类模板参数扣除类，并带有其他非类型参数

Question

在这种情况下，是否可以使CTAD适用于t？

enum class boundary_type:int{inclusive, exclusive};

template<class T, boundary_type left, boundary_type right>
struct interval
{
    using value_type = T;

    T begin;
    T end;
};

我试图添加分点指南

template<boundary_type left, boundary_type right, class T>
interval(T, T) -> interval<T, left, right>;

，但仍然有错误

模板参数数量错误（2，应为3）

尝试时

interval<boundary_type::inclusive, boundary_type::exclusive>{0, 2}

Answer 1

CTAD无法与明确指定的模板参数一起使用 。但是您可以使用普通模板功能来执行此操作

template<boundary_type left, boundary_type right, class T>
auto make_interval(T begin, T end) {
  return interval<T, left, right>{begin, end};
}

int main() {
  auto intval = make_interval<boundary_type::inclusive, 
                              boundary_type::exclusive>(0, 2);
}

Answer 2

有了更多的代码，您可以通过为包含有关这些包装器提供的信息的边界信息的边界提供包装器类型来使用类似的语法：使用：

enum class boundary_type :int { inclusive, exclusive };

template<class T>
struct exclusive
{
    static constexpr ::boundary_type boundary_type = ::boundary_type::exclusive;
    using value_type = T;

    explicit exclusive(T bound)
        : m_bound(bound)
    {
    }

    operator T() const
    {
        return m_bound;
    }

    T m_bound;
    
};

template<class T>
struct inclusive
{
    static constexpr boundary_type boundary_type = ::boundary_type::inclusive;
    using value_type = T;

    inclusive(T bound)
        : m_bound(bound)
    {
    }

    operator T() const
    {
        return m_bound;
    }

    T m_bound;
    
};

template<class T, boundary_type left, boundary_type right>
struct interval
{
    using value_type = T;

    T begin;
    T end;
};

template<class Left, class Right>
interval(Left, Right) -> interval<
    std::common_type_t<typename std::remove_cvref_t<Left>::value_type, typename std::remove_cvref_t<Right>::value_type>,
    std::remove_cvref_t<Left>::boundary_type,
    std::remove_cvref_t<Right>::boundary_type
>;

使用

interval intrvl{ inclusive(0), exclusive(2) };
static_assert(std::is_same_v<decltype(intrvl), interval<int, boundary_type::inclusive, boundary_type::exclusive>>);