【C++ TMP】The void_t idiom

Since C++ 17, we can use the variadic template class std::void_t to implement class specialization. Typically, it is used to detect whether a type supports some map of type.

Example: remove_all_extents for array-like types

#include <type_traits>
#include <utility>

template <typename T, typename = void>
struct remove_all_extents {
  typedef std::remove_reference_t<T> type;
};

template <typename T>
struct remove_all_extents<T, std::void_t<decltype(std::declval<T>()[0])>> {
    typedef typename remove_all_extents<decltype(std::declval<T>()[0])>::type type;
};

#include <vector>
#include <iostream>

int main() {
    std::cout << std::boolalpha;
    std::cout << std::is_same_v<remove_all_extents<int[10]>::type, int> << "
";    
    std::cout << std::is_same_v<remove_all_extents<double[10]>::type, double> << "
";
    std::cout << std::is_same_v<remove_all_extents<std::vector<int>>::type, int> << "
"; 
    using v2 = std::vector<std::vector<int>>;   
    std::cout << std::is_same_v<remove_all_extents<v2>::type,int> << "
";
}

https://godbolt.org/z/hvG3chYfx

std::void_t is used here to detect whether the given type T supports the subscript operator.

References

https://en.cppreference.com/w/cpp/types/void_t