问题描述
我即将设计和实现一种智能指针工具包 - 一组类来定义各种类型的智能指针,如 unique_ptr、intrusive_ptr、shared_ptr、observation_ptr、tagged_ptr 等。顺便提一下,我在独立环境中工作我没有可用的 C++ 库。我的干预是避免代码重复,使其遵循优雅的设计原则。让我在那里描述我的想法。
设计注意事项: 我想使用可变参数 CRTP 方法来混合所需的指针特征和特征。对于每个功能集,都应该有一个特征类,如:
/* Functions to support move semantics like move constructor,move assignment operator */
template<typename DERIVED>
class MoveablePointer { ... };
/* Functions to manage pointee lifetime - using the deleter class */
/* EDIT: template<typename DERIVED,typename DELETER = DefaultDeleter<typename DERIVED::element_type>> */
template<typename DERIVED,typename DELETER> /* <-------- Default removed here as EDIT */
class Owning {
public:
using deleter_type = DELETER;
/* ... */
};
/* Functions to use least significant bits effectively unused due to alignment to hold additional payload information */
template<typename DERIVED,typename TAGS,size_t ALIGNMENT>
class Tagged { ... };
然后使用可变参数CRTP模式将特征混合到一个智能指针类中:
template<template<typename> class... TRAITS>
struct make_traits {
template<typename DERIVED>
struct Traits : public TRAITS<DERIVED>... {};
};
template<typename T,template<typename> class POINTER_TRAITS>
class SmartPointer : public POINTER_TRAITS<SmartPointer<T,POINTER_TRAITS>> {
public:
using pointer = T*;
using element_type = T;
/* ... */
};
template<typename T,typename DELETER = DefaultDeleter<T>>
using unique_ptr = SmartPointer<T,make_traits<MoveablePointer,Owning>>;
template<typename T>
using observer_ptr = SmartPointer<T,make_traits<Observing,NoNowning,copyable,Moveable>>;
/* ... and to continue defining type aliases to cover varIoUs smart pointer variations... */
基本上,一旦特定特征具有相同的模板参数集(仅 template<typename DERIVED>
),这个想法似乎就可以工作。但如上例所示,情况并非如此,例如 Tagged
trait 或 Owning
trait 分别需要更多模板参数,如 ALIGNMENT
或 DELETER
。
所以我的问题是 - 如何更改 make_traits 和 SmartPointer 类模板的实现以支持可变参数 trait 的模板参数?
非常感谢任何愿意帮助我的人!马丁
编辑:随着讨论在答案下方流动而调整代码...
解决方法
您可以使用可变参数模板:
template<template<typename,typename...> class... TRAITS>
// ^^^^^^^^^^^^^
struct make_traits {
template<typename DERIVED>
struct Traits : public TRAITS<DERIVED>... {};
};
template<typename T,template<typename> class POINTER_TRAITS>
class SmartPointer : public POINTER_TRAITS<SmartPointer<T,POINTER_TRAITS>> {
public:
using pointer = T*;
using element_type = T;
/* ... */
};
template<typename T,typename DELETER = DefaultDeleter<T>>
using unique_ptr = SmartPointer<T,make_traits<MoveablePointer,Owning>::template Traits>;
template<typename T>
using observer_ptr = SmartPointer<T,make_traits<Observing,NonOwning,Copyable,MoveablePointer>::template Traits>;