namespaces, classes and free functions - when do you need fully qualified names

Question

In my example below, why do I have to fully qualify the name of the free function in the cpp to avoid linker errors and why does it work for the class function without? Can you

Answer 1

int FreeFunction(void);  

is just a declaration whereas the below is a definition.

class CTest 
{ 
    public: 
        CTest(); 
        ~CTest(); 
}; 

If you want to provide definition for an already declared entity in a namespace (e.g. in an enclosing namespace), it has to be fully qualified name.

EDIT2:

Here is something that would give you some more clarity. Note no using directive in this code.

namespace Test { 
    int FreeFunction(void);   // declare

    class CTest;              // declare
} 

int Test::FreeFunction(){return 0;} // define
class Test::CTest{            // define
};

int main(){}

EDIT 3: Declaration vs Definition (C++0x) $3.1/2-

A declaration is a definition unless it declares a function without specifying the function’s body (8.4), it contains the extern specifier (7.1.1) or a linkage-specification25 (7.5) and neither an initializer nor a function-body, it declares a static data member in a class definition (9.4), it is a class name declaration (9.1), it is an opaque-enum-declaration (7.2), or it is a typedef declaration (7.1.3), a using-declaration (7.3.3), a static_assert-declaration (Clause 7), an attribute-declaration (Clause 7), an empty-declaration (Clause 7), or a using-directive (7.3.4).