c++ - 避免在此模板代码中创建变量

Question

我有一个模板:

template<typename... Ts> //T1,T2,T3,...
struct foo {
  //my struct
};

我想对 T1、T3、T5...(“奇数类型”)和 T2、T4、T6...(“偶数类型”)进行 static_assert 检查) 分开。

我找到了这个简单的解决方案:

template<size_t N, typename... Ts>
struct perform_checks {};

template<size_t N, typename T, typename U, typename... Ts>
struct perform_checks<N, T, U, Ts...> : perform_checks<N, Ts...>
{
  //check for odd types
  static_assert(std::is_default_constructible<T>::value,"failure");

  //check for even types
  static_assert(std::is_copy_constructible<U>::value,"failure");
};

N 参数允许它结束。我这样使用它:

template<typename... Ts>
struct foo {
  perform_checks<0,Ts...> hello;
};

这似乎工作正常。但是有没有可能避免 hello 变量呢？我从不将它用于任何其他目的。

Answer 1

派生 foo来自 perform_checks<>私下:

template <typename... Ts> struct foo : private perform_checks<Ts...> {
 // stuff
};

哦，去掉 N您不需要的参数:

template <typename... Ts> struct perform_checks {};
template <typename T> struct perform_checks<T> {
  template <typename U> struct dependent_name_hack : std::false_type {};
  static_assert(dependent_name_hack<T>::value,
                "Odd number of parameters not acceptable.");
};
template <typename T, typename U, typename... Ts>
struct perform_checks<T, U, Ts...> : perform_checks<Ts...> {
  //check for odd types
  static_assert(std::is_default_constructible<T>::value,"failure");

  //check for even types
  static_assert(std::is_copy_constructible<U>::value,"failure");
};