C++20 Concepts: Element iterable concept

Question

I am trying to create a concept ElementIterable which can determine the type is nested ranges or not. For example, the elements in std::vector<int> is not iterable, but the elements (std::vector<int>) in std::vector<std::vector<int>> is iterable. The idea about using std::iterator_traits<T> comes up in my mind and the experimental code is as following. However, this ElementIterable concept doesn't work as the expected behavior. Is there any idea to fix this ElementIterable concept?

template<typename T>
concept ElementIterable = requires(typename std::iterator_traits<T>::value_type x)                        //  requires-expression
{
    x.begin();          // must have `x.begin()`
    x.end();            // and `x.end()`
};

The usage of this ElementIterable is here.

template<typename T> requires ElementIterable<T>
void Foo(T input);

template<typename T> requires ElementIterable<T>
void Foo(T input)
{
    std::cout << "Element iterable" << std::endl;
}

template<typename T>
void Foo(T input);

template<typename T>
void Foo(T input)
{
    std::cout << "Element not iterable" << std::endl;
}

The usage of the function Foo.

int number = 1;
    
std::vector<decltype(number)> vector1;
vector1.push_back(number);
Foo(vector1);           //  Element not iterable

std::vector<decltype(vector1)> vector2;
vector2.push_back(vector1);
Foo(vector2);           //  Element not iterable
                        //      but expected behaviour is: Element iterable

All suggestions are welcome.

Answer 1

If you want to ask if a type is a range which itself contains a range, that's simply applying the std::range type twice:

template<typename T>
concept nested_range = std::ranges::range<T> && std::ranges::range<std::ranges::range_value_t<T>>

range_value_t extracts the value_type from the iterator type of the range. Here's a live example.