Dereferencing an array of void pointers

Question

Say I have an array of void pointer pointing to objects of different types and I want to pass them through a function and cast them to pointers of their respective type, how would I go about doing this? I've tried this current code, but I get error:

‘void*’ is not a pointer-to-object type

objectA myObjectA;
objectB myObjectB;
objectC myObjectC;
objectD myObjectD;

void *data[4];

data[0] = static_cast<void *>( &myObjectA );
data[1] = static_cast<void *>( &myObjectB );
data[2] = static_cast<void *>( &myObjectC );
data[3] = static_cast<void *>( &myObjectD );

void dereference( void *data )
{
    objectA *objA_ptr = static_cast<objectA *>( data[0] );
    objectB *objB_ptr = static_cast<objectB *>( data[1] );
    objectC *objC_ptr = static_cast<objectC *>( data[2] );
    objectD *objD_ptr = static_cast<objectD *>( data[3] ); 
}

Anyone know the correct way to implement this?

edit: For context, I'm using opencv and trying to create a simple gui interface with a trackbar. Opencv allows for userdata to be fed into the TrackbarCallback which is called when the slider is moved, but only in the form of a void pointer.

 int createTrackbar( const string& trackbarname, const string& winname,
                           int* value, int count,
                           TrackbarCallback onChange=0,
                           void* userdata=0);

My question is; why in the world are you using void* at all? This is C++, you have templates. — Ed Swangren
– Ed Swangren, Commented Mar 13, 2012 at 18:52

James M · Accepted Answer · 2012-03-13 18:59:19Z

8

Say I have an array of void pointer

void deference( void *data )

That's not an array of void pointers, it's a single void pointer.

Change that to void **, and then since you're mixing C and C++ styles up anyway, why not just use a C style cast? :

objectA *objA_ptr = (objectA *) data[0];

edited Mar 13, 2012 at 18:59

answered Mar 13, 2012 at 18:53

James M

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Comments

Not_a_Golfer · Accepted Answer · 2012-03-13 18:56:00Z

3

Derive them all from the same base class.
make the destructors virtual if your function needs to delete them.
make your array BaseClass *data[4]
make your function void deference(BaseClass **data)

answered Mar 13, 2012 at 18:56

Not_a_Golfer

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2 Comments

John Bode Over a year ago

Alternately, use a vector<BaseClass*> instead of an array.

Not_a_Golfer Over a year ago

@JohnBode ...and use shared_ptr instead of putting "plain" pointers in a vector

Matthias · Accepted Answer · 2012-03-13 18:57:41Z

2

You could use a reinterpret_cast<>() instead of static_cast<>.

answered Mar 13, 2012 at 18:57

Matthias

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3 Comments

UncleBens Over a year ago

If static_cast doesn't work, then obviously reinterpret_cast can only be more wrong here?

Matthias Over a year ago

It depends what you mean with "doesn't work". <code>reinterpret_cast<></code> it the most mighty and therefore most dangerous cast. It works in the sense that it throw down all compiler type checks and assure "I know what I ask for - Do it!".

Rafal R Over a year ago

And that is good, because with void* compiler has no information to perform any typecheck.

John Bode · Accepted Answer · 2012-03-13 19:04:41Z

1

The correct function prototype needs to be

void dereference(void **data)

since you're passing an array of void *, and as we all know in C and C++ an array expression is replaced with a pointer expression in most circumstances.

Is there any particular reason why you must use an array of pointers to objects of differing classes? This smells like a C solution to a C++ problem.

answered Mar 13, 2012 at 19:04

John Bode

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