Variable Templates in C++14

Question:

While studying the new C++ standard, I came across the innovation “ variable templates ”

The template syntax is as follows:

template < typename T >
constexpr T value = T(1234);

About the application of the template it is written:

This feature allows you to create and use constexpr variable templates, for a more convenient combination with template algorithms.

I don't quite understand how such a variable would be used in a template algorithm . Could you give some examples of using this mechanism? I also don’t understand why it is impossible to replace such a template constexpr expression with a non- template one, because we specify the value explicitly, respectively, and we can write (deduce) the type explicitly.

Answer:

There is an example in the standard:

template<class T>
constexpr T pi = T(3.1415926535897932385L);

template<class T>
T circular_area(T r) {
  return pi<T> * r * r;
}

Here the variable template allows you to get a constant of the desired size – float/double/etc.

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Another popular use is to replace is_some<T>::value with is_some_v<T> , e.g.

template< class T, class U >
inline constexpr bool is_same_v = is_same<T, U>::value;  // C++17