Question

I’ve been learning C++, coming from C#, where I’ve gotten used to using service providers: basically a Dictionary<Type, object>. Unfortunately, I can’t figure out how to do this in C++. So the questions are basically:

1. How would I make a dictionary in C++.

2. How would I use ‘Type’ with it, as far as I know there is no ‘Type’ in C++.

3. Same as above, but with ‘object’.

Thanks!

Answer 1

I’m assuming that you’re trying to map a type to a single object instance. You could try something along these lines:

#include <typeinfo> #include <map> #include <string> using namespace std;  class SomeClass { public:     virtual ~SomeClass() {} // virtual function to get a v-table };  struct type_info_less {     bool operator() (const std::type_info* lhs, const std::type_info* rhs) const     {         return lhs->before(*rhs) != 0;     } };  class TypeMap {     typedef map <type_info *, void *, type_info_less> TypenameToObject;     TypenameToObject ObjectMap;  public:     template <typename T>      T *Get () const     {         TypenameToObject::const_iterator iType = ObjectMap.find(&typeid(T));         if (iType == ObjectMap.end())             return NULL;         return reinterpret_cast<T *>(iType->second);     }     template <typename T>      void Set(T *value)      {         ObjectMap[&typeid(T)] = reinterpret_cast<void *>(value);     } };  int main() {     TypeMap Services;     Services.Set<SomeClass>(new SomeClass());     SomeClass *x = Services.Get<SomeClass>(); } 

In C++ types are not first-class objects in their own right, but at least the type-name is going to be unique, so you can key by that.

Edit: The names aren’t actually guaranteed to be unique, so hold on to the type_info pointers and use the before method to compare them.