vs2017里用mfc写画图功能后，为什么不点击鼠标左键就会开始绘图

Question

跟着老师敲完LBUTTONDOWN,MOUSEMOVE,LBUTTONUP后，本来是想鼠标左键按着不动来画图，但是程序运行时，没点鼠标左键,直接在客户区移动鼠标就开始画线了。
老师用的是vc++6.0,我用的vs2017

void CDrawingboardView::OnPenChoice(UINT id)
{
    // TODO: 在此处添加实现代码.
    switch (id)
    {
    case ID_RED:color = RGB(255, 0, 0); break;
    case ID_GREEN:color = RGB(0, 255, 0); break;
    case ID_BLUE:color = RGB(0, 0, 255); break;
    case ID_YELLOW:color = RGB(255, 255, 0); break;
    case ID_BLACK:color = RGB(0, 0, 0); break;
    }
    pen.DeleteObject();
    pen.CreatePen(0, 1, color);

}

void CDrawingboardView::OnLButtonDown(UINT nFlags, CPoint point)
{
    // TODO: 在此添加消息处理程序代码和/或调用默认值

    spt.x = point.x;
    spt.y = point.y;

    CView::OnLButtonDown(nFlags, point);
}

void  CDrawingboardView::OnMouseMove(UINT nFlags, CPoint point)
{
    // TODO: 在此添加消息处理程序代码和/或调用默认值

    CClientDC dc(this);//定义客户区显示器对象
    dc.SelectObject(&pen);

    ept.x = point.x;
    ept.y = point.y;

    if (spt.x >= 0)
    {
        dc.MoveTo(spt);
        dc.LineTo(ept);

        spt.x = ept.x;
        spt.y = ept.y;
    }

    CView::OnMouseMove(nFlags, point);
}

void CDrawingboardView::OnLButtonUp(UINT nFlags, CPoint point)
{
    // TODO: 在此添加消息处理程序代码和/或调用默认值
    spt.x = -1;

    CView::OnLButtonUp(nFlags, point);
}

Answer 1

spt.x有初始化为-1吗