Question

I want to know why cant we create object if the constructor is in private section. I know that if i make a method static i can call that method using

<classname> :: <methodname(...)>;

But why can’t we create object is what I don’t understand.

I also know if my method is not static then also I can call function by the following:

class A
{
     A();
     public:
        void fun1();
        void fun2();
        void fun3();
};


int main()
{
     A *obj =(A*)malloc(sizeof(A));
     //Here we can't use new A() because constructor is in private 
     //but we can use malloc with it, but it will not call the constructor
     //and hence it is harmful because object may not be in usable state.
     obj->fun1();
     obj->fun2();
     obj->fun3();
}

So, my question is: why can’t we create an object when constructor is private?

Answer 1

Because it is not accessible to the program, that’s what private means. If you declared a member function or variable private, you would not be able to access them either. Creating private constructors is actually a useful technique in C++, as it allows you to say that only specific classes can create instances of the type. For example:

class A {
   A() {} // private ctor
   friend class B;
};

class B {
   public:
     A * MakeA() {
        return new A;
     }
};

Only B can create A objects – this is useful when implementing the factory pattern.