使用.find无法引用c ++ map / set迭代器

Question

I've relatively new to using maps, and I'm currently getting the Debug Assertion Failed Expression: map/set iterator not dereferencable

When I hit retry it brings me to this section of code:

auto temp = mOpenMap.find(currentNode); temp->second = false;

I assume this has to do with the .find(currentNode) returning the end of the map, as it didn't find it, but the concerning part here is that doing my manual debugging I found that the only Node in the map indeed contained the exact parts of the currentNode I had it search for.

My map is this:

std::map<PathNode*, bool> mOpenMap

Optimistically what I would like it to do is search for the row and column to ascertain that it is looking at a node that has already been searched so that I can set the accompanying boolean to false.

What I'm wondering, is how do maps generally search for objects? Or better yet, how can I go about making the map search with a custom search?

Answer 1

You should check std::map::find does find the element before dereference the iterator:

auto temp = mOpenMap.find(currentNode);
if (temp != mOpenMap.end())  // check temp is pointing to underneath element of a map
{
    temp->second = false;
}

Answer 2

If all you're doing is tracking the presence of some PathNode or other, you'd be better off using a std::set .

As for a custom search, both std::map and std::set work with a collection of values, ordered by a comparator. That comparator can be specified as the second template type when the map or set is defined. If omitted, that comparator defaults to std::less , which simply compares the objects with the less than operator, operator< . As written, your map mOpenMap is using the value of the pointer to perform this comparison, which is probably not what you want.

I suggest you declare and define PathNode::operator< , and replace mOpenMap with a member of type std::set<PathNode> . This will key off of actual PathNode values, rather than pointers (which will probably never collide under normal circumstances).

Remember that your PathNode::operator< should generate a strict ordering of PathNode objects. This is a requirement of the comparator for std::map and std::set. if you don't follow this rule, it will behave erratically, but it will compile and run, so make sure you pay attention to this detail.

• Documentation for std::set

Answer 3

You should check the result as billz said. The most likely reason that the find failed is that your map is keyed on PathNode * meaning that it will only find nodes with an exact pointer match. Searching for a pathnode with the same member values as one in the map will not work.

If you need the map to be on PathNode * , then you will need to also supply a predicate as the third parameter of the map. The predicate will need to be written to compare two PathNode * parameters by their member values.