编译时的 C++ 类型注册技巧

2022-01-11 00:00:00 macros templates c++

我有以下情况:假设我有一堆类型(函子)，我想在编译期间注册/编译，最好是像 boost::mpl::vector 这样的类型.你知道有什么技巧可以做得很好吗?

I have the following situation: suppose I have a bunch of types (functors) which I want to register/compile in during compilation, preferably into something like boost::mpl::vector. Do you know any trick to do so nicely?

我的愿望是有一个实现函子类型和注册文件的 hpp 文件，其中宏将类型引入编译.

My desire is to have hpp file which implements functor type and registration file, where a macro brings in type into compilation.

例如

// registered.hpp
REGISTER("functor1.hpp") // implementation
REGISTER("functor2.hpp")
...
boost::mpl::vector<...> types; // full registration vector

希望这是有道理的.谢谢

Hopefully it makes sense. Thank you

推荐答案

有一种方法可以逐个注册类型，然后以 mpl::vector 或类似的形式检索所有类型.我在 boost 邮件列表上学到了这个技巧(也许是从 Dave Abrahams 那里学到的，虽然我记不太清了).

There is a way to register types one by one and then retrieve all of them in the form of mpl::vector or similar. I've learned this trick on the boost mailing lists (perhaps from Dave Abrahams, although I can't recall for sure).

我是从 https:///github.com/boostcon/2011_presentations/raw/master/thu/Boost.Generic.pdf.

我不会在代码中使用 MPL 来使其自包含.

I won't use MPL in the code to make it self contained.

// The maximum number of types that can be registered with the same tag.
enum { kMaxRegisteredTypes = 10 };

template <int N>
struct Rank : Rank<N - 1> {};

template <>
struct Rank<0> {};

// Poor man's MPL vector.
template <class... Ts>
struct TypeList {
  static const int size = sizeof...(Ts);
};

template <class List, class T>
struct Append;

template <class... Ts, class T>
struct Append<TypeList<Ts...>, T> {
  typedef TypeList<Ts..., T> type;
};

template <class Tag>
TypeList<> GetTypes(Tag*, Rank<0>) { return {}; }

// Evaluates to TypeList of all types previously registered with
// REGISTER_TYPE macro with the same tag.
#define GET_REGISTERED_TYPES(Tag) 
  decltype(GetTypes(static_cast<Tag*>(nullptr), Rank<kMaxRegisteredTypes>()))

// Appends Type to GET_REGISTERED_TYPES(Tag).
#define REGISTER_TYPE(Tag, Type)                              
  inline Append<GET_REGISTERED_TYPES(Tag), Type>::type        
  GetTypes(Tag*, Rank<GET_REGISTERED_TYPES(Tag)::size + 1>) { 
    return {};                                                
  }                                                           
  static_assert(true, "")

使用示例:

struct IntegralTypes;
struct FloatingPointTypes;

// Initially both type lists are empty.
static_assert(std::is_same<GET_REGISTERED_TYPES(IntegralTypes), TypeList<>>::value, "");
static_assert(std::is_same<GET_REGISTERED_TYPES(FloatingPointTypes), TypeList<>>::value, "");

// Add something to both lists.
REGISTER_TYPE(IntegralTypes, int);
REGISTER_TYPE(FloatingPointTypes, float);
static_assert(std::is_same<GET_REGISTERED_TYPES(IntegralTypes), TypeList<int>>::value, "");
static_assert(std::is_same<GET_REGISTERED_TYPES(FloatingPointTypes), TypeList<float>>::value, "");

// Add more types.
REGISTER_TYPE(IntegralTypes, long);
REGISTER_TYPE(FloatingPointTypes, double);
static_assert(std::is_same<GET_REGISTERED_TYPES(IntegralTypes), TypeList<int, long>>::value, "");
static_assert(std::is_same<GET_REGISTERED_TYPES(FloatingPointTypes), TypeList<float, double>>::value, "");

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