How can I check if the monitor is connected in Windows / Linux?

edit

Here's the solution for Linux. You need to use the Xlib library:

#include <X11/Xlib.h>

int main(int argc, char *argv[])
{
    bool hasgui=true;
    Display *display;
    display=XOpenDisplay(NULL);
    if(!display)
        hasgui=false;
    else XCloseDisplay(display);
    //... your code here...
    if(hasgui) imshow("Yipee", image");
    else imwrite( "output.jpg", image );
    return 0;
}

You need to install xlib11-xcb-dev package (on ubuntu/debian) and link your program to libx11 (-lx11)

I don't know for Windows, if it can run without graphical interface (even if there is no screen attached) - but probably there is no simple access to low-level features. Maybe you can try to query screen resolution or something similar to see if there's a screen. I have no idea how to deal with this case.

The following version for Windows uses a global variable to keep track of the number of monitors (real and virtual):

#include <windows.h>
#include <iostream>
using namespace std;

size_t monitor_count = 0;

BOOL monitor_enum_proc(HMONITOR Arg1, HDC Arg2, LPRECT Arg3, LPARAM Arg4)
{
    monitor_count++;
    return TRUE;
}

int main(void)
{
    EnumDisplayMonitors(NULL, NULL, monitor_enum_proc, NULL);
    cout << monitor_count << endl;

    return 0;
}

... or if you abhor global variables, then you can use the following version instead. It isn't intentionally obfuscated, it's just plain old pointer use. I've included comments regarding this pointer use:

#include <windows.h>
#include <iostream>
using namespace std;

BOOL monitor_enum_proc(HMONITOR Arg1, HDC Arg2, LPRECT Arg3, LPARAM Arg4)
{
    // Regarding pointer use and the fourth parameter:
    // 1) Reinterpret the LPARAM as a size_t*
    // 2) Dereference the size_t* to get the variable
    // 3) Add one to the value of the variable
    (*reinterpret_cast<size_t *>(Arg4))++;

    return TRUE;
}

int main(void)
{
    size_t monitor_count = 0;

    // Regarding pointer use and the fourth parameter:
    // 1) Get address of variable
    // 2) Reinterpret the size_t* as an LPARAM
    EnumDisplayMonitors(NULL, NULL, monitor_enum_proc, reinterpret_cast<LPARAM>(&monitor_count));

    cout << monitor_count << endl;

    return 0;
}

In Linux you should be able to check the display availability by retrieving the environment variable DISPLAY

#if __linux__
    has_display = !(std::getenv("DISPLAY") == nullptr);
#endif