What may cause losing object at the other end of a pointer in c++?

Question

EDIT: I have found the error: I did not initialize an array with a size. question can be closed.

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I have a class V, and another class N. An object of N will have an array of pointers to objects of class V (say V **vList). So, N has a function like V **getList(); Now in some function of other classes or simply a driver function, if I say V **theList = (N)n.getList(); Q1: theList would be pointing at the 1st element of the array? Given that the size of array is known, can I loop through with index i and say V *oneV = *vList[i]? Please correct me if what I'm doing above is wrong.

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I have been using debugger to trace through the whole process of my program running, the thing I found was that after using V *oneV = vList[i], the value of the pointers in the array, vList, were the same as when they were created, but if I follow the pointer to where it is pointing at, the object was gone. I'm guessing that might be the reason why I am getting seg fault or bus error. Could it be the case? WHY did I 'loose' the object at the other end of a pointer? What did I do wrong?

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and yes, I am working on a school assignment, that's why I do not want to print out my codes, I want to finish it myself, but I need help finding a problem. I think I still need explanation on array of pointers. Thank you

Answer 1

Class N has to manage the number of elements in a list somehow. The usual approaches are to make a public function which returns the number of elements in the array, or to provide an iterator function which loops over all the list's elements.

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An array with N elements are stored at array[0] through array[N-1]. You're accessing one past the end of the array.

Answer 2

Q1 is right. For the second part, V *oneV = vList[i] would be the correct syntax. In your syntax you are dereferencing one more time (treating an object of type V as a pointer to such an object) which obviously is crashing your code.

EDIT: Since you are using the correct syntax, the reason of segfaults would depend on your memory management of the objects of type V. If you have inserted addresses of objects created on the stack (automatic vars, not by new or malloc) inside a function and are trying to access them outside of it, then the pointers would be dangling and your code will crash.

Answer 3

theList would be pointing at the 1st element of the array? Given that the size of array is known, can I loop through with index i and say V *oneV = *vList[i]?

Yes, that is correct.

I'm guessing that might be the reason why I am getting seg fault or bus error. Could it be the case?

Yes, if you have an invalid pointer and try to dereference it you'll get a segfault.

WHY did I 'loose' the object at the other end of a pointer? What did I do wrong?

That is difficult to predict without seeing the actual code. Most probable causes are that either you are not filling the V** correctly or after putting a V* pointer inside V** array you are deleting that object from some other place. BTW, I am assuming that you are allocating memory using new, is this assumption correct?

Answer 4

First rule out the initial ones:

• you are initializing correctly (new instead of automatic/local variables)
• you are accessing the elements correctly (not like in the typo you posted in the question - based on your comment)
• you are using the right size

If you go through all the normal ones and everything is k, then make sure to pay special attention to your loops / size calculations / and anything else that could be causing you to write to unintended addresses.

It is possible to write garbage at unintended locations & then get the error in unexpected places ... the worst I saw like that, was some file descriptors's variables being corrupted because of an array gone wrong right before those variables - it broke on file related functions, which seemed v. crazy.

Answer 5

what do you mean?