static_cast vs reinterpert_cas t

[Rahul]

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_instanc e->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_cas t

The books suggests
static_cast= "For "well-behaved" and "reasonably
well-behaved" casts,including things you might now do without a cast
reinterpret_cas t=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 :-)

[tragomaskhalos]

Rahul wrote:

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

Don't cast to void* before calling process(), it's not needed and the
less casts you have the better - just pass the A* as is like this:
A* pa = new A();
factory_instanc e->process(pa);
Casting back to A* inside process() should be fine.
It'd obviously be better if process() took an A* (or some Base-of-A*)
in the first place.

[Rolf Magnus]

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_instanc e->process(p);

Here p is passed to a function, which accepts void ptr.

No need for a static_cast. You can convert a pointer to an object into a
pointer to void implicitly.

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_cas t

static_cast. Generally, one could say that you should choose static_cast
over reinterpret_cas t if it does the job.

The books suggests
static_cast= "For "well-behaved" and "reasonably
well-behaved" casts,including things you might now do without a cast
reinterpret_cas t=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 :-)

You could translate it to "reinterpret_ca st is more evil than
static_cast" ;-)

[sonison.james]

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

Typically reinterpret_cas t 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_instanc e->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_cas t

The books suggests
static_cast= "For "well-behaved" and "reasonably
well-behaved" casts,including things you might now do without a cast
reinterpret_cas t=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 :-)

[Ron Natalie]

Rahul wrote:

The books suggests
static_cast= "For "well-behaved" and "reasonably
well-behaved" casts,including things you might now do without a cast
reinterpret_cas t=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.

If your books say that, burn them. That's not the definition of
static cast.

There are two meanings for static cast:

1. Forcing conversions that could happen anyway:

static_cast<int >(5.5)

2. With the exception of const conversions, reversing defined conversions:
class B { };
class D : public B { };

B* b = new D; // defined conversion D* ->B*
D* d = static_cast<B*> (b);

It is still incumbent on the application to determine the
validity of the second time type of cast. In this case
making sure that b contains an value that really is
convertable to D*.

[Frederick Gotham]

I have a simple rule-of-thumb:

If you can use static_cast, then use static_cast.

If you can't use static_cast, then use reinterpret_cas t.
--

Frederick Gotham

[Victor Bazarov]

Frederick Gotham wrote:

I have a simple rule-of-thumb:

If you can use static_cast, then use static_cast.

If you can't use static_cast, then use reinterpret_cas t.

I have a simple preceding rule:

If you can do it without a cast, do not use any cast.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Frederick Gotham]

Victor Bazarov posted:

Frederick Gotham wrote:

>I have a simple rule-of-thumb:

If you can use static_cast, then use static_cast.

If you can't use static_cast, then use reinterpret_cas t.

I have a simple preceding rule:

If you can do it without a cast, do not use any cast.

Naturally ; )
I tend to use the old-style casts now and again too, e.g.:

enum { len = 64U };

int array[len];

for(size_t i = len - 1; (size_t)-1 != i; --i)
Or if I need a reinterpret_cas t and a const_cast at the same time:

char const *pc = 0;

int *pi = (int*)pc;

/* A very contrived example, I realise! */
The major drawback of the new-style casts is that they take up far too much
horizontal screen space.
--

Frederick Gotham

[dio]

Frederick Gotham <fg*******@SPAM .comwrote:

The major drawback of the new-style casts is that they take up far too much
horizontal screen space.

Yes, which should serve as a visual reminder that a design that relies
heavily on casts is perhaps not ideal.

--
like a rainbow in the dark.

[Frederick Gotham]

dio posted:

Frederick Gotham <fg*******@SPAM .comwrote:

>The major drawback of the new-style casts is that they take up far too
much horizontal screen space.

Yes, which should serve as a visual reminder that a design that relies
heavily on casts is perhaps not ideal.

I'm out of nappies now though, and I can judge sensibly as to when I should
use what kind of cast. Would you prefer:

int *p = reinterpret_cas t<int*>(
const_cast<char *>(pcc)
);

or:

int *p = (char*)pcc;
And,
size_t i = static_cast<siz e_t>(-1);

or:

size_t i = (size_t)-1;
(Second example is intended to supress a compiler warning)
--

Frederick Gotham