c++ - 虚拟基类的虚拟继承和转换

Question

我遇到了一个关于虚拟继承的具体问题。下面的代码解释了结构。

// Header

struct ITarget
{
    virtual void SetValue() = 0;
};

struct ISource
{
    virtual void GetValue() const = 0;
};

struct IBase
{
    virtual void Stuff() = 0;
};

struct IInput :
    virtual public IBase, public ITarget
{
    // Publicly acts only as Target
};

struct IOutput :
    virtual public IBase, public ISource
{
    // Publicly acts only as Source
};

// Implementation

struct SBase :
    virtual public IBase, virtual public ISource, virtual public ITarget
{
    // IBase
    virtual void Stuff();

    // ISource
    virtual void GetValue() const;

    // ITarget
    virtual void SetValue( );
};

struct SInput :
    public IInput, public SBase
{
    // Defines ITarget and ISource per SBase
};

struct SOutput :
    public IOutput, public SBase
{
    // Defines ITarget and ISource per SBase
};

int main()
{
  // To demonstrate the behavior
  ISource* input = new SInput();
  ((SInput*)input)->GetValue();
  return 0;
}

现在，如果我想将ISource * 转换为SInput *，我会收到错误 C2635：

“无法将 'ISource*' 转换为 'SInput*'；隐含从虚拟基类的转换”

我究竟该如何设置继承才能使这成为可能？