using #if to run a function - c++

Question

I'm trying to run the expression _setmode only if I'm using Windows, and setlocale only if I'm using Linux, but I can't manage to make them work with a simple if-else inside a function due to Linux having errors with the Windows libraries, and vice versa.
#if defined(_WIN32) || defined(_WIN64) || (defined(__CYGWIN__) && !defined(_WIN32))
#define PLATFORM_NAME 0
#include <io.h>
#include <fcntl.h>
#elif defined(__linux__)
#define PLATFORM_NAME 1
#include <locale>
#elif defined(__APPLE__) && defined(__MACH__)
#include <TargetConditionals.h>
#if TARGET_OS_MAC == 1
#define PLATFORM_NAME 2
#endif
#else
#define PLATFORM_NAME NULL
#endif
#if PLATFORM_NAME == 0
_setmode(_fileno(stdout), _O_U8TEXT);
#endif
#if PLATFORM_NAME == 1
setlocale(LC_CTYPE, "");
#endif

Answer 1

If you write OS-dependent* code (as in this case), you can manage it at compile-time**. To do this, we need two parts:
Define OS-dependent constants (optional, if condition simple, this part can be omitted):
#if defined(_WIN32)
#define PLATFORM_NAME 0
#include <fcntl.h>
#include <io.h>
#elif defined(__linux__)
#define PLATFORM_NAME 1
#include <locale>
#endif
In needed place call OS-dependent code with preprocessor conditions:
#if PLATFORM_NAME == 0
//Windows code here
#endif
You can write more complex conditions:
#if PLATFORM_NAME == 0
//Windows code here
#elif PLATFORM_NAME != 0
//Non-Windows code here
#if PLATFORM_NAME == 1 || PLATFORM_NAME == 2
//Linux or unknown OS code here
#endif
#endif
See restrictions of conditions here
Tip: If your code have entrypoint (main function as example), you can call most of OS-dependent code at main if this help to reduce code. In library you can place OS-dependent code to dedicated source-file functions like there. Usage preprocessor-time code is good method for writing zero-cost runtime code because of preprocessor just remove all sources,if they're not met conditions.
* - or whatever-dependent 😃
** - more precisely, preprocessor-time
Source: GNU, Microsoft docs.