variable-templates.cpp

#include <iostream>

using namespace std;

template <typename T, typename = typename std::enable_if<std::is_floating_point<T>::value, void>::type>
constexpr T pi = 3.14159265358979323846264338327950288419716939937510582097494459230781640628620899862803482534211706798;

int main()
{
  float radiusf;
  cin >> radiusf;
  float areaf = pi<float> * radiusf * radiusf;

  double radius;
  cin >> radius;
  double area = pi<double> * radius * radius;

  cout << radiusf << endl;
  cout << radius << endl;
}