C ++：C struct的继承或类成员？

Question

I have a plain old C struct :

struct eggplant {
    double *ptr1;
    double *ptr2;
    ... // many more
}

and a function that manages the allocation the memory pointed to by the pointers, and returns a new eggplant instance:

struct eggplant *create_eggplant(int n);

The above function allocates a chunk of memory (including space for the newly created struct) and distributes it to the pointers.

---

I want to extend struct eggplant in C++11. I could do that by keeping a pointer to the struct as a

member:

class EggPlant {
    ...
    private :
        struct eggplant *plant;
}

or I could try via

inheritance:

class EggPlant : private struct eggplant {
    ...    
}

The first option allows me to use the create_eggplant function. However, the second option looks more straightforward from a conceptual point of view (the class EggPlant is an eggplant, it doesn't have one).

I tried

this = create_eggplant(...);

in the constructor but that does not work for the obvious reason that you cannot overwrite a pointer to a class that you are constructing ( lvalue required ).

Can I inherit the struct but still use my create_eggplant function in some useful way? Or, is it anyway better to keep a pointer to the struct?

Answer 1

Your choice to have struct eggplant *create_eggplant(int n); manage its own memory is in conflict with C++ inheritance. Inheritance also implies management of the location of the base object.

If you changed your C function to return a copy:

struct eggplant create_eggplant(int n);

You could also inherit from that class:

class EggPlant : private eggplant {
    EggPlant(int n) : eggplant{ create_eggplant(n) }
    {
    }
    ...    
};

Answer 2

No, you would need a new function if you wish to inherit. If you must use the existing function a member is the best you will be able to manage.

Answer 3

 struct eggplant *create_eggplant(int n); 

Should be just:

eggplant *create_eggplant(int n);

This isn't C, you don't have to type struct everywhere.

However, the second option looks more straightforward from a conceptual point of view (the class EggPlant is an eggplant, it doesn't have one).

That's wrong. Only public inheritance models is-a . Private inheritance models has-a or is-implemented-in-terms-of .

Can I inherit the struct but still use my create_eggplant function in some useful way?

No, not in a useful way, especially as your description of create_eggplant sounds like malloc is used in it.

Or, is it anyway better to keep a pointer to the struct?

No, pointers make code more complicated.

If you want to use private inheritance, then either add a constructor to eggplant that more or less does what create_eggplant does now, except of allocating memory for the eggplant itself, or modify create_eggplant to become something like void init_eggplant(eggplant& e) , then do this in the constructor of EggPlant :

EggPlant::EggPlant() :
    eggplant()
{
    init_eggplant(*this);
}

Or just don't use private inheritance but a normal data member:

class EggPlant
{
    // ...
private:
    eggplant my_eggplant;
}

As you can see, the common goal of all solutions is to get rid of the dynamic memory allocation for eggplant .