Why must a method be declared in a C++ class definition?

Question

If only define this method it will have a compiler error.

void classA::testMethod() {    
}

And so it must be declared first:

class classA {    
   void testMethod();
};

Why should these be declared?

I know for common C method methods it is not needed declare, but they can just be defined:

void test() {
}

Answer 1

You don't need to declare the method before you define it, but you need to declare class methods in the class. Else it wouldn't be a class method.

That may seem like a contradiction, but a definition is also a declaration. So what this means is that the definition may appear in the class itself:

class A {
  void testMethod() { /*...*/ } 
};

[edit] Also, practically speaking, inside the class declaration there are private, protected and public parts. This is needed to for encapsulation. If you could declare methods outside the class, you would lose encapsulation. Anybody could access private members merely by defining extra getters and setters, even when those would not make sense. Class invariants would become meaningless.