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I am am using std::unordered_map and I would like to use it to store pointers to an object that I dynamically create with new.

I can create the objects fine and the insert method succeeds; however when I attempt to resolve a specific object from the map using the index it appears that all of the items in the list are mapped to the last item I point in the map.

typedef std::unordered_map<__int64, Session*> HashDict; HashDict dict; void Sessions::Insert() { Session* newSession = new Session; newSession->setId(sessionCounter++); dict.insert(HashDict::value_type(newSession->Id(), newSession)); } void Sessions::Find(__int64 id) { HashDict::iterator it = dict.find(id); if(it != dict.end()) { Session* s = it->second; std::cout << "Search for: " << id << std::setw(3) << " found @ location: " << it->second << std::setw(3) << " with value: " << s->Id() << std::endl; } } 001D4F50 0 001D5038 1 001D50D0 2 001D5378 3 001D5410 4 001D54A8 5 001D5540 6 001D55D8 7 001D5670 8 001D4E50 9 Search for: 5 found @ location: 001D54A8 with value: 9

Notice that 5 points to the location at 5 (from the insert), however it says that the value is 9.

What am I doing wrong?

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    Can you post the definition of Session? Commented Aug 21, 2011 at 23:19
  • @downvoters I hope you aren't downvoting because you dislike the chairman of Google! Commented Aug 21, 2011 at 23:27
  • Is it possible for Id to change after creation? Commented Aug 21, 2011 at 23:30
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    You can narrow this problem down. Create two Session objects and note that setting the value on the ID of the first affects the ID of the second. Nothing to do with unordered_map. Commented Aug 21, 2011 at 23:39
  • I'm with Kerrek -- what does Session (and session's Id()) look like? Commented Aug 22, 2011 at 0:06

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If I had to guess from the output you're getting, by any chance is the ID field of Session marked static? If so, this would manifest itself with exactly the behavior you're seeing here. In particular, if the field is static, then every time you create and update the ID of a new session, you'd be overwriting the ID field shared across all Session objects and only the last Session ID would persist.

If I'm totally wrong, let me know and I'll delete this answer.

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5 Comments

This was the case (red face). I had the variables declared at the global level of the class (outside of the class scope). I moved them to private: members and the results are now correct. This lesson I learned here is that members declared outside of the class definition are by default static as I did not have them marked as such. The one follow on question I have is the following - "Is there a different mechanism to hide the internal implementation of class when defining it's header file?"
Not really. In C++ the class definition must be completely available to use the class in most contexts, since the compiler has to know the size of the object, it's layout, etc. There are techniques for trying to hide the implementation (do a Google search for "pImpl idiom" or "compiler firewall" for examples), but ultimately they come at the cost of efficiency.
@Eric: Your statement, "members declared outside of the class definition are by default static", is a bit misleading. Unfortunately, in C++ static means many different things depending on context. In particular, inside a class it means something else as outside a class. Variables defined outside a class are globals. Globals, as the name suggests, are not specific to one object.
@templatetypedef: Thanks for the pointer. I will just keep it simple.
@MSlalters: Understood, I think. Outside of the class is Global, so once included in a process and object can have access. In my case and it seems in most cases this is a poor way to manage scoping. Static inside of the class is static for that instance. Good for things like counters. Am I on the right track?

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