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-   -   void* pointer to object in C++ (http://www.chiefdelphi.com/forums/showthread.php?t=72102)

byteit101 01-14-2009 03:43 PM
void* pointer to object in C++
 
I have been at this for some time now, and this won't compile:
Code:
(function(int data))
void * MyPointer
if (data=0)
{
    Gyro gy(1);
    MyPointer = &gy;
}
if (data=1)
{
    Ultrasonic gy(1);
    MyPointer = &gy;
}
if (etc...
etc...
(function2(int data2))
if (data2=0)
    return MyPointer->GetValue(); //this produces error: 'void*' is not a pointer-to-object type (:eek:)
etc...
You can probably see what i am doing, (accessing different function based on sensor type), and Nothing has worked :(

virtuald 01-14-2009 03:51 PM
Re: void* pointer to object in C++
 
Don't use a void pointer, use a pointer to the base class. So for those I think its SensorBase.

jee7s 01-14-2009 03:58 PM
Re: void* pointer to object in C++
 
To be in line with the strictest and most proper C++ design principles (a very good idea), you don't want to use void* in the scenario you describe. Rather, you want to take advantage of polymorphism and use an abstract base class pointer instead.

For example, if all your sensors derive from a single abstract base class (like the WPILib classes do), then you can use a pointer to the base class to access the inherited class functions.

I assume that GetValue is inherited from SensorBase and is virtual. ( I don't have the library in front of me at the moment.) Therefore, rather than using a void* you can use a SensorBase* like this:

Code:
Gyro gy(1);
Ultrasound us(2);
SensorBase* mySensor;
mySensor = &gy;
mySensor->GetValue(); //mySensor calls Gyro::GetValue() here
mySensor = &us;
mySensor->GetValue();//mySensor calls Ultrasound::GetValue() here
If you want to use the void* you need to recast it for the compiler to know what you are doing. So, you need a recast for each different sensor. This leads to all sorts of extra code that you avoid by going the polymorphic route.

-Jeff Erickson, FRC 41

byteit101 01-14-2009 03:59 PM
Re: void* pointer to object in C++
 
Thanks! I'll Try that!

slavik262 01-14-2009 04:28 PM
Re: void* pointer to object in C++
 
And for future reference, void pointers need to be casted to the correct pointer type in order to be used. This should fix your error.

byteit101 01-14-2009 04:41 PM
Re: void* pointer to object in C++
 
like (Gyro)MyPointer->GetValue();?
or ((Gyro)MyPointer)->GetValue();?
or ((Gyro)(*MyPointer))->GetValue();?

heydowns 01-14-2009 05:15 PM
Re: void* pointer to object in C++
 
Quote:
Originally Posted by byteit101 (Post 800949)
like (Gyro)MyPointer->GetValue();?
or ((Gyro)MyPointer)->GetValue();?
or ((Gyro)(*MyPointer))->GetValue();?
None of the above.
Your pointer is the wrong type. So it's the pointer you have to cast to the correct type of pointer:

((Gyro *)MyPointer)->GetValue()

But you don't want to do this :) Do as suggested in the prior posts as it is much cleaner and less error-prone.


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