使用 Base Class 对象调用 Dervied Class 函数

Question

我正在尝试绘制 CMFCRibbonTab 一个略微不同的图，其中文本的位置稍微高一点。

为了实现这一点，我创建了一个名为 DCRibbonTab 的派生 class，它扩展了 CMFCRibbonTab 并重写了 virtual void OnDraw(CDC * pDC) 函数，如下所示：

#pragma once
#include "StdAfx.h"

class DCRibbonTab : public CMFCRibbonTab
{
public:
    DCRibbonTab ();

    virtual ~DCRibbonTab ();
    virtual void OnDraw(CDC* pDC);
};

DCRibbonTab::~DCRibbonTab()
{
}

void DCRibbonTab::OnDraw(CDC * pDC)
{
    CMFCRibbonTab::OnDraw(pDC);
    ASSERT_VALID(this);
    ASSERT_VALID(m_pParent);
    ASSERT_VALID(m_pParent->GetParentRibbonBar());

    if (m_rect.IsRectEmpty())
    {
        return;
    }

    COLORREF clrText = CMFCVisualManager::GetInstance()->OnDrawRibbonCategoryTab(pDC, this, m_pParent->IsActive() || GetDroppedDown() != NULL);
    COLORREF clrTextOld = pDC->SetTextColor(clrText);

    CRect rectTab = m_rect;
    CRect rectTabText = m_rect;

    pDC->DrawText(L"test", rectTabText, DT_CALCRECT | DT_SINGLELINE | DT_VCENTER);

    const int cxTabText = rectTabText.Width();
    const int cxTabTextMargin = max(4, (rectTab.Width() - cxTabText) / 2);

    rectTab.DeflateRect(cxTabTextMargin, 0);
    //rectTab.top += nPanelMarginTop;

    pDC->DrawText(GetParentCategory()->GetName(), rectTab, DT_SINGLELINE | DT_VCENTER);
    pDC->SetTextColor(clrTextOld);
}

为了能够调用 DCRibbonTab 的新 virtual void OnDraw(CDC * pDC)，我必须重写 CMFCRibbonBar Class 的 void OnPaint()，如下所示：

void DCRibbonBar::OnPaint()
{
    CMFCRibbonBar::OnPaint();

    CPaintDC dc(this); // device context for painting

    int i = 0;

    CMemDC memDC(dc, this);
    CDC* pDC = &memDC.GetDC();

    for (i = 0; i < (int)m_arCategories.GetSize(); i++)
    {
        CMFCRibbonCategory* pCategory = m_arCategories[i];
        ASSERT_VALID(pCategory);

        if (pCategory->IsVisible())
        {
            CMFCRibbonTab * CurrentTab = pCategory->GetTab();

            //Need to Call DCRibbonTab OnDraw function using CMFCRibbonTab object CurrentTab

        }
    }
}

但是，我似乎无法实现的是调用派生的 class DCRibbonTab virtual void OnDraw(CDC * pDC) 使用基础 class 对象，其中CMFCRibbonTab 对象。

那么，如何从基础 class 对象 CMFCRibbonTab 调用派生 class 函数 DCRibbonTab？

Answer 1

class DCRibbonTab : public CMFCRibbonTab
{
public:
    DCRibbonTab ();

    virtual ~DCRibbonTab ();
    void OnDraw(CDC* pDC) override;
};

如果您的 OnDraw() 方法版本实际上并未覆盖基础 class 中定义的方法，则此版本应提供明确的错误消息。如果您看到该错误，您就知道您需要仔细检查您是否与基 class 中的函数签名完全匹配，当然还要确认该函数在基 'virtual' 中声明。

如果添加 'override' 后没有出现错误，那么调用正确的版本应该很简单：

CurrentTab->OnDraw(pDC);

