confusing malloc error problem with chunks example

[mibes]

I've been messing around with the stepper tool in the chunks example of WDL-ol. I've made it so the number of steps displayed is dynamically adjustable. Fine.

But when the destructor runs (closing the plugin) I get an error about trying to delete an unallocated pointer. I don't understand why because I am testing for nulls before deleting.


this is the significant part of the constructor:

  {
  ...

    float sliderWidth = ((float) mDrawRECT.W() / (float) mStepsDisplayed);

    mSteps = new double[numSliders];
    for(int i=0; i<numSliders; i++)
    {
        mSteps[i] = 0.;
    }

    for(int i=0; i<mStepsDisplayed; i++)
    {
      int lpos = (i * sliderWidth);
      mSteps[i] = 1.;

      mSliderBounds[i] = new IRECT(mDrawRECT.L + lpos , mDrawRECT.T, mDrawRECT.L + lpos + sliderWidth, mDrawRECT.B);
    }

    mHandleWidth = handleWidth;
  }

...

and this is the destructor:

  ~MultiSliderControlV()
  {
    delete [] mSteps;
      

    for(int i=0; i<mNumSliders; i++)
    {
        if (mSliderBounds[i] != NULL)
        {
            delete mSliderBounds[i];
        }
    }
  }

[Xenakios]

Checking for null pointers won't help if you have pointers that were not initialized to begin with. (C++ generally does not initialize anything automatically to zeros, so maybe you have garbage pointers initialized to some random values. Attempting to use delete on such pointers is not going to end well.)

In any case, you should redesign the code to use safer C++ idioms. (Use objects like IRECT as values instead of using them via pointers/heap allocations and use container classes instead of new[n]/delete[].)

[mibes]

Gotcha! Thanks

I'll have a crack at redesigning it. I was just reworking WDL-ol example, so wasn't my chosen technique to start with really!

If I did want to just patch this up I guess initialising all the pointers to null in the constructor would do the trick though.

[earlevel]

In addition to the remarks from Xenakis (and yes, I'd just make it an array of IRECTs instead of an array of pointers to IRECTs, then there is no delete needed):

In the constructor, you're looking through the mSliderBounds array from 0 to mStepsDisplayed (and not zeroing out the rest). In the destructor, you're loop 0 to mNumSliders—why not mStepsDisplayed? Anyway, the combination of those two problems ensures you'll try to deallocate garbage.

But if you use an array of IRECTs (not pointers), you won't need to deallocate them.