Detect if a container has iterator type or not

Question

Consider the following pseudocode:

template<class Container>
int some_function(const Container& container)
{
      if (container has iterator) { // 
            // get an element by its iterator
      } else {
            // do another action
      }
}

So, we have a function template which takes a container type(e.g., vector, list, valarray or something else). Is it possible to determine (runtime) if a given container has iterator type? Compile time?

Answer 1

You use the detection idiom

#include<experimental/type_traits>

template<typename T>
using iterator_t = typename T::iterator;

template<typename T>
constexpr bool has_iterator = std::experimental::is_detected_v<iterator_t, T>;

template<class Container>
int some_function(const Container& container)
{
      if constexpr (has_iterator<Container>) {
            // get an element by its iterator
      } else {
            // do another action
      }
}