std::tuple 作为模板参数？

Question

我正在尝试编写一个std::sort 模板比较类，它应该接收未知数量的元组（可变参数模板）。每个元组应该由一个列（我们的代码中的某种类型）和一个 bool 组成，指定该列是按升序还是降序排序。

基本上，我想要类似的东西：

// doesn't compile - conceptual code
template <typename std::tuple<Col, bool>>
struct Comparator
{
    bool operator() (int lhs, int rhs)
    {
         // lhs and rhs are row indices. Depending on the columns
         // and the bools received, decide which index should come first
    } 
}

这种事情在 C++ 11 中可能吗？

Answer 1

是的，有可能 - 您想要 Comparator 的部分专业化：

template <typename T>
struct Comparator;

template <typename Col>
struct Comparator<std::tuple<Col, bool>>
{
    // ...
};

Answer 2

这可能吗？是的，但你需要一些相当丑陋的模板技巧。

//a trait for checking if a type is of the form std::tuple<T,bool>
template <class Tuple>
struct is_col_bool_tuple : false_type{};

template <typename Col>
struct is_col_bool_tuple<std::tuple<Col,bool>> : true_type{};

//a helper struct for checking if all the values in a boolean pack are true
template<bool...> struct bool_pack;
template<bool... bs> 
using all_true = std::is_same<bool_pack<bs..., true>, bool_pack<true, bs...>>;

//a trait to check if a list of types are all of the form std::tuple<T,bool>
template <class... Tuples>
using are_col_bool_tuples = all_true<is_col_bool_tuple<Tuples>::value...>;

//an incomplete type for when we pass incorrect template arguments
//this impl helper is needed because variadic parameters need to be last
template <typename Enable, class... Tuples>
struct ComparatorImpl;

//our specialized implementation for when the template arguments are correct
template <class... Tuples>
struct ComparatorImpl<std::enable_if_t<are_col_bool_tuples<Tuples...>::value>,
                      Tuples...>
{
     bool operator() (int lhs, int rhs)
    {
         //do your comparison
    } 
};

//a nice alias template for forwarding our types to the SFINAE-checked class template
template <class... Tuples>
using Comparator = ComparatorImpl<void, Tuples...>;