Determining the type from a smart pointer

Question

I have a function that currently takes in two template parameters. One is expected to be the smart pointer, and the other is expected to be the object type. For example, SmartPtr<MyObject> as the first template parameter and MyObject as the second template parameter.

template <typename T, typename TObject>

I would like to know whether I can determine the second parameter, MyObject , automatically from the first parameter SmartPtr<MyObject> or not so that my template function is written like this:

template <typename T>

And the type TObject in the original template function is automatically determined from T which is expected to be a smart pointer.

As requested, here is the function declaration and its use:

template <typename T, typename TObject>
T* CreateOrModifyDoc(T* doc, MyHashTable& table)
{
    T* ptr = NULL;

    if (!table.FindElement(doc->id, ptr))
    {
        table.AddElement(doc->id, new TObject());
        table.FindElement(doc->id, ptr);
    }       

    return ptr;
}

Answer 1

If you know that the first template parameter will be the smart pointer type, why not declare your function with only one parameter and use it as such:

template<typename T>
void WhatIsIt(SmartPtr<T> ptr)
{ 
    printf("It is a: %s" typeid(T).name());
}

Answer 2

If the classes that can serve as the first template parameter can be made to provide a handy typedef by a common name, you can do this:

template <typename T>
class SmartPtr
{
  public:
    typedef T element_type;

  // ...
};


template <typename PtrType, typename ObjType=PtrType::element_type>
void your_function_here(const PtrType& ptr)
{
  // ...
}

Answer 3

Did you write SmartPtr? If so, add this to it

  typedef T element_type;

Answer 4

All smart pointers I know of support the member ::element_type . For example boost's shared_ptr: http://www.boost.org/doc/libs/1_46_1/libs/smart_ptr/shared_ptr.htm#element_type , but also the std smart pointers support this convention.

template <typename T> class SharedPtr {
public:
    typedef T element_type;

    // ...
};