Question

I am having a bit of a problem with Qt’s foreach function. I have a class Phrase, which is a subclass of QList. In ~Phrase I delete all of the GlossItem pointers.

In iterating through the GlossItem pointers in Phrase, I would like to use Qt’s foreach:

    // phrase is a pointer to a Phrase object, 
    // which is a subclassed QList<GlossItem*>
    foreach( GlossItem *glossItem , *phrase )
    {
        // use glossItem
    }

For some reason foreach is performing a deep copy on Phrase (I know this because it required me to implement the copy constructor). But if there is a copy of Phrase — and if I don’t want to create a deep copy of each GlossItem — that means that those pointers will be deleted twice. (Or, deleted once, and then crash.) So I have to use this, which works but is less pretty.

    for(int i=0; i<phrase->count(); i++ )
    {
        GlossItem *glossItem = phrase->at(i);
        // use glossItem
    }

Is there a way around this or do I just need to live with it?

Answer 1

From the docs

Qt automatically takes a copy of the container when it enters a foreach loop.

and

Since foreach creates a copy of the container, using a non-const reference for the variable does not allow you to modify the original container.

So I think for your specific use case foreach isn’t suitable otherwise you end up with a fresh copy of the Phrase with additional pointers, rather than returning the actual original Phrase that you want.