C++ function callback

Question

This a continuation of my concern in this post: How do I get the dimensions (resolution) of each display?

I wanted to contains the solution inside a class. But it's retaining this error upon compilation:

error C2065: 'MonitorEnumProc' : undeclared identifier.

ScreenManager::ScreenManager() {
     BOOL monitorInitialized = EnumDisplayMonitors( NULL, NULL, MonitorEnumProc, reinterpret_cast<LPARAM>(this) );
}

BOOL CALLBACK MonitorEnumProc(  HMONITOR hMonitor,
                                HDC hdcMonitor,
                                LPRECT lprcMonitor,
                                LPARAM dwData ) {

    reinterpret_cast<ScreenManager*>(dwData)->callback(hMonitor,hdcMonitor,lprcMonitor);
    return true;
}

bool ScreenManager::callback(HMONITOR hMonitor, HDC hdcMonitor, LPRECT lprcMonitor) {

    screenCounter++;

    MONITORINFO  info;
    info.cbSize =  sizeof(MONITORINFO);

    BOOL monitorInfo = GetMonitorInfo(hMonitor,&info);

    if( monitorInfo ) {
        std::vector<int> currentScreenVector;
        currentScreenVector.push_back( screenCounter );
        currentScreenVector.push_back( abs(info.rcMonitor.left - info.rcMonitor.right) );
        currentScreenVector.push_back( abs(info.rcMonitor.top - info.rcMonitor.bottom) );
        screenVector.push_back( currentScreenVector );
    }

    return true;
}

Thanks in advance!

Answer 1

At the point you call EnumDisplayMonitors, the compiler doesn't know that MonitorEnumProc exists. You have two options:

Change the order of these two functions, so MonitorEnumProc comes first:

BOOL CALLBACK MonitorEnumProc(  HMONITOR hMonitor,
                                HDC hdcMonitor,
                                LPRECT lprcMonitor,
                                LPARAM dwData ) {

    reinterpret_cast<ScreenManager*>(dwData)->callback(hMonitor,hdcMonitor,lprcMonitor);
    return true;
}

ScreenManager::ScreenManager() {
     BOOL monitorInitialized = EnumDisplayMonitors( NULL, NULL, MonitorEnumProc, reinterpret_cast<LPARAM>(this) );
}

Or,

Add a forward declaration before you try to reference MonitorEnumProc:

BOOL CALLBACK MonitorEnumProc(  HMONITOR hMonitor,
                                HDC hdcMonitor,
                                LPRECT lprcMonitor,
                                LPARAM dwData );

ScreenManager::ScreenManager() {
     BOOL monitorInitialized = EnumDisplayMonitors( NULL, NULL, MonitorEnumProc, reinterpret_cast<LPARAM>(this) );
}

BOOL CALLBACK MonitorEnumProc(  HMONITOR hMonitor,
                                HDC hdcMonitor,
                                LPRECT lprcMonitor,
                                LPARAM dwData ) {

    reinterpret_cast<ScreenManager*>(dwData)->callback(hMonitor,hdcMonitor,lprcMonitor);
    return true;
}