When would I use std::integral_constant over constexpr?

Question

#include <iostream> #include <type_traits>  int main(){      //creating an integral constant with constexpr     constexpr unsigned int speed_of_light{299792458};      //creating an integral constant with std::integral_constant     typedef std::integral_constant<unsigned int, 299792458> speed_of_light_2;      //using them     std::cout << speed_of_light/2 << '\n';     std::cout << speed_of_light_2::value/2 << '\n';  } 

What's special about std::integral_constant that I would choose to use it over constexpr?
Their behaviour and use cases look identical to me. I'm trying to think of some kind of template scenario, where constexpr may not suffice.

Answer 1

Template integral_constant defines a type, keyword constexpr defines a constant. For example std::true_type is std::integral_constant<bool, true>.

One of the usage examples is tag-dispatching.

template<typename T> void use_impl(const T&, std::false_type) { }  template<typename T> void use_impl(const T&, std::true_type) { }  template<typename T> void use(const T& v) {    use_impl(v, typename std::is_integral<T>::type()); } 

Live example