c++ - 指定类模板参数的要求

Question

我有几个类是这样的:

struct neg_inf {
    constexpr double operator()() { return -std::numeric_limits<double>::infinity(); }
};

struct pos_inf {
    constexpr double operator()() { return std::numeric_limits<double>::infinity(); }
};

template<typename dX, class LowerBound, class UpperBound>
class limit {
    dX dx;
    UpperBound upperBound;
    LowerBound lowerBound;
    double step_size;

    limit( dX x, LowerBound lower, UpperBound upper, double step = 1 ) :
        dx{ x }, lowerBound{ lower }, upperBound{ upper }, step_size{ step }
    {}

    dX value() const { return dx; }
    LowerBound lower() const { return lowerBound; }
    UpperBound upper() const { return upperBound; }
    double step() const { return step_size; }            
};

到目前为止，这些类(class)按预期工作。现在我想使用条件语句修改模板参数，例如 std::enable_if_t、std::is_arithemtic std::is_same。

这些是实例化限制对象需要满足的条件。

• dX must be at least arithmetic and numerical
• Lower & Upper bound must be either numerical, arithmetic or neg_inf or pos_inf.

例如，这些是有效的实例化:

dX = 1st and can be any: int, long, float, double, /*complex*/, etc.

limit< 1st, 1st, 1st >     // default template

limit< 1st, 1st, pos_inf >  // these will be specializations
limit< 1st, 1st, neg_inf >
limit< 1st, pos_inf, 1st >
limit< 1st, neg_inf, 1st >
limit< 1st, pos_inf, pos_inf >
limit< 1st, neg_inf, neg_inf >
limit< 1st, neg_inf, pos_inf >
limit< 1st, pos_inf, neg_inf >

这些是实例化我的模板的有效条件。当 UpperBound 和/或 LowerBound 是 infinity 类型时，我计划部分专门化此类。当 upper 和 lower 边界是数字 - 算术类型时，通用或默认模板将处理它们。

我的问题是，使用type_traits 库，我的类的模板声明 会是什么样子？

Answer 1

将模板类型限制为类的一种方法是为 SFINAE 添加额外的参数:

template <typename dX, class LowerBound, class UpperBound, typename Enabler = void>
class limit;

然后，提供适当的 SFINAE 特化

template <typename dX, class LowerBound, class UpperBound>
class limit<dX,
            LowerBound,
            UpperBound,
            std::enable_if_t<my_condition<dX, LowerBound, UpperBound>::value>>
{
    // ...
};

所以在你的情况下，my_condition 应该是这样的

template <typename dX, class LowerBound, class UpperBound>
using my_condition =
    std::conjunction<std::is_arithmetic<dX>,
                     std::disjunction<std::is_arithmetic<LowerBound>,
                                      std::is_same<LowerBound, neg_inf>,
                                      std::is_same<LowerBound, pos_inf>>,
                      std::disjunction<std::is_arithmetic<UpperBound>,
                                       std::is_same<UpperBound, neg_inf>,
                                       std::is_same<UpperBound, pos_inf>>
                      >;

另一种方式是static_assert:

template <typename dX, class LowerBound, class UpperBound>
class limit
{
    static_assert(std::is_arithmetic<dX>::value, "!");
    static_assert(std::is_arithmetic<LowerBound>::value
                  || std::is_same<LowerBound, neg_inf>::value
                  || std::is_same<LowerBound, pos_inf>::value, "!");
    static_assert(std::is_arithmetic<UpperBound>::value
                  || std::is_same<UpperBound, neg_inf>::value
                  || std::is_same<UpperBound, pos_inf>::value, "!");
    // ...
};