如何编写类型特征“is_container"或“is_vector"? [英] How to write a type trait `is_container` or `is_vector`?

问题描述

是否可以编写一个值对所有常见 STL 结构(例如，vector、set、map、map、...)?

首先，我想编写一个类型特征，它对 vector 为真，否则为假.我试过这个，但它不能编译:

template结构 is_vector {static bool const value = false;};模板<T类，U类>struct is_vector>>::类型>{static bool const value = true;};

错误信息是模板参数未用于部分特化:U.

解决方案

看，另一个基于 SFINAE 的用于检测类 STL 容器的解决方案:

template结构 is_container : std::false_type {};模板结构 is_container_helper {};模板struct is_container<,std::conditional_t<错误的，is_container_helper<类型名称 T::value_type，类型名称 T::size_type,类型名称 T::allocator_type,类型名 T::iterator,类型名 T::const_iterator,decltype(std::declval().size()),decltype(std::declval().begin()),decltype(std::declval().end()),decltype(std::declval().cbegin()),decltype(std::declval().cend())>,空白>>:公共 std::true_type {};

当然，您可以更改要检查的方法和类型.

如果您只想检测 STL 容器(这意味着 std::vector、std::list 等)，您应该执行类似 这个.

更新.正如@Deduplicator 所指出的，容器可能不满足 AllocatorAwareContainer 要求(例如:std::array).这就是为什么不需要检查 T::allocator_type 的原因.但是您可以以类似的方式检查任何/所有Container要求.>

Is it possible to write a type trait whose value is true for all common STL structures (e.g., vector, set, map, ...)?

To get started, I'd like to write a type trait that is true for a vector and false otherwise. I tried this, but it doesn't compile:

template<class T, typename Enable = void>
struct is_vector {
  static bool const value = false;
};

template<class T, class U>
struct is_vector<T, typename boost::enable_if<boost::is_same<T, std::vector<U> > >::type> {
  static bool const value = true;
};

The error message is template parameters not used in partial specialization: U.

解决方案

Look, another SFINAE-based solution for detecting STL-like containers:

template<typename T, typename _ = void>
struct is_container : std::false_type {};

template<typename... Ts>
struct is_container_helper {};

template<typename T>
struct is_container<
        T,
        std::conditional_t<
            false,
            is_container_helper<
                typename T::value_type,
                typename T::size_type,
                typename T::allocator_type,
                typename T::iterator,
                typename T::const_iterator,
                decltype(std::declval<T>().size()),
                decltype(std::declval<T>().begin()),
                decltype(std::declval<T>().end()),
                decltype(std::declval<T>().cbegin()),
                decltype(std::declval<T>().cend())
                >,
            void
            >
        > : public std::true_type {};

Of course, you might change methods and types to be checked.

If you want to detect only STL containers (it means std::vector, std::list, etc) you should do something like this.

UPDATE. As @Deduplicator noted, container might not meet AllocatorAwareContainer requirements (e.g.: std::array<T, N>). That is why check on T::allocator_type is not neccessary. But you may check any/all Container requirements in a similar way.

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