Re: Help to remove reinterpret_cast from c++ code

From:
=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Wed, 24 Apr 2013 06:54:06 CST
Message-ID:
<kl7tes$n14$1@dont-email.me>
On 2013-04-24 02:15, nvangogh wrote:

Hi, I have been reading a class that I found online that creates an
object to represent a joystick. Now i recall reading that any use of
'reinterpret cast' is dangerous as it is highly machine
dependent. The code I found unfortunately uses this and i was hoping
to improve it, before attempting to use the class in my
program. Note, I have not tested the class as yet - even on the
assumption it works on my machine, i need it to be as general as
possible as my program will run on other machines as well.

This is the offending member function definition:

//loop function attempts to read an event from the device, and, if
//an event is waiting to be processed,
//updates the state structure based on the event type.

void* cJoystick::loop(void *obj)
{
    while (reinterpret_cast<cJoystick *>(obj)->active)
reinterpret_cast<cJoystick *>(obj)->readEv();
}


What does this function return?

- so how can this be re-written so there are no re-interpret casts?


Technically you could rewrite this as:

void* cJoystick::loop(void* obj)
{
     cJoystick& pJ = *static_cast<cJoystick*>(obj);
     while (pJ.active)
       pJ.readEv();
     return nullptr; // Or whatever is right here
}

I find this more readable, but it still has the basically the same
meaning as the alternative version with the reinterpret_cast (modulo
some minor details that I think are not relevant in the context of
your question) and also cannot validate what you are doing here.

I assume you wrote this function with opaque parameters, because you
have to adapt to some low-level API. Instead of investing much efforts
to replace the reinterpret_cast here by static_cast, I recommend to
invest time to design a public API that does not depend on such
low-level details, instead wrapping this part into a layer that has
full control over the input and output of such functions. std::thread
is a good example for this: It completely hides the OS layer that
typically has exactly similar internal APIs as your cJoystick class.

HTH & Greetings from Bremen,

Daniel Kr?gler

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