Issue with C++ B-tree

Question

I am using the Google's C++ B-tree and I have an issue that someone might be able to answer.

First of all I get the following error:

In file included from ref_impl/../include/btree_map.h:31:0,
             from ref_impl/core.cpp:48:
ref_impl/../include/btree.h: In instantiation of ‘btree::btree_node<Params>::reference btree::btree_node<Params>::value(int) [with Params = btree::btree_map_params<unsigned int, unsigned int, std::less<unsigned int>, std::allocator<std::pair<const unsigned int, unsigned int> >, 256>; btree::btree_node<Params>::reference = std::pair<const unsigned int, unsigned int>&]’:
ref_impl/../include/btree.h:809:33:   required from ‘btree::btree_iterator<Node, Reference, Pointer>::pointer btree::btree_iterator<Node, Reference, Pointer>::operator->() const [with Node = btree::btree_node<btree::btree_map_params<unsigned int, unsigned int, std::less<unsigned int>, std::allocator<std::pair<const unsigned int, unsigned int> >, 256> >; Reference = std::pair<const unsigned int, unsigned int>&; Pointer = std::pair<const unsigned int, unsigned int>*; btree::btree_iterator<Node, Reference, Pointer>::pointer = std::pair<const unsigned int, unsigned int>*]’
ref_impl/core.cpp:539:18:   required from here 
ref_impl/../include/btree.h:557:57: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]

btree.h:

pointer operator->() const {
    return &node->value(position);
 }

That seems to be the cause of the error.

Part of my code is the following:

auto range = duplicates.equal_range(query_ids_temp[i]);
    for (auto it = range.first; it != range.second; ++it) {
//error     
       std::vector<unsigned int>::iterator pos = std::find(deleted_queries.begin(), deleted_queries.end(), it->second);
    //error
        if (pos != deleted_queries.end()) {
            duplicates.erase(it);
            deleted_queries.erase(pos);

        } else {
            query_ids.push_back(it->second);
        }
    } 

The error seems to be the it->second

Additionally I tried:

for (btree_multimap<unsigned int, unsigned int>::iterator it = range.first; it != range.second; ++it) {
                    //error
        auto temp = it->second;
                    //error
        std::vector<unsigned int>::iterator position = std::find(deleted_queries.begin(), deleted_queries.end(), temp);
        if (position != deleted_queries.end()) {
            duplicates.erase(temp);
            deleted_queries.erase(position);

        } else {
            query_ids.push_back(it->second);
        }
    }

Please keep in mind that the error seems to be at auto temp = it->second;

I have also tried:

auto p = duplicates.find(query_ids_temp[i]);
      if(p != duplicates.end()) { // found a name
        do {
                    //error
            auto temp = p->second;
                    //error
            auto pos = std::find(deleted_queries.begin(), deleted_queries.end(), temp);
            if (pos != deleted_queries.end()) {
                duplicates.erase(p->second);
                deleted_queries.erase(pos);

            } else {
                query_ids.push_back(p->second);
            }

          p++;
        } while (p != duplicates.upper_bound(query_ids_temp[i]));
      }
      else{
        cout << "Name not found.\n";
      }

And the problem seem to be again the same: auto temp = p->second;

Any suggestions?

Answer 1

The central caveat with Google's B-tree is that mutations invalidate iterators. The call to duplicates.erase() inside the loop invalidates "p".

The erase() method returns an iterator for this reason, so you can write something like:

if (...) {
  p = duplicates.erase(...);
  ...
} else {
  ...
  p++;
}

BTW, please provide details about which compiler version gives the warning.