os_check.cpp

#include <iostream>

using std::cout;
using std::string;

int main() {
    // Uses C++11 features.
    string os{};

    // Detect operating system.
    #if defined(__linux__)
        os = "Linux";
        #if defined(__ANDROID__)
            os = "Android";
        #endif
    #elif defined(_WIN32)
        os = "Windows";
    #elif defined(__APPLE__)
        // Detect iOS before macOS (__MACH__ is also defined for iOS)
        #if defined(IPHONE)
            os = "iOS";
        #elif defined(__MACH__)
            os = "macOS";
        #endif
    #elif defined(__EMSCRIPTEN__)
        os = "Emscripten";
    #else
        os = "Unable to determine operating system";
    #endif

    cout << os;
    return 0;
}