Can anybody explain to the meaning of this expression?

Question

 void* GetData()
 {
    return reinterpret_cast<unsigned char*>(this);
 }

Is there a case of automatic type coercion happening in this case ??? How could I convert the object of my class to an unsigned char* ??

Answer 1

I assume you refer to return reinterpret_cast<unsigned char*>(this);

This is a nasty chunk that is probably taking the wrong approach in solving the wrong problem. Technically, it asks the compiler to re-interpret the type of the this pointer as an unsigned char* pointer. No work is done here: the statement only instructs the compiler to think of the pointer as of a different type. This is disastrous if it's wrong to interpret the type as such.

If you wish to inspect your object bitwise then you might try something like this:

union inspect_klass {
    klass obj;
    char bits[sizeof(klass)];
};

EDIT: My confidence about the above union is shaken now. Don't use it just yet, I'm looking for confirmation that this trick is indeed broken. :s

Answer 2

I agree with the other posts: this is almost certainly not what you intend to do.

However, IIRC, you are guaranteed by the C++ standard to be able to convert a pointer of any type (not including function or member pointers) to an unsigned char * or a void * and back without entering the realm of undefined behavior. Additionally, you may access any object through an lvalue of type char * or unsigned char * (see ISO 14882:2003 section 3.10.15). I have made use of this before in order to inspect the internal representation of arbitrary object types. This results, if my reading of the standard is correct, in "implementation-defined behavior" (obviously, the internal representation of types depends upon the implementation).

For example,

template<class T> std::vector<unsigned char> to_bytes (const T& t)
{
    const unsigned char *p = reinterpret_cast<const unsigned char *>(&t);
    return std::vector<unsigned char>(p, p + sizeof(T));
}

is a template function that will yield a std::vector<unsigned_char> which is a copy of the object's internal representation.

However, the fact that you cast to void * when you return makes me suspect you are doing something perhaps more unsavory and more undefined.

Answer 3

First off, your GetData() return type does not match the return type of the expression.

reinterpret_cast is similar to old an old style C cast - you are simply returning a pointer to a unsigned char*. It is completely unsafe. In effect, you are creating a view into the data of a class, which can be accessed byte-wise.

Answer 4

Strictly speaking, unsigned char* is a byte array. It seems like a perfectly logical thing to do if the caller is going to serialize the data.