关于类CRect＆Rect，Width = right - left

It is a problem about C++ and mfc. For example, left = 3, right = 8. Doesn't it mean there are 6 pixel from left to right? Why the width = right - left? If I know a rect which represents the image rect, when I allocate memory for the image data, which one should I use? Width = right-left, or Width = right-left+1? I am a beginner of image process. It really confuses me. Thank you for your help!It is a problem about C++ and mfc. For example,