c++ - 你能 static_assert 一个元组只有一种类型满足特定条件吗？

Question

此代码获取满足测试的第一个元组元素(在本例中，如果它派生 Base )。 如何 static_assert 认为只有一个满足该条件的元素可能存在？(很像 C++17 的 std::get<class> 如果提供的类型名存在多个元素，则无法编译。)

template<class...>
struct voider { using type = void; };

template<class...Ts>
using void_t = typename voider<Ts...>::type;

template<template<class...> class Test, class Tuple>
struct get_first_pass;

template<template<class...> class Test, class Tuple>
using get_first_pass_t = typename get_first_pass<Test, Tuple>::type;

template<template<class...> class, class, class...>
struct first_pass {};

template<template<class...> class Test, class T0, class...Ts>
struct first_pass<Test, std::enable_if_t<!Test<T0>::value>, T0, Ts...> :
  first_pass<Test, void, Ts...>
{};

template<template<class...> class Test, class T0, class...Ts>
struct first_pass<Test, std::enable_if_t<Test<T0>::value>, T0, Ts...> {
  using type = T0;
};

template<template<class...> class Test, template<class...> class Tuple, class...Ts>
struct get_first_pass<Test, Tuple<Ts...>> : first_pass<Test, void, Ts...>
{};

template<class Base>
struct is_derived_from {
  template<class Derived>
  using test = std::is_base_of<std::decay_t<Base>, std::decay_t<Derived>>;
};

template<class Base, class Tuple>
using get_first_derived =
  get_first_pass_t<is_derived_from<Base>::template test, Tuple>;

template<class Base, class Tuple>
auto get_from_base(Tuple&& tuple)
-> decltype(std::get< get_first_derived<Base, std::decay_t<Tuple>> >(
                                                  std::forward<Tuple>(tuple))) {
  return std::get< get_first_derived<Base, std::decay_t<Tuple>> >(
                                                  std::forward<Tuple>(tuple));
}

Answer 1

不确定您到底想要什么。

但我想，如果你写一个“计数器”如下

// count is derived from
template <typename, typename>
struct count_idf : public std::integral_constant<std::size_t, 0U>
 { };

template <typename Base, template <typename...> class C, typename ... Ts>
struct count_idf<Base, C<Ts...>>
   : public std::integral_constant<std::size_t,
     (is_derived_from<Base>::template test<Ts>::value + ...)>
 { };

您可以在get_from_base()中添加一个static_assert()，如下所示

template<class Base, class Tuple>
auto get_from_base(Tuple&& tuple)
-> decltype(std::get< get_first_derived<Base, std::decay_t<Tuple>> >(
                                                  std::forward<Tuple>(tuple))) {
  static_assert( 1U == count_idf<Base, std::decay_t<Tuple>>{}, 
                 "not one and only one" );
  return std::get< get_first_derived<Base, std::decay_t<Tuple>> >(
                                                  std::forward<Tuple>(tuple));
}