Using #define to check operating system

[sarano]

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?

//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;
}