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: 
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](http://ideone.com/469YTq)
It can be used with the ternary operator
eg

```
void gotoN_impl(std::integral_constant<int,0>::type)
{
    std::cout << "GoTo 0" << '\n';
}

void gotoN_impl(std::integral_constant<int,1>::type)
{
    std::cout << "GoTo 1" << '\n';
}

void gotoN_impl(std::integral_constant<int,2>::type)
{
    std::cout << "GoTo 2" << '\n';
}

void gotoN_impl(std::integral_constant<int,3>::type)
{
    std::cout << "GoTo 3" << '\n';
} 

template<int N>
void gotoN()
{
    gotoN_impl(typename std::integral_constant<int, N>::type());
}

int main()
{
    gotoN<0>();
    gotoN<1>();
    gotoN<2>();
    gotoN<3>();

    constexpr auto x = 99;

    gotoN<x<4?x:3>(); // with a ternary operator
}

```