forward declare a type previously declared as a class as a struct

Question

Is there any interest to forward a class as a struct and vice versa ? It seems to be perfectly legal, but some use cases exist ?

struct Bar;
class Foo
{
Bar * bar_;
};

class Bar
{

};

Answer 1

There isn't a use case any more or less than there is a use case for forward declaring with the same keyword. It makes no difference to the meaning of the program. The class-key identifier only makes a difference when defining the class.

The above applies to standard compliant compilers. Some non compliant ones might handle the declarations differently in which case there is a case for using the same keyword in particular.

---

Ok, here is a practical use case. Let's say you've implemented a class with the struct keyword. Over time, the class is widely used across multiple code bases and it is declared in many headers using the same keyword. At a later time, perhaps after adding a ton of features, you decide that classwould be more appropriate and you refactor the code. Now, there isn't a point in refactoring all the unrelated depending code bases to use the new keyword.