Answer 2

经过一些研究，我找到了解决方案，使用函数重载和运行时多态性。这是解决方案的源代码：

class DCRibbonTab : public CMFCRibbonTab
{
public:
    DCRibbonTab();
    virtual ~DCRibbonTab();
    void OnDraw(CDC* pDC, BOOL Redraw);
};

static const int nPanelMarginTop = 3;
// DCRibbonTab

DCRibbonTab:: DCRibbonTab ()
{
}

DCRibbonTab::~ DCRibbonTab ()
{
}

void DCRibbonTab::OnDraw(CDC* pDC, BOOL Redraw)
{
    ASSERT_VALID(this);
    ASSERT_VALID(pDC);
    ASSERT_VALID(m_pParent);
    ASSERT_VALID(m_pParent->GetParentRibbonBar());

    if (m_rect.IsRectEmpty())
    {
        return;
    }

    COLORREF clrText = CMFCVisualManager::GetInstance()->OnDrawRibbonCategoryTab(pDC, this, m_pParent->IsActive() || GetDroppedDown() != NULL);
    COLORREF clrTextOld = pDC->SetTextColor(clrText);

    CRect rectTab = m_rect;
    CRect rectTabText = m_rect;

    pDC->DrawText(m_pParent->GetName(), rectTabText, DT_CALCRECT | DT_SINGLELINE | DT_TOP);

    const int cxTabText = rectTabText.Width();
    const int cxTabTextMargin = max(4, (rectTab.Width() - cxTabText) / 2);

    rectTab.DeflateRect(cxTabTextMargin, 0);
    rectTab.top += nPanelMarginTop;

    pDC->DrawText(m_pParent->GetName(), rectTab, DT_SINGLELINE | DT_TOP);
    pDC->SetTextColor(clrTextOld);
}

void DCRibbonBar::OnPaint()
{
    CPaintDC dc(this); // device context for painting

    int i = 0;

    CMemDC memDC(dc, this);
    CDC* pDC = &memDC.GetDC();

    CRect rectClip;
    dc.GetClipBox(rectClip);

    CRgn rgnClip;

    if (!rectClip.IsRectEmpty())
    {
        rgnClip.CreateRectRgnIndirect(rectClip);
        pDC->SelectClipRgn(&rgnClip);
    }

    pDC->SetBkMode(TRANSPARENT);

    CRect rectClient;
    GetClientRect(rectClient);

    OnFillBackground(pDC, rectClient);

    CFont* pOldFont = pDC->SelectObject(GetFont());
    ENSURE(pOldFont != NULL);
    // Draw tabs:
    CRect rectTabs = rectClient;
    rectTabs.top = m_rectCaption.IsRectEmpty() ? rectClient.top : m_rectCaption.bottom;
    rectTabs.bottom = rectTabs.top + m_nTabsHeight;

    COLORREF clrTextTabs = CMFCVisualManager::GetInstance()->OnDrawRibbonTabsFrame(pDC, this, rectTabs);

    for (i = 0; i < (int)m_arCategories.GetSize(); i++)
    {
        CMFCRibbonCategory* pCategory = m_arCategories[i];
        ASSERT_VALID(pCategory);

        if (pCategory->IsVisible())
        {
            DCRibbonTab *TempVnRibbonTab = new DCRibbonTab ();
            CMFCRibbonTab * TempMFCRibbonTab = pCategory->GetTab();
            TempDCRibbonTab = (DCRibbonTab*)TempMFCRibbonTab;
            TempDCRibbonTab->OnDraw(pDC, TRUE);
        }
    }
    // Draw elements on right of tabs:
    COLORREF clrTextOld = (COLORREF)-1;
    if (clrTextTabs != (COLORREF)-1)
    {
        clrTextOld = pDC->SetTextColor(clrTextTabs);
    }
    pDC->SelectObject(pOldFont);
    pDC->SelectClipRgn(NULL);
}

使用 Base Class 对象调用 Dervied Class 函数

Call Dervied Class function Using Base Class object

c++

mfc

ribbon