std :: sort在模板函数中

Question

This is the part of my code that doesn't work:

template<typename T>
list<T> f(list<list<T> >& lst,
          void (*op)(list<T>&, list<T>&))
{
    list<list<T> >::iterator itr = lst.begin();

    int count = 0;
    for_each(lst.begin(), lst.end(),
             [&count](list<T> l){ count += l.size(); });

    list<T> res(count);
    res.insert(res.end(), itr->begin(), itr->end());

    sort(res.begin(), res.end());

    return res;
}

VS2012 compiler says error C2784:

error C2784: ''unknown-type' std::operator -(std::move_iterator<_RanIt> &,const 
std::move_iterator<_RanIt2> &)' : could not deduce template argument for 
'std::move_iterator<_RanIt> &' from 'std::_List_iterator<_Mylist>'

I tried calling

sort<T>(res.begin(), res.end());

but got the message:

cannot convert parameter 1 from 'std::_List_iterator<_Mylist>' to 'int'

Answer 1

While I cannot tell you why those particular error messages, what I can tell you is that std::sort requires random access iterators, and std::list<>::iterator are not such.

You can use std::list<>::sort()

---

Actually the first error message is not that hard to understand. The implementation has attempted to substract two list iterators, and there is no operator to do that. ADL brought the std namespace into the search and the compiler found the overload of operator- to substract two std::move_iterator<> (of random iterators), but the template type _RanIt is in a non-deducible context and that failed. Just focus on the core issue: the compiler could not find a valid operator-

The second error message is related to the fact that you provided the wrong template argument. The type argument to the std::sort template is the iterator type, and the signature is:

template <typename Iterator>
std::sort(Iterator begin, Iterator end);

When you provided T (which seems to be int in this case) the compiler generated the specialization:

template <>
std::sort(int begin, int end)

But in your call you are trying to pass std::list<int>::iterator as argument, and the compiler cannot convert from that into int .

Answer 2

You can't use std::sort on a std::list because a list is not random-access (more specifically, its iterators are not RandomAccessIterators ).

If you want to sort a list , use its member function std::list<T>::sort() instead:

list<T> res(count);
res.insert(res.end(), itr->begin(), itr->end());
res.sort();