5c4ca604   
   From: s.gesemann@googlemail.com   
      
   On Mar 14, 12:25 pm, Bobby wrote:   
   > [...] type X [...] an "incomplete type" [...]   
   >   
   > But why then is it possible (both with g++ and VC) to   
   > define a function returning a X?   
      
   [...]   
      
   > class X {   
   > public:   
   >     X() {}   
   >     X foo(); // <-- why does this compile?   
   > private:   
   >     // X x_; // x_ has incomplete type   
   >     std::string s_;   
   > };   
      
   First of all, this is a DECLARATION, not a DEFINITION. If you just   
   declare a function, the return type and parameter types don't need to   
   be complete. But they need to be complete where you want to invoke or   
   DEFINE it -- usually.   
      
   However, this does not apply to member function definitions that are   
   inside a class definition with respect to the class type. You can   
   think of   
      
      class X {   
      public:   
          X(int v) : member(v) {}   
          X foo() {return X(member+1);};   
      private:   
          int member;   
      };   
      
   just being a shortcut syntax for   
      
      class X {   
      public:   
          X(int v);   
          X foo();   
      private:   
          int member;   
      };   
      
      // now X is complete :-)   
      
      inline X::X(int v) : member(v) {}   
      
      inline X X::foo() {return X(member+1);};   
      
   With this in mind, you see why referencing 'member' in X::X and X::foo   
   and creating an X object in X::foo actually works.   
      
   Cheers!   
   SG   
      
      
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