cast void* to class-pointer

themts

Hey guys,

I have a question that is more c++ related then QT.

I'm currently exporting a c++ class to a library.
I have some functions where I want to return a pointer to an object that I can pass to another function as a handler. This was a common procedure I used in C.

e.g.

bool getDevice(NDevice *networkDevice);

void startConnection(NDevice *networkDevice);

As I don't want to export all the needed classes (in the example NDevice), I would change the arguments to void*
In this case I would have to cast void* to my real class NDevice* back and forth.
Is there a save way of casting which checks if void* is NDevice* ?
dynamic_cast is not working on void* and static_casting is not safe as I could pass anything.

CU

JonB

@themts said in cast void* to class-pointer:

bool getDevice(NDevice *networkDevice)

Because you are only using a pointer to a class instance here, the compiler does not need the definition of NDevice. (You must simply precede this line with class NDevice; somewhere, to tell compiler "there is a class named NDevice".)

That would not be the case If, say, the declaration were bool getDevice(NDevice networkDevice) or bool getDevice(NDevice &networkDevice). (Then it would need the full class definition for NDevice.)

You can use this to allow .h files to use external classes without needing their definition. Selected .cpp files which need the internals can include the full definition where needed.

JonB

@themts said in cast void* to class-pointer:

As I don't want to export all the needed classes (in the example NDevice), I would change the arguments to void*

Are you aware that all you need to write is

class NDevice;`

and not go via void * here?

themts

? Sorry, don't get it

NDevice is a class.
In this sample:
bool getDevice(NDevice *networkDevice) would create an object of NDevice and return it via argument.

But in this case I also need to supply the NDevice header.
Or am I totally wrong here?

I want to avoid exporting unnecessary header files as I would have to clean them up by using PIMPL first. I simply want to avoid this overhead-work as it is not necessary to know the Class-definition as long as I'm only using it as a handler.

SGaist

Hi,

What @JonB talks about is forward declaration.

JonB

@themts said in cast void* to class-pointer:

bool getDevice(NDevice *networkDevice)

Because you are only using a pointer to a class instance here, the compiler does not need the definition of NDevice. (You must simply precede this line with class NDevice; somewhere, to tell compiler "there is a class named NDevice".)

That would not be the case If, say, the declaration were bool getDevice(NDevice networkDevice) or bool getDevice(NDevice &networkDevice). (Then it would need the full class definition for NDevice.)

You can use this to allow .h files to use external classes without needing their definition. Selected .cpp files which need the internals can include the full definition where needed.

themts

ah ok, wasn't aware of it.

but when I make my .h file like this:

class NDevice;
bool getDevice(NDevice *networkDevice);

and my .cpp file like this:

#include "ndevice.h"

Class::getDevice(NDevice *networkDevice) 
{
  networkDevice->doSomething();
}

I get the error:

error: variable has incomplete type 'Class::NDevice'
note: forward declaration of  'Class::NDevice'

SGaist

If you have it in a namespace, you have to forward déclaré it within that namespace.

But in your case, the forward declaration must be done outside of the class definition.

themts

forget what I just wrote (stupid mistake). The forward declaration works fine.

Thank you guys!