how to detect if a type is an iterator or const_iterator

Question

I\'m wondering, if there is a way to check at compile time whether a type T of some iterator type is a const_iterator, or not. Is there some difference in the types that ite

Answer 1

One method that works at least on gcc is via the reference typedef:

struct true_type { };
struct false_type { };

template
struct is_const_reference
{
    typedef false_type type;
};

template
struct is_const_reference
{
    typedef true_type type;
};

template
struct is_const_iterator
{
    typedef typename is_const_reference<
        typename std::iterator_traits::reference>::type type;
};

You can verify that it works by using

inline bool test_internal(true_type)
{
    return true;
}

inline bool test_internal(false_type)
{
    return false;
}

template
bool test(T const &)
{
    return test_internal(typename is_const_iterator::type());
}

bool this_should_return_false(void)
{
    std::list l;
    return test(l.begin());
}

bool this_should_return_true(void)
{
    std::list const l;
    return test(l.begin());
}

With a sufficiently high optimization level, the last two functions should be reduced to return false; and return true;, respectively. At least they do for me.