问题描述
如下例所示,我需要我的 Component
结构才能访问子结构中定义的所有“字段”结构。但我有一些限制:
Component
和 Field
结构的模板参数可以随意修改,只要 Field
具有其 TType
参数。重要提示,我使用的是 C++20。
// * CRTP stands for CurIoUsly recurring template pattern
template <typename TCrtp>
struct Component
{
template <typename TType>
struct Field
{
using Type = TType;
using Component = TCrtp;
};
using ComponentType = TCrtp;
// Because TCrtp is the inheriting class,the 'TCrtp::Fields' aka 'TestComponent::Fields' type
// can be accessed from here to do anything I need to
};
struct TestComponent : Component<TestComponent>
{
struct Field1: Field<bool> {};
struct Field2: Field<float> {};
// Problem: Can we find a way to fill this tuple automatically
// either from this class or the parent one without using header tool
// or even macros if this is possible
// The goal here is to avoid the programer that is creating this class to repeat itself
// by having to fill manually this tuple and thus potentially forgetting a field,that would cause him
// some troubles (bugs) later on...
using Fields = std::tuple<Field1,Field2>;
};
不幸的是,C++ 不允许在模板参数中进行类型声明。
我也已经尝试使用和修改这个 answer 来生成我的代码,但这里的问题是宏需要考虑 2 个参数而不是一个(一个用于字段名称,并且一个用于类型),这使得它非常棘手,因为需要相当多的逻辑才能实现我所需要的。
解决方法
使用 MACRO,达到硬编码限制,您可以执行以下操作:
#define COUNT_N(_1,_2,_3,_4,_5,_6,_7,_8,_9,_10,N,...) N
#define COUNT(...) COUNT_N(__VA_ARGS__,10,9,8,7,6,5,4,3,2,1)
#define CONCAT(A,B) A ## B
#define FIELDS_VAR(N) CONCAT(FIELDS_,N)
#define USING_VAR(N) CONCAT(USING_,N)
#define TAKE_FIRST(_1,...) _1
#define FIELD(name,type) struct name : Field<type>{};
#define FIELDS_1(_1) FIELD _1
#define FIELDS_2(_1,_2) FIELD _1 FIELD _2
#define FIELDS_3(_1,_3) FIELD _1 FIELD _2 FIELD _3
#define USING_1(_1) using Fields = std::tuple<TAKE_FIRST _1>
#define USING_2(_1,_2) using Fields = std::tuple<TAKE_FIRST _1,TAKE_FIRST _2>
#define USING_3(_1,_3) using Fields = std::tuple<TAKE_FIRST _1,TAKE_FIRST _2,TAKE_FIRST _3>
#define FIELDS(...) FIELDS_VAR(COUNT(__VA_ARGS__)) (__VA_ARGS__) \
USING_VAR(COUNT(__VA_ARGS__)) (__VA_ARGS__)
struct TestComponent : Component<TestComponent>
{
FIELDS((Field1,bool),(Field2,float));
};