Re: Downcasting base-class objects to a derived-class

"Daniel T." <>
Sat, 06 Dec 2008 13:48:22 -0500
In article
 vsk <> wrote:

In my AP Comp. class, we wrote a Symbolic Algebra program in Java that
is completely based on one interface: IExpression.
I want to port my Java code to C++, for experience, and I'm having a
few issues.

C++ doesn't (to my knowledge) have an equivalent of an Interface, so

class IExpression {
        IExpression() {};
        virtual bool hasVar() ;
        virtual double eval(double);
        virtual string getStr();
        virtual string getSmart();
        virtual bool equals(IExpression&);
        virtual IExpression simplify();
        virtual IExpression derivative();

Once the "interface" or base-class was done, I wanted to implement it
with a simple class from my project: Number;

class Number : public virtual IExpression {
        double value;
        void init();
        bool equals(Number &that);

        bool hasVar();
        double eval(double);
        string getStr();
        string getSmart();
        bool equals(IExpression&);
        IExpression simplify();
        IExpression derivative();

I wrote the implementation of Number's methods in the header, and I
wont bother posting (most of) them.
The one that's giving me hell is;

bool Number::equals(IExpression &that) {
    if (typeid(this) == typeid(that)) {
        return this->equals(reinterpret_cast<Number&> (that));
    } else {
        return false;

bool Number::equals(Number &that) {
    return this->value == that.value;

C++ has given me arcane error messages, and I don't know what I'm
doing that's so horribly incorrect.
I think it's a down-casting problem in equals(), but it's also telling
me that I have an "undefined reference to vtable".

How can I fix this?

1) IExpression, in all probability, needs a virtual destructor.

2) As it stands, every member-function (method) in IExpression needs to
be defined. If you don't want to define them (because this is an
interface after all) you need to put "=0" between the ')' and the ';'.
As in:
   virtual bool hasVar() = 0;

3) 'simplify()' and 'derivative()' both return IExpressions by value,
that is probably wrong. If these function are supposed to return some
sub-type of IExpression, they need to be returning by pointer or
reference. In that case, be careful not to return a pointer/reference to
a temporary variable.

4) Number::equals() should use dynamic_cast, since you mention Java, you
can think of it like this:

   // Java
   boolean result = false;
   Number n = (Number)that;
   if (n != null)
      result = equals(n);
   return result;

   // C++
   bool result = false;
   Number* n = dynamic_cast<Number*>(&that);
   if (n)
      result = equals(*n);
   return result;

The above are the outright errors in the code. Design issues include

1) const correctness

2) The fact that you are using a cast in the first place. After all,
according to the above, the number '5' is not equal to the expression '2
+ 3'. Is that really what you want?

3) C++ and Java have very different philosophies, especially when it
comes to object creation a straight port probably isn't wise. You would
learn more by attempting to re-implement the behavior of the Java
program in C++ without trying to directly port the code.

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