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I have the following class used to access global registry variables.

template <typename T> using RegistryMap = std::unordered_map <std::string, T *>;
template <typename T> class Registry {
  static RegistryMap<T> registry;

public:
  static T* get(const std::string& name) {
    auto it = registry.find(name);
    return it == registry.end() ? nullptr : it->second;
  }

  static const RegistryMap<T>& getAll() {
    return registry;
  }

 static bool add(const std::string &name, T *object) {
    T* &store = registry[name];
    if (store)
      return false;
    else {
      store = object;
      return true;
    }
  }
};

template <typename T> 
RegistryMap<T> Registry<T>::registry = RegistryMap<T>();

I find that calls to the getAll method return different memory addresses, and I think my understanding of static members and object construction is faulty. I had thought that by declaring registry as static, each Registry template class would allocate memory for a RegistryMap, which would just be a pointer to the STL container on the heap. If I saved a reference to the map (the return value of getAll), I could refer to the STL container even after it gets modified.

Instead, getAll returns a reference of the current state of the map, but as things are added/deleted from the RegistryMap, getAll returns new addresses (specifically, old saved references of the map do not show the additional values.

Why would this be?

Edit:

auto t1 = Registry<void>::getAll();
Registry<void>::add("TESTING", nullptr);
auto t2 = Registry<void>::getAll();

Stepped through the preceding test in VS2013 debugger. t1 has size 0. t2 has size 1 and I can see ("TESTING", nullptr) inside.

user2411693
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  • Also (I will repost as a follow up question after 90 min, but thought I'd ask in case someone had a one-line answer), why must I define the Registry::registry like that? I get linking errors/unresolved external symbols if I try to initialize the value of registry in declaration or via a constructor. – user2411693 May 30 '15 at 03:35
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    http://stackoverflow.com/help/mcve – Benjamin Lindley May 30 '15 at 03:40
  • Cannot reproduce: http://ideone.com/NS3eFh – Benjamin Lindley May 30 '15 at 03:46
  • Thanks for your code sample. I see the problem. I have to specify that I'm assigning the return value of getAll to a reference. Otherwise, it creates a copy of the STL container? – user2411693 May 30 '15 at 03:57

1 Answers1

3
auto t1 = Registry<void>::getAll();

This makes a copy of the map. You want a reference.

auto& t1 = Registry<void>::getAll();
//  ^
Benjamin Lindley
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  • Thank you. I see I've been using references wrong throughout my code. – user2411693 May 30 '15 at 04:01
  • Any thoughts on why I can't define the static map at the declaration or via a constructor? Isolating the problem to template classes, static class members, or constructors would be really helpful. – user2411693 May 30 '15 at 04:08
  • Got it: http://stackoverflow.com/questions/195207/unresolved-external-symbol-on-static-class-members – user2411693 May 30 '15 at 04:16