Getting the refrence of a template iterator refrence

Question

Hello all :)

I need to get a reference to an iterator of a reference. However, my compiler is choking on this code:

template <typename InputIterator> size_t iLongestBegin(InputIterator first, InputIterator last)
{
    typedef typename std::iterator_traits<InputIterator>::reference SequenceT;
        //Problem is next line
    typedef typename std::iterator_traits<typename SequenceT::iterator>::reference T;
    for(size_t idx; idx < first->length(); idx++)
    {
     T curChar = (*first)[idx];
     for (InputIterator cur = first; cur != last; cur++)
     {
      if (cur->length() < idx)
       return idx;
      if (_tolower(cur->at(idx)) != _tolower(curChar))
       return idx;
     }
    }
    return first->length();
}

Any ideas on how to fix it? The error is

error C2825: 'SequenceT': must be a class or namespace when followed by '::'

Thanks! Billy3

Answer 1

Did you miss specifying SequenceT in the template argument list - Where is it defined? Or maybe you must indicate that it's a type with typename SequenceT.

Answer 2

You need to write typename SequenceT::iterator instead of just SequenceT::iterator. This is because SequenceT is a type that is derived from your template parameters (a "dependent type", in standard lingo), and iterator is a nested type in SequenceT, not a function or variable. When both of these things are true, the compiler can't figure out what you mean and needs to be told that SequenceT::iterator is a type with typename.

Answer 3

The following compiles with g++ 4.4.0:

#include <iterator>
using namespace std;

template <typename InputIterator> size_t iLongestBegin(InputIterator first, InputIterator last) {
   typedef typename std::iterator_traits<InputIterator>::reference SequenceT;
   typedef typename std::iterator_traits<typename SequenceT::iterator>::reference T;
   for(size_t idx; idx < first->length(); idx++)
   {
      T curChar = (*first)[idx];
      for (InputIterator cur = first; cur != last; cur++)
      {
         if (cur->length() < idx)
             return idx;
         if (_tolower(cur->at(idx)) != _tolower(curChar))
             return idx;
      }
   }
    return first->length();
}

Answer 4

Actually, just solved it :)

Problem is that SequenceT is a reference, not a type. Since you can't generally take the address of a reference type, the compiler won't generate iterators for it. I need to use value_type instead of reference:

template <typename InputIterator> size_t iLongestBegin(InputIterator first, InputIterator last)
{
    typedef typename std::iterator_traits<InputIterator>::reference SequenceT;
    typedef typename std::iterator_traits<std::iterator_traits<InputIterator>::value_type::iterator>::reference T;
    for(size_t idx; idx < first->length(); idx++)
    {
     typename T curChar = (*first)[idx];
     for (InputIterator cur = first; cur != last; cur++)
     {
      if (cur->length() < idx)
       return idx;
      if (_tolower(cur->at(idx)) != _tolower(curChar))
       return idx;
     }
    }
    return first->length();
}