Error cause by inheritance

I tried the following code but i couldnt figure out why does the error occurs. Can someone let me know whats wrong with this code ??

Code:

---

class Expression {
 public:
        Expression() { }
        virtual ~Expression() { }
};


class aaa : public Expression{
  public:
//test3.cpp:18: error: syntax error before `*' token
//test3.cpp:18: error: missing ';' before right brace
          aaa(bbb *x,Expression *y){ }
         
  private:
//test3.cpp:20: error: semicolon missing after declaration of `aaa'
                bbb *lh;
        Expression *rh;
};
//test3.cpp:23: error: syntax error before `}' token
class bbb : public Expression{
  public:
          bbb(){ }


};

---

its because aaa has no clue what bbb does so your creating a pointer of a class which it has no clue of

But i thought i can create a bbb object and pass it to the aaa constructor ???

Change the order of declaration:

Code:

---

class Expression {
 public:
        Expression() { }
        virtual ~Expression() { }
};

class bbb : public Expression{
  public:
          bbb(){ }
};

class aaa : public Expression{
  public:
        aaa(bbb *x,Expression *y){ }
         
  private:
        bbb *lh;
        Expression *rh;
};

---

Or add a forward declaration:

Code:

---

class Expression {
 public:
        Expression() { }
        virtual ~Expression() { }
};

class bbb;

class aaa : public Expression{
  public:
        aaa(bbb *x,Expression *y){ }
         
  private:
        bbb *lh;
        Expression *rh;
};

class bbb : public Expression{
  public:
          bbb(){ }
};

---

Either solution allows the compiler to know what bbb is when it is compiling aaa.