如下，取 _SYSTEM_ 的值即可。
#include <iostream>
#include <cstdlib>
#include <string>
const int N = 100;
using namespace std;
// system check
#if defined(_WIN32)
#define _SYSTEM_ 1 // Windows
#include<windows.h>
#elif defined(__ANDROID__)
#define _SYSTEM_ 5 // Android
#elif defined(__APPLE__)
#include <TargetConditionals.h>
#if TARGET_IPHONE_SIMULATOR || TARGET_OS_IPHONE
#define _SYSTEM_ 6 // iOS
#else
#define _SYSTEM_ 4 // macOS
#include <termios.h>
#include <unistd.h>
#endif
#elif defined(__linux__)
#define _SYSTEM_ 2 // Linux
#include <termios.h>
#include <unistd.h>
#elif defined(__unix__)
#define _SYSTEM_ 3 // Other Unix (except Apple,Linux&Android)
#include <termios.h>
#include <unistd.h>
#endif
/*
System list
type code
Windows 1
Linux 2
Other Unix 3
MacOS 4
Android 5
IOS 6
Get code from "_SYSTEM_"
*/