Question

Here’s my example:

template<typename TContainer>
class MyClass
{
public:
   typedef typename SomeUnknownHelper<TContainer>::iterator iterator;
};

std::vector<int>::iterator i = MyClass<std::vector<int>>::iterator;
int *pi = MyClass<int[20]>::iterator;

Basically, I don’t know how to write SomeUnknownHelper.

I know I could specialize MyClass itself, but in my real-world case it would be a hassle because the class is large.

Answer 1

That’s easily doable with decltype and std::begin:

#include <iterator>
#include <utility>

namespace tricks{
  using std::begin; // fallback for ADL
  template<class C>
  auto adl_begin(C& c) -> decltype(begin(c)); // undefined, not needed
  template<class C>
  auto adl_begin(C const& c) -> decltype(begin(c)); // undefined, not needed
}

template<typename TContainer>
class MyClass
{
public:
   typedef decltype(tricks::adl_begin(std::declval<TContainer>())) iterator;
};

std::vector<int>::iterator i = MyClass<std::vector<int>>::iterator;
int *pi = MyClass<int[20]>::iterator;

An even better option might be using Boost.Range:

#include <boost/range/metafunctions.hpp>

template<typename TContainer>
class MyClass
{
public:
   typedef typename boost::range_iterator<TContainer>::type iterator;
};

std::vector<int>::iterator i = MyClass<std::vector<int>>::iterator;
int *pi = MyClass<int[20]>::iterator;