Why mandatory to include stdafx.h at the start? - c++

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)

Answer 1

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.