C++ Insights

Source:

//clang 6.0.0

#include <iostream>
#include <utility>
#include <type_traits>
using namespace std::literals;
class Ball
{
public:
    template <typename T, typename T1>
    static constexpr auto best_match(T1 const& a, T const& b) { return b; }
    template <typename T, typename T2>
    static constexpr auto best_match(T const& a, T2 const& b) { return a; }
    
    template <typename T1 = std::string
            , typename T2 = std::conditional_t<std::is_convertible_v<T1, std::string>, double, std::string>
            , typename = std::enable_if_t<
                  (std::is_convertible_v<T1, std::string> && std::is_convertible_v<T2, double>)
               || (std::is_convertible_v<T2, std::string> && std::is_convertible_v<T1, double>)
              >
    >
    Ball(T1 f = best_match<T1>("red"s, 20.0), T2 s = best_match<T2>("red"s, 20.0))
        : m_color(best_match<std::string>(f, s)), m_radius(best_match<double>(f, s))
    { }
    
    std::string const& color() const {
        return m_color;    
    }
    double radius() const {
        return m_radius;    
    }    
private:
    std::string m_color;
    double m_radius;
};

std::ostream& operator<<(std::ostream& out, Ball const& ball) {
    return out << "c: " << ball.color() << " r: " << ball.radius();
}

int main()
{
    std::cout << Ball(20.0, "red"s) << std::endl;
    std::cout << Ball("red"s) << std::endl;
    std::cout << Ball(20.0) << std::endl;
    std::cout << Ball("red"s, 20.0) << std::endl;
    std::cout << Ball() << std::endl;
}

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