Using #define to check operating system

Hi again, got a new one here. Kind of surprised I've never hit up against this before.

I'd like to be able to specify a #define for the OS used in the build. That way, in various included header files, I can specify different versions of functions which contain OS-dependent code. Here's a non-working example of what I'd like the code to look like, though I suspect that this kind of thing is impossible using #if checks, even if I were to cloak them in macros. Is there another way?

Code:

---

//This is what I would like the code to resemble.

#include <iostream>
#include <string>

using std::cout;
using std::cerr;
using std::endl;
using std::string;

int OS_Table(const string& str)
{
        if(str == "windows")
                return 0;
       
        else if(str == "mac")
                return 1;
       
        else if(str == "*nix")
                return 2;
}

#define OS_BUILD OS_Table("windows")

#if OS_BUILD == OS_Table("windows")
        void Function() { cout << "Windows" << endl; }
#elif OS_BUILD == OS_Table("mac")
        void Function() { cout << "Mac" << endl; }
#elif OS_BUILD == OS_Table("*nix")
        void Function() { cout << "*nix variant" << endl; }
#else
        void Function() { cerr << "Invalid operating system." << endl; }
#endif

int main()
{
        Function();
        return 0;
}

---

You have to determine the OS based on preprocessor symbols defined by the compiler. If you're primarily worried about just windows, mac and linux, you can usually do something like:

Code:

---

#define PLATFORM_WIN32 1
#define PLATFORM_APPLE 2
#define PLATFORM_LINUX 3

#if defined(__WIN32__) || defined(_WIN32)
#define PLATFORM PLATFORM_WIN32

#elif defined(__APPLE_CC__)
#define PLATFORM PLATFORM_APPLE

#else
#define PLATFORM PLATFORM_LINUX

#endif

---

Which you'd then use like so:

Code:

---

std::string platformName()
{
#if PLATFORM == PLATFORM_WIN32
  return "Win32";

#elif PLATFORM == PLATFORM_APPLE
  return "Mac";

#else
  return "Linux";

#endif
}

---

Os_table only returns something at run-time, but (#if condition) requires a condition at compile time!

Code:

---

#ifdef MY_WINDOWS_DEF
        void Function() { cout << "Windows" << endl; }

#elif defined(MY_MAC_DEF)
        void Function() { cout << "Mac" << endl; }

#elif defined(MY_NIX_DEF)
        void Function() { cout << "*nix variant" << endl; }

#else
        void Function() { cerr << "Invalid operating system." << endl; }
#endif

---

where one of MY_WINDOWS_DEF, MY_MAC_DEF and MY_NIX_DEF needs to be #defined.