Non-negative signed integer type (C++14)

non_negative.hpp

#pragma once

#include <cstdint>
#include <stdexcept>
#include <type_traits>

template<
  class T,
  typename = std::enable_if_t<std::is_integral_v<T> && std::is_signed_v<T>>
>
class non_negative {
public:
  // Can't be a valid default construction, I think (?)
  non_negative() = delete;
  
  non_negative(T _number) {
    if (_number < static_cast<T>(0)) {
      throw std::runtime_error("Please enter non-negative number.");
    }
    number = _number;
  }
  
  // Implicit conversion to underlying integral type
  inline operator T() const { return static_cast<T>(number); }

private:
  T number;
};

/*
// Example use

void foo(int64_t cant_be_negative);

non_negative<int64_t> num = INT64_C(5);
foo(num); // implicitly convert to 

// compile error
non_negative<double> num = 5.;

// runtime error
non_negative<int64_t> num = -5;
*/