嵌套模板类中的std :: conditional

Question

I'm trying to implement a RingBuffer in the style of the STL. This means I'm also implementing an iterator for it that has to work as either const or non-const. This is just the iterator part:

#include <iterator>
#include <type_traits>

template <typename T> class RingBuffer {
public:
    class Iterator;
    // actual RingBuffer implementation here
};    

template <typename T, bool is_const=false> 
class RingBuffer<T>::Iterator {
public:
    typedef std::ptrdiff_t difference_type;
    typedef T value_type;
    typedef typename std::conditional<is_const, const value_type*, value_type*>::type pointer ;
    typedef typename std::conditional<is_const, const value_type&, value_type&>::type reference ;
    typedef std::random_access_iterator_tag iterator_category;

    // a bunch of functions here
    ...
};

GCC 4.8.0 gives errors for every line where I try to access an iterator, saying something like

no type named 'type' in 'struct std::conditional<is_const, const int*, int*>'

Substituting int for the type that RingBuffer<T> has been instantiated with. I don't get it. is_const has a default value. Why doesn't this work? And why doesn't GCC subsitute in false in the error message, like it substituted int for value_type ?

The solution is probably obvious but all the googling in the world didn't get me anywhere. Templates are still kind of confusing to me.

Answer 1

If you want Iterator to also be templated by bool is_const , you have to declare it as such:

template <typename T> class RingBuffer {
public:
    template <bool is_const = false>
    class Iterator;
    // actual RingBuffer implementation here
};    


template <typename T>
template <bool is_const> 
class RingBuffer<T>::Iterator {
public:
    typedef std::ptrdiff_t difference_type;
    typedef T value_type;
    typedef typename std::conditional<is_const, const value_type*, value_type*>::type pointer ;
    typedef typename std::conditional<is_const, const value_type&, value_type&>::type reference ;
    typedef std::random_access_iterator_tag iterator_category;

    // a bunch of functions here
    ...
};

Explanation: Iterator is a member of a class template, but in your original code, Iterator itself was a non-template class. RingBuffer had one template parameter, T ; Iterator was a non-template class; there just wasn't anywhere for is_const to appear. It will become clearer if we remove the outer class for a moment:

class Foo;

template <bool b = false>
class Foo
{
  // something
};

I believe it's obvious the above wouldn't work.