Advanced template autocast problem

[AdrianH]

ok, i've got a template that takes a pointer:

template<class T>
struct deref {
    T* pObj;
    deref(T* pObj) : pObj(pObj) {}
    operator T const & () const { return *pObj; }
};
 

and I have the following 2 code snippits:

    deref<int> a(new int(2));
    cout << endl << string("hello: ") << a << endl;
 

    deref<string> a(new string("there"));
    cout << endl << string("hello: ") << a << endl;
 

The first snippet works fine, the second has a compile error saying there is no function that corrisponds.

If I declare a new function like so:

ostream& fn(ostream& os, string const & rhs)
{
    os << rhs;
    return os;
}
 

and change my test to the following:

    deref<string> a(new string("there"));
    cout << endl << string("hello: ");
    fn(cout, a) << endl;
 

then it works.

I change the function to a template like so:

template<class T>
ostream& fn(ostream& os, T const & rhs)
{
    os << rhs;
    return os;
}
 

and I get the same error.

As I understand it, in my test, the type T is a deref<string>. That said, it has an autocast that should generate a T const & which is string const &.

So my question is why is the autocast not being done?


Adrian

[AdrianH]

Well, I've looked in to it by removing all templates thus having a concreate class deref and function fn() and the problem still exists.

It is like the function ostream& operator << (ostream&, string const &) has been declared as explicit, but explicit is only available for constructors.

Here is my new code:

#include<iostream>
#include<string>
using namespace std;
 
#define T string 
//template<class T>
struct deref {
    T* pObj;
    deref(T* pObj) : pObj(pObj) {}
    //T & operator ()() const { return *pObj; }
    operator T & () const { return *pObj; }
};
#undef T
 
#define T deref
//template<class T>
ostream& fn(ostream& os, T const & rhs)
{
    os << rhs;  //< Error occurs here!
    return os;
}
#undef T
 
int main() {
    deref/*<string>*/ a(new string("there"));
    cout << endl << string("hello: ");
    fn(cout, a) << endl;
}
 

The error messages are:
../main.cpp: In function `std::ostream& fn(std::ostream &, const deref&)':
../main.cpp:20: error: no match for 'operator<<' in 'os << rhs'
then spouts a bunch of candidates, none of which take a string const &. And I do know that one does exist as I've seen it in the header files.

Man this is frustrating.

Does anybody have an idea what is going on here?


Adrian