Deriving the type of a value in a class template

From: Sean Parent (sparent_at_[hidden])
Date: 2001-11-02 17:42:34

I'd like to extend auto_ptr<> to parameterize how the pointer is deleted -

The basic form for this would be:

---
template<class T, class DelFun>
class auto_ptr
    {
public:
    explicit auto_ptr(T* x, DelFun del = DelFun ()) :
        fDelMem(del), fVal(x) { }
    ~auto_ptr() { fDelMem(fVal); }
    ...
private:
    DelFun  fDelMem;
    T*      fVal;
    };
---
Ignoring empty member opts and such. The problem with this is that it is
fairly rare that you have a class to do the delete (usually you have a
function) - and if you use the fun_ptr adaptor then you don't have a default
constructor and you have to explicitly pass one each time you construct an
auto_ptr. Not what I want. That is, instead of this:
----
auto_ptr<int, pointer_to_unary_function<void, int*> >
    my_ptr(p, fun_ptr(&MyDelete));
----
I want to be able to do this:
-----
auto_ptr<int, function_object<&MyDelete> > my_ptr(p);
-----
The challenge is coming up with a definition of function_object<> that works
statically by value. Here is what I've managed so far (slightly abbreviated
to just the interesting bits):
-----
template <typename T, T V>
class function_object
    { };
template <typename Result, typename Argument, Result (*CallValue)(Argument)>
struct function_object<Result (*)(Argument), CallValue>
    {
    Result operator () (Argument a) { return CallValue(a); }
    };
-----
Using this you can do the following:
-----
auto_ptr<int, function_object<void (*)(int*), &MyDelete> > my_ptr(p);
-----
Not bad, but specifying the function type seems unnecessary. I know I can
derive the type in a function template but how do I do it in a class
template?
And yes I know I can always write the adapter for every instance (it is
small) but that gets to be a pain and I would like to use this technique for
more complicated operations.
Sean
-- 
Sean Parent
Sr. Computer Scientist II
Advanced Technology Group
Adobe Systems Incorporated
sparent_at_[hidden]