C ++函数回调

Question

这是我在这篇文章中继续关注的话题： 如何获得每个显示器的尺寸（分辨率）？

我想将解决方案包含在一个类中。 但是在编译时会保留此错误：

错误C2065：“ MonitorEnumProc”：未声明的标识符。

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;
}

提前致谢！

Answer 1

在您调用EnumDisplayMonitors ，编译器不知道MonitorEnumProc存在。 您有两种选择：

更改这两个函数的顺序，因此MonitorEnumProc首先出现：

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) );
}

要么，

在尝试引用MonitorEnumProc之前，添加一个前向声明：

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;
}