使用可变参数 CRTP 的智能指针工具包

Question

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、observing_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。让我在里面描述一下我的想法。

设计注意事项： 我想使用可变 CRTP 方法来混合所需的指针特性，即特征。对于每个特征集，都应该有一个特征类，例如：

/* 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 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>），这个想法似乎就有效。但如上所示，情况并非如此，例如 Tagged 特征或 Owning 特征分别需要更多模板参数，例如 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...

Answer 1

你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>;

Demo