensures that all the code referenced up to the point of the precompiled header is pre-compiled, thus the compiler has less work to do at compilation time.

What happened when you switched that around was that it assumed that the contents of "abc.h" were already pre-compiled when they actually weren't. One easy solution would be to put #include "abc.h" inside stdafx.h. See here for a more details explanation of how the stdafx.h header works.

The precompiled headers option can be easily turned off in the compiler options. There should be a "precompiled headers/precompilation" category in the options, was in earlier Visual C++ IDE's I've encountered.

Why mandatory to include stdafx.h at the start?

1 Answers