Question

Is the type check a mere integer comparison? Or would it make sense to have a GetTypeId virtual function to distinguishing which would make it an integer comparison?

(Just don’t want things to be a string comparison on the class names)

EDIT: What I mean is, if I’m often expecting the wrong type, would it make sense to use something like:

struct Token
{
    enum {
        AND,
        OR,
        IF
    };
    virtual std::size_t GetTokenId() = 0;
};

struct AndToken : public Token
{
    std::size_t GetTokenId() { return AND; }
};

And use the GetTokenId member instead of relying on dynamic_cast.

Answer 1

The functionality of the dynamic_cast goes far beyond a simple type check. If it was just a type check, it would be very easy to implement (something like what you have in your original post).

In addition to type checking, dynamic_cast can perform casts to void * and hierarchical cross-casts. These kinds of casts conceptually require some ability to traverse class hierarchy in both directions (up and down). The data structures needed to support such casts are more complicated than a mere scalar type id. The information the dynamic_cast is using is a part of RTTI.

Trying to describe it here would be counterproductive. I used to have a good link that described one possible implementation of RTTI… will try to find it.