Re: static_cast vs reinterpert_cast

From:

sonison.james@gmail.com

Newsgroups:

comp.lang.c++

Date:

10 Jul 2006 02:33:14 -0700

Message-ID:

<1152523994.665451.236650@m79g2000cwm.googlegroups.com>

Hi,

static_cast is meant to be used for cases which the compiler would
automatically be able to convert, such as char to int and in your case
A* to void*. reinterpret_cast is used, as the book mentions, for
low-level hacks, especially when you know what you are doing, eg:

struct S
{
    int a, b;
};

int main()
{
    S s;
    s.a = 10;
    s.b = 20;

    int* p = reinterpret_cast<int*>(&s);
    cout << "a=" << *p << endl;
    ++p;
    cout << "b=" << *p << endl;
}

Typically reinterpret_cast should work where a static_cast works.
"Ideally" in C++ we should avoid using void* as much because we loose
type safety. Also in your case you do seem to be having a class
hierarchy (A,B), so using virtual functions with base ptrs would be
cleaner (if virtual fn overhead is not an issue).

Thanks and regards
Sonison James

Rahul wrote:

Hi,

I have a
class A : public B {...member functions......data members};

and am doing the following
    A *p=new A();
    void *p=static_cast<void *>(p);
    factory_instance->process(p);

Here p is passed to a function, which accepts void ptr. That function
need to cast it back
A *pp=static_cast<A *>(p);

The function is in the factory which accepts void *p only, the specific
implementations need to cast the pointer back to the expected class
and use it.

Question:Though both works fine, yet I want to know what is more
appropriate in this situation static_cast OR reinterpert_cast

The books suggests
static_cast=> "For "well-behaved" and "reasonably
well-behaved" casts,including things you might now do without a cast
reinterpret_cast=> To cast to a completely different meaning. The key
is that you'll need to cast back to the original type to use it
safely.

But I am not able to interpret the sentences in this context :-)