Declaration Vs. Definition for classes

Question

I have a quick question about class declaration vs. class definition.

So, I can think of four different scenarios of class code:

Which ones qualify as declarations and which as definitions? Why?

TIA.

1)

class foo_empty{};

2,3)

class foo_NoMemDef{
   int f1(); //Member function declaration(?), no body
  //int i; (consider including this as a separate case 3)
};

4)

class foo_final{
   int f2(){}
};

Answer 1

Which ones qualify as declarations and which as definitions?

All examples show class definitions (that are also declarations). A class declaration that is not a definition looks like this:

class Foo;

Once you "open/close" a class by { and }, you are providing a definition.