Inheritance / Return type

**_Elixia_** · 06-21-2003

Say I have a class like this:

Code:

class Ta {
private:
// some class-local vars here
public:
   void *GetFromStore (int bucket, int id);
// other functions
}

This is the base class for several other classes that need to store data in this type of way. However, each other class uses a different type of pointer, and so has different return types.

Instead of using casting throughout the program, I overload the function and change the return type:

Code:

class Tb : Ta {
public:
   char *GetFromStore (int bucket, int id);
}

The class is then used as follows:

Code:

   Tb cvar;
   char *p;

   p = cvar.GetFromStore (0, 1);

Where the function GetFromStore would be defined in .cpp file as:

Code:

char * Tb::GetFromStore (int bucket, int id)
{
   return ((char *)Ta::GetFromStore (bucket, id));
}

Now, is this the best of dealing with it? I'm suspect not, and think there must be an easier/better way of doing this or calling the previous function (before the overloaded function took over it's name).

Looking through a few of my C++ books simply says that overloading and just changing the return type is invalid and can't be done, but the above compiles ok with no errors or warnings. If I place code in the Ta class to return a value, then the main code received that value, so the compiler is obviously interpreting it correctly, but it just seems so ugly that there must be a better way of doing this.

**Prelude** · 06-21-2003

>Now, is this the best of dealing with it?
Far from it. If you find yourself using casts, then you're design is broken or you neglected to use a feature of C++ that lets you avoid casting. In this case you have a base class function that needs to be overloaded, but the type of the return value will change with the derived classes. This strikes me as a good time to employ class templates:

Code:

#include <iostream>

template <class T>
class Ta {
public:
    virtual T* getData() = 0;
};

template <class T>
class Tb: public Ta<T> {
    T data;
public:
    Tb(T init = T()): data(init) {}
    T* getData()
    {
        return &data; // Just to test
    }
};

int main()
{
    Tb<int> myT(10);

    std::cout<< *myT.getData() <<std::endl;
    
    std::cin.get();
}

**Zach L.** · 06-21-2003

Templates are a much more type-safe way (void* should usually be avoided where possible). Here is an example:

Code:

template<typename T>
class Ta
{
public:
   virtual T* GetFromStore (int bucket, int id);
};

class Tb : Ta<char>
{
public:
   char* GetFromStore (int bucket, int id);
};

(Also note I made the function virtual).

The main drawback is that Ta<char> and Ta<int> (for example) are now different types.

**_Elixia_** · 06-21-2003

Thanks, I think that'll do the job nicely.

However, how come Zach uses "template<typename T>" and Prelude uses "template <class T>", but they both seem to do exactly the same thing?

Also, I assume the "= 0" on the function definition means the function must be overloaded in a derived class or the compilation will fail?

**Zach L.** · 06-21-2003

>> However, how come Zach uses "template<typename T>" and Prelude uses "template <class T>", but they both seem to do exactly the same thing?

They do. I like typename, though most people use class.

>> Also, I assume the "= 0" on the function definition means the function must be overloaded in a derived class or the compilation will fail?

Yep

**Prelude** · 06-21-2003

>but they both seem to do exactly the same thing?
They do. I prefer to use class where the parameter may not be a built-in type, and typename where it must be. Of course, this is all a matter of style.

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