使用可变 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 classes 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，我在没有可用的 C++ 库的独立环境中工作。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功能，特征。对于每个特征集，应该有一个特征class，比如：

/* 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中class：

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（只是 template<typename DERIVED>），这个想法似乎就起作用了。但正如上面的示例，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<> class 模板的实现以支持可变特征的模板参数？

非常感谢愿意帮助我的人！马丁

编辑：代码随着讨论在答案下方进行调整...

Answer 1

您可能会使用可变参数模板：

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

使用可变 CRTP 的智能指针工具包

Smart pointer toolkit using variadic CRTP

c++

variadic-templates

smart-pointers

c++20

crtp