C++17中模板参数auto的优点

Question

auto v1 = constant<5>;      // v1 == 5, decltype(v1) is int
auto v2 = constant<true>;   // v2 == true, decltype(v2) is bool
auto v3 = constant<'a'>;    // v3 == 'a', decltype(v3) is char

Answer 1

在芬兰奥卢举行的ISO C++2016会议上，模板 特性(P0127R1)被C++接受。

模板参数中的auto关键字可用于指示非类型参数，其类型是在实例化时推导出来的。把这看作是一种更方便的写作方式是有帮助的：

template <typename Type, Type value>

例如，

template <typename Type, Type value> constexpr Type constant = value;
constexpr auto const IntConstant42 = constant<int, 42>;

template <auto value> constexpr auto constant = value;
constexpr auto const IntConstant42 = constant<42>;

template <auto ... vs> struct HeterogenousValueList {};
using MyList1 = HeterogenousValueList<42, 'X', 13u>;

template <auto v0, decltype(v0) ... vs> struct HomogenousValueList {};
using MyList2 = HomogenousValueList<1, 2, 3>;

template <typename T, T ... vs> struct Cxx14HomogenousValueList {};
using MyList3 = Cxx14HomogenousValueList<int, 1, 2, 3>;

template <typename ... ValueTypes> struct Cxx14HeterogenousValueList {};
using MyList4 = Cxx14HeterogenousValueList<constant<int, 42>,
                                           constant<char, 'X'> >;