Best Working

void GetDesktopResolution(int& width, int& height, int& posX, int& posY)
{
	int index = 0;
	DISPLAY_DEVICE dd;
	dd.cb = sizeof(DISPLAY_DEVICE);

	bool secondDisplay = false;

	while (EnumDisplayDevices(NULL, index++, &dd, 0))
	{
		if (dd.StateFlags & DISPLAY_DEVICE_ATTACHED_TO_DESKTOP) {
			DEVMODE mode;
			mode.dmSize = sizeof(mode);
			EnumDisplaySettings(dd.DeviceName, ENUM_CURRENT_SETTINGS, &mode);

			width = mode.dmPelsWidth;
			height = mode.dmPelsHeight;
			posX = mode.dmPosition.x;
			posY = mode.dmPosition.y;

			if (dd.StateFlags & DISPLAY_DEVICE_PRIMARY_DEVICE) printf("* ");
			cout << "Device Name: " << dd.DeviceName << "; Device String: " << dd.DeviceString << endl;
			cout << "5: Width: " << width << "; height: " << height << '\n';

		}
	}
}

C++ Get Desktop Resolution

void GetDesktopResolution1(int& width, int& height)
{
	RECT desktop;
	const HWND hDesktop = GetDesktopWindow();
	GetWindowRect(hDesktop, &desktop);

	width = desktop.right;
	height = desktop.bottom;

        cout << "1: Width: " << width << "; height: " << height << '\n';
}

void GetDesktopResolution2(int& width, int& height)
{
	HMONITOR hMon = MonitorFromPoint({ 0, 0 }, MONITOR_DEFAULTTOPRIMARY);
	MONITORINFO monInfo;
	monInfo.cbSize = sizeof monInfo;

	GetMonitorInfo(hMon, &monInfo);

	width = monInfo.rcMonitor.right - monInfo.rcMonitor.left;
	height = monInfo.rcMonitor.bottom - monInfo.rcMonitor.top;

	cout << "2: Width: " << width << "; height: " << height << '\n';
}

void GetDesktopResolution3(int& width, int& height)
{
	width = GetSystemMetrics(SM_CXSCREEN);
	height = GetSystemMetrics(SM_CYSCREEN);

	cout << "3: Width: " << width << "; height: " << height << '\n';
}

void GetDesktopResolution4(int& width, int& height)
{
	RECT rc;
	SystemParametersInfo(SPI_GETWORKAREA, 0, &rc, 0);
	
	width = rc.right - rc.left;
	height = rc.bottom - rc.top;

	cout << "4: Width: " << width << "; height: " << height << '\n';
}

// Only function that returns correct resolution for primary monitor
void GetDesktopResolution5(int& width, int& height)
{
	DEVMODE mode;
	mode.dmSize = sizeof(mode);
	EnumDisplaySettings(NULL, ENUM_CURRENT_SETTINGS, &mode);

	width = mode.dmPelsWidth;
	height = mode.dmPelsHeight;

	cout << "5: Width: " << width << "; height: " << height << '\n';
}

struct MonitorInfo {
	unsigned monitor = 0;
	unsigned primary = 0;
	unsigned index = 0;
	unsigned width;
	unsigned height;
	string displayName;
};

static BOOL CALLBACK 
MonitorEnumProc(HMONITOR hMonitor, HDC hdcMonitor, LPRECT lprcMonitor, LPARAM dwData) {
	MonitorInfo& info = *(MonitorInfo*) dwData;
	MONITORINFOEX mi;
	memset(&mi, 0, sizeof(MONITORINFOEX));
	mi.cbSize = sizeof(MONITORINFOEX);

	GetMonitorInfo(hMonitor, &mi);
	string displayName = (const char*)mi.szDevice;
	if (displayName.find(R"(\\.\DISPLAYV)") == 0) return TRUE;  //ignore pseudo-monitors
	if (mi.dwFlags & MONITORINFOF_PRIMARY) info.primary = info.index;
	if (info.monitor == info.index) {
		info.width = lprcMonitor->right - lprcMonitor->left;
		info.height = lprcMonitor->bottom - lprcMonitor->top;
		info.displayName = displayName;
	}

	info.index++;
	return TRUE;
}

void GetDesktopResolution6(unsigned monitor) {
	MonitorInfo info;
	info.monitor = monitor;

	EnumDisplayMonitors(NULL, NULL, MonitorEnumProc, (LPARAM)&info);

	cout << "Name:" << info.displayName << "; Width: " << info.width << "; height " << info.height << endl;
}
void ListDisplayDevices()
{
	int index = 0;
	DISPLAY_DEVICE dd;
	dd.cb = sizeof(DISPLAY_DEVICE);

	while (EnumDisplayDevices(NULL, index++, &dd, 0))
	{
		if (dd.StateFlags & DISPLAY_DEVICE_PRIMARY_DEVICE) printf("* ");
		cout << "Device Name: " << dd.DeviceName << "; Device String: " << dd.DeviceString << endl;
	}
}

void ListDisplaySettings(int index)
{
	DISPLAY_DEVICE dd;
	dd.cb = sizeof(DISPLAY_DEVICE);
	if (!EnumDisplayDevices(NULL, index, &dd, 0))
	{
        	cerr << "EnumDisplayDevices failed:" << GetLastError() << endl;
         	return;
	}

	DISPLAY_DEVICE monitor;
	monitor.cb = sizeof(DISPLAY_DEVICE);
	if (!EnumDisplayDevices(dd.DeviceName, index, &monitor, 0))
	{
		pcerr << "EnumDisplayDevices failed:" << GetLastError() << endl;
		return;
	}

	DEVMODE dm;
	dm.dmSize = sizeof(DEVMODE);

	if (!EnumDisplaySettings(dd.DeviceName, ENUM_CURRENT_SETTINGS, &dm))
	{
		cerr << "EnumDisplaySettings failed" << GetLastError() << endl;
		return;
	}

	cout << "Device name:" << dd.DeviceName << endl;
	cout << "Monitor name:" << monitor.DeviceID << endl;
	cout << "Refresh rate, in hertz:" <<  dm.dmDisplayFrequency << endl;
	cout << "Color depth: %d\n" << dm.dmBitsPerPel << endl;
	cout << "Screen resolution, in pixels:" << dm.dmPelsWidth << " x " << dm.dmPelsHeight << endl;
}