For a system I need to convert a pointer to a long then the long back to the pointer type. As you can guess this is very unsafe. What I wanted to do is use dynamic_cast to do the conversion so if I mixed them I'll get a null pointer. This page says http://publib.boulder.ibm.com/infocenter/lnxpcomp/v7v91/index.jsp?topic=/com.ibm.vacpp7l.doc/language/ref/clrc05keyword_dynamic_cast.htm
The dynamic_cast operator performs
type conversions at run time. The
dynamic_cast operator guarantees the
conversion of a pointer to a base
class to a pointer to a derived class,
or the conversion of an lvalue
referring to a base class to a
reference to a derived class. A
program can thereby use a class
hierarchy safely. This operator and
the typeid operator provide run-time
type information (RTTI) support in
C++.
and I'd like to get an error if it's null so I wrote my own dynamic cast
template<class T, class T2> T mydynamic_cast(T2 p)
{
assert(dynamic_cast<T>(p));
return reinterpret_cast<T>(p);
}
With MSVC I get the error "error C2681: 'long' : invalid expression type for dynamic_cast". It turns out this will only work with classes which have virtual functions… WTF! I know the point of a dynamic cast was for the up/down casting inheritance problem but I also thought it was to solve the type cast problem dynamically. I know I could use reinterpret_cast but that doesn't guarantee the same type of safety.
What should I use to check if my typecast are the same type? I could compare the two typeid but I would have a problem when I want to typecast a derived to its base. So how can I solve this?
Best Answer
I've had to do similar things when loading C++ DLLs in apps written in languages that only support a C interface. Here is a solution that will give you an immediate error if an unexpected object type was passed in. This can make things much easier to diagnose when something goes wrong.
The trick is that every class that you pass out as a handle has to inherit from a common base class.