Error when casting a base pointer: `Cannot instantiate abstract class`

Question

I'm working with PhysX in C++ and am getting the error,cannot instantiate abstract class in the following code:

std::vector < PxJoint* > joints;
PxD6Joint* j = PxD6JointCreate(*gPhysics, a0, t0, a1, t1);
joints.push_back (j);
(PxD6Joint)(joints[0])->setDrivePosition(PxTransform()); //error here

The code is just an example. PxD6joint is an abstract class that inherits from PxJoint. I figure there must be some way to access the public members of &j via joints[0], but, if there is, I'm not doing it right.

You try to cast the *return value* of `setDrivePosition` to an *instance* of the abstract class. — Some programmer dude, Jun 04 '15 at 12:43
@Joachim How do I do it properly? like this `((PxD6Joint*)(joints[1]))->setDrivePosition(PxTransform());` ? — Matt Munson, Jun 04 '15 at 12:51
No need for the parentheses around `joints[1]` but other wise a correct C-style cast. But since you're programming in C++ I would suggest you use `static_cast` instead. — Some programmer dude, Jun 04 '15 at 12:57
[This old question and its answers](http://stackoverflow.com/questions/1609163/what-is-the-difference-between-static-cast-and-c-style-casting) might be of help. :) — Some programmer dude, Jun 04 '15 at 13:01

score 0 · Answer 1 · answered Jun 04 '15 at 12:52

As told by Joachim, you are trying to cast the result of setDrivePosition to PxD6Joint type, which is wrong (especially since the method does nots return anything. I guess you are trying somehow to cast the object pointed to by joints[0] to PxD6Joint type, which is useless, it is already of the correct class.

But this class is a pure abstract class : some methods are not implemented (setDrivePosition (const PxTransform &pose)=0). You need to use an object that is of a subclass of PxD6Joint, with all methods implemented.

Error when casting a base pointer: `Cannot instantiate abstract class`

1 Answers1