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_.