CODEWITHSUNDEEP
c++templatesiteratorc++11
moswald
moswald

Reputation: 11677

Can I get the "iterator" for a template type, regardless if that type is an array or STL-like container?

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.

Upvotes: 2

Views: 357

Answers (2)

Xeo
Xeo

Reputation: 131809

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;

Upvotes: 2

Kerrek SB
Kerrek SB

Reputation: 477160

That's only one single specialization, how bad could that be?

template <typename T> struct ContainerTrait
{
    typedef typename T::iterator iterator;
    typedef typename T::const_iterator const_iterator;
};

template <typename T, unsigned int N> struct ContainerTrait<T[N]>
{
    typedef T * iterator;
    typedef T const * const_iterator;
};

Alternatively, you can use the free std::begin/std::end and auto:

auto it = std::begin(x);  // x could be vector<int> or float[10]...

Upvotes: 1

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