C ++如何使用派生对象中的独立基础对象

Question

I have 3 classes. One is a base, one is a parent and one is a child. The parent is instantiated at some point and assigned data. In some cases it is used directly but in others there is a need for functionality implemented in its child class.

What I would like to do is to cast the parent object into child object so its virtual functions would be used with parent's data:

class UndoBase : public QUndoCommand
{
public:
    UndoBase() {}    

    virtual void undo() = 0;
    virtual void redo() = 0;
};

class CommandParent : public UndoBase
{
public:
    CommandParent() {}

    virtual void undo()
    virtual void redo()

protected:
    someType data
};

class CommandChild : public CommandParent
{
public:
    CommandChild() {}

    virtual void undo() //overrides parent
    virtual void redo() //overrides parent
};

I have tried this:

CommandParent *parent = new CommandParent;
//assign data
UndoBase *command = static_cast<CommandChild*>(parent);
command->redo();

And it worked (used the CommandChild implementation and CommandParent data) although I would swear it should be undefined behaviour (that could work too, I know). Since I doubt this is the right thing to do I would like to ask if this is ok or if there is other ways beside composition.

Answer 1

UndoBase *command = static_cast<CommandChild*>(parent);

This is all hell at every inch of this line. You are telling to static_cast to cast to CommandChild whereas you are storing it in UndoBase .

Also you are trying to cast CommandParent to CommandChild means Base to Derived where Base class has no knowledge of derived cast. Even if you write it properly.

CommandChild  *child = static_cast<CommandChild*>(parent);

it would be erroneous.Apart from that always use dynamic_cast for these cases. It explicitly checks whether cast is possible or not.dynamic_cast will cast properly if cast is safe OR returns NULL (in case of pointers, for references it throws bad_cast exception ) if its not able to cast to target type.

Answer 2

您必须使用复制构造函数使用CommandParent中的数据创建CommandChild的新实例。

UndoBase *command = new CommandChild(*parent);