你给的代码报错form2.cpp:126:13: Out-of-line definition of 'mousePressEvent' does not match any declaration in 'Form2'

这个错误提示是因为在 form2.cpp 文件中重新定义了 mousePressEvent 函数，但是其声明与 Form2 类中的声明不一致，因此编译器无法匹配。解决方案是在 Form2 类中声明该函数，并将其设为 virtual：

class Form2 : public QWidget
{
    Q_OBJECT

public:
    explicit Form2(QWidget *parent = nullptr);
    virtual ~Form2();

protected:
    virtual void mousePressEvent(QMouseEvent *event) override;
    virtual void mouseMoveEvent(QMouseEvent *event) override;

private:
    QPoint m_pressPos;
    QPoint m_windowPos;
};

然后在 form2.cpp 文件中重新定义该函数：

void Form2::mousePressEvent(QMouseEvent *event)
{
    m_pressPos = event->pos();
    m_windowPos = this->pos();
}

void Form2::mouseMoveEvent(QMouseEvent *event)
{
    if (event->buttons() & Qt::LeftButton)
    {
        QPoint delta = event->pos() - m_pressPos;
        this->move(m_windowPos + delta);
    }
}

这样就可以实现拖动无边框窗口的功能了。