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change "class Derived; friend Derived;" to "friend class Derived;". will compile

code will not compile,
though we are creating base class object (Base b) inside derived class, we cant access the private member 'a' of the base class inside the derived class.
'b.a' inside the derived class is the problem in code. 'a' is private member of base class.

the code will not and should not compile due to:

1. a being private and being accessed by privately derived b
2. friend declaration should error as class key is not there

it will not compile since base object b tries to access its private member 'a'.

It can be compiled after the following changes (I tried it):
1. change "class Derived; friend Derived;" to "friend class Derived;"
2. move "friend class Derived;" to the first line of class Base

Here class derived is declared within base which means derived is a nested class within base.
The class defined outside is a different class with the same name(of course to confuse). I hope the code given below will help you
class Base{
int a;
public:
int b;
Base(){a = 10; b = 11;}
virtual ~Base(){};
class Derived;
friend Derived;
};
class Base::Derived{
public:
//Derived(){cout<<b<<endl;}
void p(Base b){cout<<b.a<<endl;};
virtual ~Derived(){};
};



int main(){
Base b;
Base::Derived d;
d.p(b);
return 0;
}

As per the standards, there should be a elaborated type (containing class or struct keywords)in the friend declaration.
That's why "friend Derived" will not work.
The solution given by Ashish is incorrect (please try it on g++). Also you changed the inheritance to containment which will change the original problem definition.

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