删除使用 C++17 pmr 内存资源分配的多态对象

Question

我想构建一棵由 Node 类型的多态对象组成的树，这些对象是使用自定义 PMR 分配器分配。

到目前为止，一切正常，但我不知道如何正确删除使用非标准分配器分配的多态对象？我只是想出了一个解决方案来声明一个静态对象持有对 std::pmr::memory_resource.. 的引用，但这很讨厌。是否有任何 "right" 删除自定义分配的多态对象的方法？

这是一个自包含的例子：

#include <iostream>
#include <string>
#include <vector>
#include <array>
#include <functional>
#include <memory_resource>
        
struct Node {
    // NOTE: this is actually not necessary..
    using allocator_type = std::pmr::polymorphic_allocator<Node>;

    void operator delete(void *ptr, std::size_t sz) noexcept {
        Node::deleter(ptr, sz);
    }
    // don't bother with getters/setters so far..
    std::pmr::string name;

    template <class TNode >
    static TNode *create(std::string_view name, std::pmr::memory_resource *res) {

        std::pmr::polymorphic_allocator<TNode> alloc(res);
        auto ptr = alloc.allocate(1);
        ::new (ptr) TNode(alloc);
        ptr->name = name;
        return ptr;
    }
    virtual ~Node() {
        std::cerr << "Destructing node: " << name << std::endl;
    }

    // NASTY: pointer to memory resource to delete polymorphic objects..
    static std::pmr::memory_resource *s_deleterResource;

protected:
    Node(const allocator_type& alloc) : name(alloc) {}

    static void deleter(void *ptr, std::size_t sz) noexcept {
        if (s_deleterResource != nullptr) {
            std::cerr << "Deleting mem: " << ptr << " of size: " << sz << " using PMR resource\n";
            std::pmr::polymorphic_allocator< char >(s_deleterResource)
                .deallocate((char *)ptr, sz);
        }
        else {
            std::cerr << "Deleting mem: " << ptr << " of size: " << sz << " using default\n";
            ::operator delete(ptr, sz);
        }
    }
};

decltype (Node::s_deleterResource) Node::s_deleterResource = nullptr;

struct CompoundNode : Node {

    friend struct Node;
    using allocator_type = std::pmr::polymorphic_allocator<CompoundNode>;

    void operator delete(void *ptr, std::size_t sz) noexcept {
        Node::deleter(ptr, sz);
    }

    void addChild(Node *child) {
        m_children.push_back(child);
    }

    ~CompoundNode() override {
        for(auto child : m_children) {
            delete child;
        }
    }

protected:
    explicit CompoundNode(const allocator_type& alloc) :
        Node(alloc), m_children(alloc)
    { }

    std::pmr::vector< Node * > m_children;
};

struct LeafNode : Node {

    friend struct Node;
    using allocator_type = std::pmr::polymorphic_allocator<LeafNode>;

    void operator delete(void *ptr, std::size_t sz) noexcept {
        Node::deleter(ptr, sz);
    }
    ~LeafNode() override {
        // NOTE: this is probably won't work since the object m_payload is not yet destroyed
        //allocator_type alloc(m_payload.get_allocator());
        //alloc.deallocate(this, 1);
    }

protected:
    explicit LeafNode(const allocator_type& alloc) :
        Node(alloc), m_payload(77, alloc) { }

    std::pmr::vector< uint8_t > m_payload;
};

// adding verbosity to the existing memory resource
struct VerboseMemResource : public std::pmr::memory_resource {

    VerboseMemResource(const char *name, std::pmr::memory_resource *base)
            : m_name(name), m_resource(base) {  }

private:

    void *do_allocate(std::size_t bytes, std::size_t alignment) override {

        auto ptr = m_resource->allocate(bytes, alignment);
        std::cerr << "Allocated: " << bytes << " bytes with '" << m_name << "': " << ptr << std::endl;
        return ptr;
    }

    void do_deallocate(void *ptr, std::size_t bytes, std::size_t alignment) override {

        std::cerr << "Deallocating " << bytes << " bytes with '" << m_name << "': " << ptr << std::endl;
        return m_resource->deallocate(ptr, bytes, alignment);
    }

    bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override {
        return m_resource->is_equal(other);
    }

    std::string m_name;
    std::pmr::memory_resource *m_resource;
};

int main() try
{
    std::array< uint8_t, 1000 > buf;
    std::pmr::monotonic_buffer_resource bufferRes(buf.data(), buf.size(),
                                       std::pmr::null_memory_resource());
    VerboseMemResource res("buffered resource", &bufferRes);

    auto root = Node::create<CompoundNode>("root", &res);
    root->addChild(Node::create<LeafNode>("child1", &res));
    root->addChild(Node::create<LeafNode>("child2", &res));

    // set the pointer to our memory resource for deletion:
    Node::s_deleterResource = &res;
    std::cerr << "Beginning tree deletetion..\n";
    delete root;
    Node::s_deleterResource = nullptr;
    return 0;
}
catch(std::exception& ex) {
    std::cerr << "Exception: " << ex.what() << std::endl;
    return 1;
}

