Multiple Instances of a class deriving from on instance of the SuperClass

[SnowBlind]

Hi,

I have a program with a class (lets call it class A) that has very large data members. I also have another class (class B) that derives its data members from the data members in class A. I will also need more than one instant of class B. Does any body know of a way for multiple objects of class B to derive from one object of class A?

I thought about public inheritance, but every time I create an object of class B, an object of class A will be created also. I cannot afford to create more than one class A.

Any help is appreciated.

[kempofighter]

I thought about public inheritance, but every time I create an object of class B, an object of class A will be created also. I cannot afford to create more than one class A.

Then you need to redesign class A. Are there parts of class A that do not need to be duplicated for every object? Then you need to identify those parts and create a different class which the base class instance can have a pointer to. Derived classes always construct an instance of their base classes. The base class can also have static members but static member functions cannot be virtual.

[moorecm]

Exactly my thoughts, as well. Consider using composition, rather than inheritance, to model a "has a", rather than an "is a", relationship. To limit the instances of the member, you could make it static or use the Singleton pattern.

[Galik]

Its difficult to know without having more details. From what I understand of what you said it might be worth you considering havinf both A and B derive from a common base class.

Something a bit like this:

class D
{
protected:
    size_t id;
    std::string name;
    std::string bar_code;
};

class A
: public D
{
    double average;
    int sign;
    // ... etc... 
    // ... etc... 
    // ... etc... 
};

class B
: public D
{
    double qty;
};

[Galik]

Alternatively you could group the common elements into a struct and keep one in each class.

A bit like this:

struct std_info
{
protected:
    size_t id;
    std::string name;
    std::string bar_code;
};

class A
{
    std_info inf;
    double average;
    int sign;
    // ... etc... 
    // ... etc... 
    // ... etc... 
};

class B
{
    std_info inf;
    double qty;
};

[SnowBlind]

Thanks Guys that helps a lot.