c++ - 将整数常量映射到类型

Question

我正在经历Alexandrescu's book它演示了典型整数值以允许编译时调度的有用概念:

template <int Val>
struct Elastic
{
    enum {V = Val};
};

template <class Object, bool isElastic>
class ImpactMomentum
{
    double calc_momentum(double v_in, Elastic<true> /*  */)
    {
        // compute elastic ...
    }

    double calc_momentum(double v_in, Elastic<false> /*  */)
    {
        // compute rigid ...
    }
 public:
    double calc_momentum(double v_in)
    {
        calc_velocity(v_in, Elastic<isElastic>());
    }
};

是否有取代这个习语的现代 C++ 实现？当函数的参数列表中有多个标志可以切换时，可以很好地扩展。

Answer 1

就在这里。这个想法是使用类型而不是 bool (或数字)表达式。您的案例相当琐碎，但在处理更复杂的属性时很有帮助，并且对以后的扩展更加开放。

我将添加另一个虚构的弹性类型“Alien”，只是为了演示扩展。

考虑一下:

// empty types
struct Elastic { enum {V = Val}; };
struct Rigid   {};
struct Alien   {};

template <class Object, class Elasticity>
class ImpactMomentum
{
    // for libraries wanting to give users even more expansion options, 
    // without modifying the library, these specializations could be 
    // regular free functions also taking *this as a parameter, for 
    // example.

    double calc_momentum(double v_in, Elastic)  // or const & if property has data
    {
        // ...
    }

    double calc_momentum(double v_in, Rigid)
    {
        // ...
    }

    double calc_momentum(double v_in, Alien)
    {
        // ...
    }

 public:
    double calc_momentum(double v_in)
    {
        return calc_velocity(v_in, Elasticity{});
    }
};