An example using c++ constraints and concept templates

constraint_template_example.cc

// Function template constraint (implicit or "ad-hoc")

template<typename T> requires 
requires (T a) {  a + a;  } &&
requires (T b) {  b * b;  } &&
requires (T c) { sqrt(c); }
T Magnitude(vector<T>&& vec) {
  T sum;
  for (auto& const i : vec) { sum += i*i; }
  return sqrt(sum);
}

// Explicit requirements (not "ad-hoc") which can be reused in function templates


template<typename T>
concept bool Addable = requires(T a) { a + a; };

template<typename T>
concept bool Multipliable = requires(T a) { a * a; };

template<typename T>
concept bool Rootable = requires(T a) { sqrt(a); };


template<typename T> requires Addable<T> && Multipliable<T> && Rootable<T>
T Magnitude(vector<T>&& vec) {
  T sum;
  for (auto& const i : vec) { sum += i*i; }
  return sqrt(sum);
}

///// Type constraints 

// We can also require a template type T to have certain properties (or traits)
template<typename T> concept bool VectorType = requires () {
    typename T::elements;   // required member
    typename Matrix<T>;     // required class template specialization
};