Check if a variable type is iterable?

Question

Is there any way to check if an arbitrary variable type is iterable?

So to check if it has indexed elements or I can actually loop over it\'s children? (Use foreach

Answer 1

cpprefence has an example answering your question. It is using SFINAE, here is a slightly modified version of that example (in case the content of that link gets changed over time):

template 
struct is_iterable : std::false_type {};

// this gets used only when we can call std::begin() and std::end() on that type
template 
struct is_iterable())),
                                  decltype(std::end(std::declval()))
                                 >
                  > : std::true_type {};

// Here is a helper:
template 
constexpr bool is_iterable_v = is_iterable::value;

Now, this is how it can be used

std::cout << std::boolalpha;
std::cout << is_iterable_v> << '\n';
std::cout << is_iterable_v> << '\n';
std::cout << is_iterable_v << '\n';
struct A;
std::cout << is_iterable_v << '\n';

Output:

true
true
false
false

Having said that, all it checks is, the declaration of begin() const and end() const, so accordingly, even following is verified as an iterable:

struct Container
{
  void begin() const;
  void end() const;
};

std::cout << is_iterable_v << '\n'; // prints true

You can see these pieces together here