Question

What is the reason for the following code that does not let me to create object.

class base
{
    public:
        void foo()
        { cout << "base::foo()"; }
};

class derived : private base
{
    public:
        void foo()
        { cout << "deived::foo()"; }
};

void main()
{
    base *d = new derived();
    d->foo();
}

It Gives me error :

” ‘type cast’ : conversion from
‘derived *’ to ‘base *’ exists, but is
inaccessible”

Thanks in advance 🙂

Answer 1

The problem is that you are using private inheritance; this means that inheritance can only be seen inside your class (in this case, derived). You cannot point a base* to a derived instance outside your class (in this case, in main()) because the inheritance (and hence, the conversion) cannot be accessed.

This is exactly the same as trying to access a private member from outside a class.

In fact, the name “private inheritance” is quite misleading, since it does not implement real inheritance. In your example, a derived instance is not a base; it is just implemented in terms of a base, and this is what “private inheritance” means. If you are tempted to use private inheritance, you should consider the possibility of using simple aggregation (i.e.: a private pointer to base inside derived) instead. In most cases (most cases, not always), private inheritance offers no advantages and has some subtle problems.