为什么在用Win32 GDI绘图时需要将句柄保存到旧的位图? [英] Why do I need to save handle to an old bitmap while drawing with Win32 GDI?

问题描述

以下是我以WndProc函数中的开关代码为例:

Here is the code from switch in WndProc function I've been given as an example:

case WM_PAINT:
 hdc = BeginPaint(hWnd, &ps);
 // Create a system memory device context.
 bmHDC = CreateCompatibleDC(hdc);
// Hook up the bitmap to the bmHDC.
 oldBM = (HBITMAP)SelectObject(bmHDC, ghBitMap);
 // Now copy the pixels from the bitmap bmHDC has selected
 // to the pixels from the client area hdc has selected.
 BitBlt(
 hdc, // Destination DC.
 0, // 'left' coordinate of destination rectangle.
 0, // 'top' coordinate of destination rectangle.
 bmWidth, // 'right' coordinate of destination rectangle.
 bmHeight, // 'bottom' coordinate of destination rectangle.
 bmHDC, // Bitmap source DC.
 0, // 'left' coordinate of source rectangle.
 0, // 'top' coordinate of source rectangle.
 SRCCOPY); // Copy the source pixels directly
 // to the destination pixels
 // Select the originally loaded bitmap.
 SelectObject(bmHDC, oldBM);
 // Delete the system memory device context.
 DeleteDC(bmHDC);
 EndPaint(hWnd, &ps);
 return 0; 

我的问题是，为什么必须在oldBM中保存和恢复SelectObject()的返回值?

My question is why is it necessary to save and restore the return value of SelectObject() in oldBM?

推荐答案

为什么必须在oldBM中保存和恢复SelectObject()的返回值?

why is it necessary to save and restore the return value of SelectObject() in oldBM?

BeginPaint()为您提供了一个 HDC ，它已经选择了默认的 HBITMAP .然后，用您自己的 HBITMAP 替换它.您没有分配原始的 HBITMAP ，也不拥有它， BeginPaint()分配了它.使用 HDC 完成操作后，必须还原原始的 HBITMAP ，以便 EndPaint()可以在销毁 HDC ，否则它将被泄漏.

BeginPaint() gives you an HDC that already has a default HBITMAP selected into it. You are then replacing that with your own HBITMAP. You did not allocate the original HBITMAP and do not own it, BeginPaint() allocated it. You must restore the original HBITMAP when you are done using the HDC so that EndPaint() can free it when destroying the HDC, otherwise it will be leaked.

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