Question

I’m getting monitor information using EnumDisplayMonitors:

BOOL CALLBACK MonitorEnumProc(HMONITOR hMonitor, HDC hdcMonitor, LPRECT lprcMonitor, LPARAM dwData){
  Class::callback(hMonitor,hdcMonitor,lprcMonitor,dwData);
  return true;
}

bool Class::callback(HMONITOR hMonitor, HDC hdcMonitor, LPRECT lprcMonitor, LPARAM dwData){
  classVar.appendData("callback");
  return true;
}

bool Class::f(){
  ...
  EnumDisplayMonitors(NULL,NULL,MonitorEnumProc,NULL);
  ...
}

Class::callback is static (if it isn’t I get error C2352: illegal call of non-static function). This however causes problems with classVar: error C2228: left of ‘.appendData must have class/struct/union’. What should I be doing here to get around this problem (I want the callback to write data to classVar)?

Answer 1

The last parameter of EnumDisplayMonitors() is an extra pointer reserved for use by the caller. It is passed uninterpreted to the callback function. Pass a pointer to the class instance.

BOOL CALLBACK MonitorEnumProc(HMONITOR hMonitor, HDC hdcMonitor, LPRECT lprcMonitor, LPARAM dwData){
  reinterpret_cast<Class*>(dwData)->callback(hMonitor,hdcMonitor,lprcMonitor);
  return true;
}

bool Class::callback(HMONITOR hMonitor, HDC hdcMonitor, LPRECT lprcMonitor){
  classVar.appendData("callback");
  return true;
}

bool Class::f(){
  ...
  EnumDisplayMonitors(NULL,NULL,MonitorEnumProc,reinterpret_cast<LPARAM>(this));
  ...
}