Re: Partial specialization bug or me?
On 5 Okt., 20:24, german diago <germandi...@gmail.com> wrote:
Hello, I would like to know why the code in main chooses the primary
template and not the specialization:
//Class
template <class T>
class MyVecIter {
public:
T * elem_;
MyVecIter(T * elem) : elem_(elem) {}
void next() { ++elem_; }
T & get() { return *elem_; }
void equals(const MyVecIter & other) { return this->elem_ ==
other.elem_; }
This function should probably const and should return
a bool.
};
template <class Iter>
struct iterator_traits_new {
...
};
template <class T>
struct iterator_traits_new<MyVecIter<T>> {
...
};
template <class FwdIter, class FwdIter2>
void copy_new(FwdIter && itbeg, FwdIter && itend, FwdIter2 &&
itbegcopy)
{
typedef std::iterator_traits_new<FwdIter> tr;
typedef std::iterator_traits_new<FwdIter2> tr2;
}
You use here a perfect forwarding signature, which has consequences,
see below.
int main(int argc, char * argv[])
{
int arr[10] = {3, 4, 5, 1, 2, 7};
std::vector<int> arr2(10);
copy_new(&arr[0], &arr[0] + 6, arr2.begin());
MyVecIter<int> veciter(arr2.data()), veciter2(arr2.data() +
arr2.size());
//This alogrithm specializes iterator_traits_new<FwdIter> with the
general template
//instead of the specialization
std::copy_new(veciter, veciter2, &arr[0]);
}
If someone can help. I'm confused why this doesn't work. I think that
MyVecIter<int> should
choose the partial specialization for iterator_traits_new.
Any help? Thanks for your time.
Let me first add that your code doesn't compile because
of many reasons and providing such snippets should actually
be banned ;-)
Your example uses lvalues are arguments to copy_new
(and I'm ignoring the std::qualifier here, because
your own specialization of copy_new is not in namespace
std and would never be considered in the second call).
According to the perfect forwarding rules, the relevant
template parameter FwdIter will be deduced to MyVecIter<int>&
(it would be deduced to MyVecIter<int> if the argument were
an rvalue). You could solve the problem, if you would either
- specialize iterator_traits_new also for MyVecIter<T>&, but
I don't recommend that
- ensure that the type provided to iterator_traits_new is
"reference-free":
typedef iterator_traits_new<typename
std::remove_reference<FwdIter>::type> tr;
HTH & Greetings from Bremen,
Daniel Kr?gler
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