#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.
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());
}