Question

what’s wrong with this code?

class matrix{ 
private:  
   friend transpose(const matrix&);  
   friend class invert;  
   public: //...
};  
matrix (*p)(const matrix&)=&transpose; //error: no transpose() in scope.

what does the statement means “a friend declaration does not introduce a name into enclosing scope”.This problem does not occur when friend keyword is removed

Answer 1

The difference between the declaration of transpose() as a friend and without the friend declaration is that if you declare “friend transpose()” all you are doing is telling the compiler that a function friend with the signature shown in the friend declaration can have access to the private members of an object of type matrix. It does not declare a function transpose() with this signature – you still have to do this outside the scope of the matrix class.

If you remove the ‘friend’ keyword, you are declaring a member function transpose() inside the class matrix, so the compiler actually has seen a function it can take the address of.