Why is protected constructor raising an error this this code?

Question

One question about protected constructor. I learnt that the protected constructor can be used in the derived class. How ever, I found the code below has an error. Why does i

Answer 1

When a base class has a protected constructor, you can't instantiate the class directly. But you can do this to call the constructor from the base class constructor:

class A {

protected:
   A() {}
};

class B: public A {
public:
   B() : A() // allowed to access constructor like this
   {
      A* f = new A(); // Not allowed to access constructor like this!
   }
};

A direct call to the constructor as shown below gives you the following error with gcc version 4.1.2:

      A* f = new A(); // Not allowed to access constructor like this!

test.cpp:4: error: A::A() is protected

However, you this call to the constructor gives no errors:

   B() : A() // allowed to access constructor like this

The reason behind this is that the second call accesses the A() constructor through inheritance, which is allowed. However, this tries to explicitly create a new instance of A() by calling the constructor directly:

      A* f = new A(); // Not allowed to access constructor like this!

This might seem unintuitive, as B should be able to access A's constructor because B inherits from A. However, if you declare a constructor protected in C++, you can't create an instance of that class except through inheritance or a friend relationship.