Question

Take two classes that both can only be created by new. One class is the base and the other is a derivative. The derived class only adds methods.

class Base
{};

class Derived : public Base
{};

Base * b = new Base{}
Derived * d = covert( b );

// - or -

Base * b = new Base{};
convert( b ); // converts Base to Derived
Derived * d = dynamic_cast<Derived *>(b);

What I want to do is take the Base class data that has been allocated and extend/wrap with the derivative via some method or function, convert.

Update:
Building for embedded systems memory is scarce, so I am doing everything I can to reduce that amount of memory allocation. I was just wondering if there was a way to just sort of extend the base class already allocated memory and wrap it with the derivative.

More Updates:
Although the embedded system is ARM and I am currently using LLVM compiler this might not be true in the future. So a standard compliant way is preferred.

Answer 1

If I understand correctly your question one possible solution is to use aggregation instead of inheritance.

class Base
{/*has only the data*/}

class Derived 
{
    Base &base;
    Derived(Base &b) : base(b) {}
    //now methods from derived will use the data members from instance passed in constructor
    //if is possible the Derived needs to be a friend class of Base in case there are no getter for all members
}

Instead of reference we can use a smart pointer if necessary.
This way you avoid the cast by constructing a new Derived object which uses the data from the a Base object.