A compile-time symbol was #defined but #ifdef does not see it - c++

Question

I have a test class declared in a header file and defined in a separate file. The class must compile differently under Windows, so i use #if defined ( _WINDOWS_ ). When I compile the cpp file which also contains #if defined ( _WINDOWS_ ), the file is compiled as if the symbol _WINDOWS_ was not defined, although it is defined in another file. When I compile the code, I am getting the following error:
Error Code : error lnk2019 unresolved external symbol public
source code
// test.h
class Test
{
public:
#if defined (_WINDOWS_)
void printwindow();
#endif
void notwindows();
};
//test.cpp
#include "test.h"
#if defined (_WINDOWS_)
void Test::printwindow()
{
cout << "i am windows ";
}
#endif
void test::notwindows()
{
cout << " not windows " ;
}
//main.cpp
#include "windows.h"
#include "test.h"
void main()
{
test t1 ;
t1.printwindow() // OK I have declared function so my _WINDOWS_ is available but when i run it i get
}
Error Code : error lnk2019 unresolved external symbol public
NOTE : if i define the function directly it works without any problem
// test.h
class Test
{
public:
#if defined (_WINDOWS_)
void printwindow(){couT << "i am window" }
#endif
void notwindows();
};
but I don't like this method. I prefer to define them in separate files (h and cpp).

Answer 1

It would be better for you to use _WIN32 instead of _WINDOWS_ for your conditional compilation test. _WINDOWS_ is defined only if windows.h has been included, while the compiler automatically defines _WIN32 for any build for a Windows target, regardless of what headers are included. In your situation, _WINDOWS_ is defined when you compile main.cpp but not when you compile test.cpp becuase test.cpp doesn't include windows.h.
Also, the _WINDOWS_ macro definition is an implementation detail of the windows.h header, and is not guaranteed to be used. For example, the MinGW version of windows.h does not define _WINDOWS_.

Answer 2

You need to include windows.h in your test.cpp file
When you compile test.cpp it does not have the _WINDOWS_ symbol defined. So it creates an object file without the windows function.
Then in test.hpp it uses the symbol because you included windows.h in main.cpp
If you change the order of the includes it will behave differentially. You should rethink the way how you want to select the windows version which does not depend on the order of includes (for example see the other answer about _WIN32_).
You can not change the code in an already compiled object file (test.cpp) using some define in another code file (main.cpp)