继承类的组成

Question

There is probably already a question about this but I couldn't find it.

I'm sure there is a simple solution but I can't see it.

Let's say you have the following:

class FooBase
{
protected:
    BarBase myInstance;
}

class FooChild : public FooBase
{
protected:
    BarChild myInstance;
}

class BarChild : public BarBase
{
}

So basically, you want to use the derived version of BarBase in the FooChild class.

I think that in the above code, there will be 2 different instances of myInstance (one BarBase and one BarChild).

I would like to have only one instance so that I can use BarBase in FooBase and I can use BarChild in FooChild.

Is there an elegant solution?

Answer 1

Can you try something like this so that only one instance is created.

#include <iostream>

class BarBase
{
}
;
class BarChild : public BarBase
{
}
;

class FooBase
{
public:
  FooBase(BarBase* baseBar);
protected:
  BarBase* myInstance;
}
;


class FooChild : public FooBase
{
public:
  FooChild(BarBase* base)
;
}
;


FooBase::FooBase(BarBase* baseBar)
{
  myInstance = baseBar;
}

FooChild::FooChild(BarBase* childBar) : FooBase(childBar)
{

}

int main()
{
  BarChild *bc = new BarChild();
  FooChild fc(bc);
}

I don't know either this is the right solution or it's elegant one ;)

Answer 2

Are templates an option?

tempalate <typename T = BarBase>
class FooBase{
protected:
    T myInstance;
};

class FooChild : public FooBase<BarChild> {};

If I use int and double to test this works as desired using only one member variable: http://ideone.com/nOfR5x

It does however require declarations of FooBase to be followed by angle brackets.

Answer 3

Virtualization is what you're looking for i believe. Give it a look. You can simply use a Constructor Method which creates those variables. If you create a derived class, then it calls that class's constructor, which creates that variable.