How to modify objects using an iterator? using <list>

Question

So here's an example. The star's mLocation and mSpeed are a Vector3 custom type.

I'v tried:

Star &star = *iStar;
Star star = *iStar;

Using iStar-> directly doesn't work with my operators, not sure why. So whats the proper way to do that?

   void UniverseManager::ApplySpeedVector()
   { 
   std::list <Star>::const_iterator iStar;

       for (iStar = mStars.begin(); iStar != mStars.end(); ++iStar)
       {
           // how to I get a hold on the object the iterator is pointing to so I can modify its values
                   // i tried  Star &star = *iStar;  this is illegal
                   // tried just using the iStar->mLocation += iStar->mSpeed this also fails due to the operator not accepting the values not sure why
                   // tried other things as well, so what is the proper way to do this?

           iStar->SetLocationData( iStar->mLocation += iStar->mSpeed);
       }
   }

James McNellis · Accepted Answer · 2011-12-28 22:51:02Z

11

std::list<Star>::const_iterator iStar;

You cannot modify the objects in a container via a const_iterator. If you want to modify the objects, you need to use an iterator (i.e., std::list<Star>::iterator).

answered Dec 28, 2011 at 22:51

James McNellis

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2 Comments

EddieV223 Over a year ago

and then how do you go about it? Can you use Star &star = *iStar; or is there a better way?

James McNellis Over a year ago

Sure, you can use a reference to the element. Or you can access the element through the iterator each time you need it.

David · Accepted Answer · 2011-12-28 23:02:34Z

As James told you, you should use a std::list<Star>::iterator so that you can modify the object by calling a method or accessing its member variables.

It would be something like this:

void UniverseManager::ApplySpeedVector()
{
    std::list <Star>::iterator iStar;

    for (iStar = mStars.begin(); iStar != mStars.end(); ++iStar)
    {
        iStar->SetLocationData(iStar->mLocation += iStar->mSpeed);
    }
}

Nevertheless, if you want to improve your code you might prefer to have a getter for accessing the location and the speed:

void UniverseManager::ApplySpeedVector()
{
    std::list <Star>::iterator iStar;

    for (iStar = mStars.begin(); iStar != mStars.end(); ++iStar)
    {
        iStar->SetLocationData(iStar->GetLocationData() + iStar->GetSpeed());
    }
}

In any case, you have to use a non-const iterator.

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