Question

I’m debugging my code and at one point I have a multimap which contains pairs of a long and a Note object created like this:

void Track::addNote(Note &note) {
    long key = note.measureNumber * 1000000 + note.startTime;
    this->noteList.insert(make_pair(key, note));
}

I wanted to look if these values are actually inserted in the multi map so I placed a breakpoint and this is what the multimap looks like (in Xcode):

It seems like I can infinitely open the elements (my actual multimap is the first element called noteList) Any ideas if this is normal and why I can’t read the actual pair values (the long and the Note)?

Answer 1

libstdc++ implements it’s maps and sets using a generic Red/Black tree. The nodes of the tree use a base class _Rb_tree_node_base which contain pointers to the same types for the parent/left/right nodes.

To access the data, it performs a static cast to the node type that’s specific to the template arguments you provided. You won’t be able to see the data using XCode unless you can force the cast.

It does something similar with linked lists, with a linked list node base.

Edit: It does this to remove the amount of duplicate code that is generated by the template. Rather than have a RbTree<Type1>, RbTree<Type2>, and so on; libstdc++ has a single set of operations that work on the base class, and those operations are the same regardless of the underlying type of the map. It only casts when it needs to examine the data, and the actual rotation/rebalance code is the same for all of the trees.