具有责任转移的转换构造函数 [英] Conversion constructor with responsibility transfer

问题描述

这是此问题的后续行动。

我需要执行一个转换操作，以将成员对象的责任从一个对象传递到另一个对象。转换操作在类层次结构中进行。有一个 Base 类和两个派生类，例如 Child1 和 Child2 。在 Base 类中，有一个动态创建的对象，我需要从 Child1 传递到在进行转换时，Child2 （以及责任）不会让 Child1 的析构函数销毁它。我已经写了一个简单的示例来说明我要实现的目标：

I need to implement a conversion operation to pass responsibility for a member object from one object to another. The conversion operation takes place in a class hierarchy. There is a Base class and two deriving classes, say Child1 and Child2. In the Base class, there is a dynamically created object I need to pass from Child1 to Child2 (along with responsibility) while the conversion happens and not let Child1's destructor destroy it. I've written a simple example to illustrate what I'm trying to achieve:

#include <iostream>
using namespace std;

class Base {
public:
    Base() {
        p_int = new int; *p_int = 0;
        cout << "In Base constructor, reserving memory." << endl;
    }
    Base(const Base& other) : p_int(other.p_int), a(other.a) {
        cout << "In Base copy-constructor." << endl;
    }
    virtual ~Base() { delete p_int; p_int = NULL; cout << "Freeing memory." << endl; }

    void setpInt(int val) { *p_int = val; }
    void setInt(int val) { a = val; }
    virtual void print() {
        cout << "Base: ";
        cout << (long)p_int << ":" << *p_int << " " << a << endl;
    }
protected:
    int* p_int;
    int a;
};

class Child1 : public Base {
public:
    Child1() : Base() {};
    Child1(const Base& base) : Base(base) {}

    void print() {
        cout << "Child1: ";
        cout << (long)p_int << ":" << *p_int << " " << a << endl;
    }
};

class Child2 : public Base {
public:
    Child2() : Base() {};
    Child2(const Base& base) : Base(base) {}

    void print() {
        cout << "Child2: ";
        cout << (long)p_int << ":" << *p_int << " " << a << endl;
    }
};

int main() {
    Child1* c1 = new Child1();
    c1->setpInt(3);
    c1->setInt(2);
    c1->print();

    Child2* c2 = new Child2(*c1);
    c2->print();

    delete c1;      //Obviously c1's destructor is called here.
    c2->print();

    delete c2;

    return 0;
}

结果为：

In Base constructor, reserving memory.
Child1: 158711832:3 2
In Base copy-constructor.
Child2: 158711832:3 2
Freeing memory.
Child2: 158711832:0 2
Freeing memory.

有没有一种方法可以干净地做我想做的事情？我无法复制构造 p_int ，因为它很重。该项目是一个嵌入式AVR项目，因此可能无法使用智能指针或Boost库（尽管我不知道）-我只是记得这可能是一种解决方案，但我从未使用过它们。 / p>

Is there a way to do what I'm trying to do in a clean way? I can't copy-construct p_int because it's very heavy. The project is an embedded, AVR-project, so smart-pointers or boost library might not be available (I don't know, though) - I just recall this might be a solution, but I've never used them yet.

推荐答案

我认为您需要查看引用计数对象。本质上，对象本身会跟踪其使用情况，而不是使用指向引用的指针来存储指向该对象的原始指针。

I think you need to look at reference-counted objects. Essentially, the object itself tracks it's usage itself, and instead of storing a raw pointer to the object, you use a reference-counted pointer.

Scott Meyers在这里谈到：

Scott Meyers talks about this here:

http：//www.aristeia .com / BookErrata / M29Source.html

在嵌入式系统中，我不确定您是否可以使用 boost :: shared_ptr<> ，但是没有什么可以阻止您按照Meyers的概述来实现。

As you're on an embedded system, I'm not sure you can use boost::shared_ptr<> but there's nothing stopping you implementing this as Meyers outlines.

因此，与其原始指向在基类中有问题的对象具有共享的指针/引用计数的指针，一旦将其复制到 Child2 ，它将防止删除该对象。按照Child2的构造，它将有2个引用，因此删除Child1时不会死亡。但是，当删除Child2时，它将自动删除。

So, instead of having a raw pointer to the object in question in the base class, have a shared pointer/reference-counted pointer that will prevent the deletion of the object once it gets copied in to Child2. By the construction of Child2, it will have 2 references so won't die when Child1 gets deleted. It will, however, when Child2 is deleted.

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