这是输出：

Allocated: 80 bytes with 'buffered resource': 0000006D21FDF740
Allocated: 80 bytes with 'buffered resource': 0000006D21FDF790
Allocated: 77 bytes with 'buffered resource': 0000006D21FDF7E0
Allocated: 8 bytes with 'buffered resource': 0000006D21FDF830
Allocated: 80 bytes with 'buffered resource': 0000006D21FDF838
Allocated: 77 bytes with 'buffered resource': 0000006D21FDF888
Allocated: 16 bytes with 'buffered resource': 0000006D21FDF8D8
Deallocating 8 bytes with 'buffered resource': 0000006D21FDF830
Beginning tree deletetion..
Deallocating 77 bytes with 'buffered resource': 0000006D21FDF7E0
Destructing node: child1
Deleting mem: 0000006D21FDF790 of size: 80 using PMR resource
Deallocating 80 bytes with 'buffered resource': 0000006D21FDF790
Deallocating 77 bytes with 'buffered resource': 0000006D21FDF888
Destructing node: child2
Deleting mem: 0000006D21FDF838 of size: 80 using PMR resource
Deallocating 80 bytes with 'buffered resource': 0000006D21FDF838
Deallocating 16 bytes with 'buffered resource': 0000006D21FDF8D8
Destructing node: root
Deleting mem: 0000006D21FDF740 of size: 80 using PMR resource
Deallocating 80 bytes with 'buffered resource': 0000006D21FDF740

Answer 1

问题

在 C++20 之前，无法调用未首先调用您的 class' 析构函数的释放函数 (operator delete)，从而使您无法清理增加你的 class 拥有的额外明确分配的资源（没有像你的静态指针这样的骇人听闻的代码）

解决方案

如果您可以访问 C++20，那么我鼓励您使用 destroying delete，它是为解决此类问题而创建的。

您的 class 可以持有 std::pmr::memory_resource* 的实例（通过构造函数注入）
将您的 operator delete 更改为例如 void operator delete(Node *ptr, std::destroying_delete_t) noexcept
- destroying_delete 是一个标签，当您使用它时，表示您将负责调用适当的析构函数。
派生的 classes 也应该实现类似的删除器。

无需对您的代码进行太多更改，我们可以在 Node 中执行以下操作：

struct Node {
    // NOTE: this is actually not necessary..
    using allocator_type = std::pmr::polymorphic_allocator<Node>;

    void operator delete(Node *ptr, std::destroying_delete_t) noexcept {
        deleter(ptr);
    }
    // don't bother with getters/setters so far..
    std::pmr::string name;

    template <class TNode >
    static TNode *create(std::string_view name, std::pmr::memory_resource *res) {

        std::pmr::polymorphic_allocator<TNode> alloc(res);
        auto ptr = alloc.allocate(1);
        ::new (ptr) TNode(alloc, res);
        ptr->name = name;
        return ptr;
    }
    virtual ~Node() {
        std::cerr << "Destructing node: " << name << std::endl;
    }

protected:
    Node(const allocator_type& alloc, std::pmr::memory_resource *res)
     : name(alloc), s_deleterResource(res) {}

    std::pmr::memory_resource *s_deleterResource = nullptr;  

    template<class TNode>
     static void deleter(TNode* ptr) noexcept {
        if (ptr->s_deleterResource != nullptr) {
            auto* deleterResource = ptr->s_deleterResource;
            ptr->~TNode();
            std::cerr << "Deleting mem: " << ptr  << " using PMR resource\n";
            std::pmr::polymorphic_allocator< TNode >(deleterResource)
                .deallocate(ptr, 1);
        }
        else {
            std::cerr << "Deleting mem: " << ptr << " using default\n";
            ::delete ptr;
        }
    }
};

然后在例如LeafNode中你可以这样写：

struct LeafNode : Node {
    friend struct Node;
    using allocator_type = std::pmr::polymorphic_allocator<LeafNode>;

    void operator delete(LeafNode *ptr, std::destroying_delete_t) noexcept {
        deleter(ptr);
    }

protected:
    explicit LeafNode(const allocator_type& alloc, std::pmr::memory_resource *res) :
        Node(alloc, res), m_payload(77, alloc) { }

    std::pmr::vector< uint8_t > m_payload;
};

Live Demo

Allocated: 88 bytes with 'buffered resource': 0x7ffebb5906d0
Allocated: 88 bytes with 'buffered resource': 0x7ffebb590728
Allocated: 77 bytes with 'buffered resource': 0x7ffebb590780
Allocated: 8 bytes with 'buffered resource': 0x7ffebb5907d0
Allocated: 88 bytes with 'buffered resource': 0x7ffebb5907d8
Allocated: 77 bytes with 'buffered resource': 0x7ffebb590830
Allocated: 16 bytes with 'buffered resource': 0x7ffebb590880
Deallocating 8 bytes with 'buffered resource': 0x7ffebb5907d0
Beginning tree deletetion..
Deallocating 77 bytes with 'buffered resource': 0x7ffebb590780
Destructing node: child1
Deleting mem: 0x7ffebb590728 using PMR resource
Deallocating 88 bytes with 'buffered resource': 0x7ffebb590728
Deallocating 77 bytes with 'buffered resource': 0x7ffebb590830
Destructing node: child2
Deleting mem: 0x7ffebb5907d8 using PMR resource
Deallocating 88 bytes with 'buffered resource': 0x7ffebb5907d8
Deallocating 16 bytes with 'buffered resource': 0x7ffebb590880
Destructing node: root
Deleting mem: 0x7ffebb5906d0 using PMR resource
Deallocating 88 bytes with 'buffered resource': 0x7ffebb5906d0

（注意 class 有点大，因为它持有一个指针而不是那个指针 static）

删除使用 C++17 pmr 内存资源分配的多态对象

Deleting polymorphic objects allocated with C++17 pmr memory resource

c++

polymorphism

memory-management

allocator

c++17

问题

解决方案

Live Demo