Question

I have following classes: Container, Element and then couple of classes that inherit from Element, eg. Button, Input, etc …

I have a problem when adding the elements to Container array, my main() looks like this:

Container c;
c.Add( Button(...) );
c.Add( Input(...) );

where “…” are some constructor parameters.

In the container class I have a array of pointers to store all elements that belong to that container:

Element ** elements;

But the problem I’m having is how to implement the Add method, I was hoping something like this would work:

void Add(const CControl & newElement){                      
    elements[elemCnt++] = &newElement;              
}

(the elements array is allocated: elements = new Element * [100];)

However I am getting this compilation error:

main.cpp: In member function ‘Container& Container::Add(const Element&)’:
main.cpp:138:23: error: invalid conversion from ‘const Element*’ to ‘Element*’

When I remove the const qualifier, I get a compilation error saying there is no suitable candidate.

The thing is, I am new to polymorphism and inheritance in C++, so I might be going wrong way about this. What would be the best approach on this?

PS: The main method must look the same, also don’t suggest any vector or STL stuff.

Answer 1

Add should take a pointer:

void Add(CControl * newElement){                       
    elements[elemCnt++] = newElement;               
} 

then you can call it like this

c.Add( new Button(...) ); 
c.Add( new Input(...) ); 

If you really cannot change the calling code, you need to somehow create a copy of the temporary.
E.g. by implementing a virtual Clone method in CControl, Input, Button an call it in Add.

void Add(const CControl & newElement){                        
    elements[elemCnt++] = newElement.Clone();
}