When would I use std::integral_constant over constexpr?

Trevor Hickey picture Trevor Hickey · Dec 4, 2013 · Viewed 11.6k times · Source
#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

ForEveR picture ForEveR · Dec 4, 2013

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