为什么shared_ptr没有虚拟解析器？ （我怎么能解决这个问题？） [英] Why doesn't shared_ptr have a virtual descructor? (and how can I get around this?)

问题描述

我想创建一个特殊版本的 shared_ptr ，在创建或销毁时执行特定操作，但我的计划似乎被 shared_ptr 的析构函数是非虚拟的，意味着当我重写它，我的指针永远不会被清除，当他们的最后一个实例被销毁。

I wanted to make a special version of shared_ptr that would perform specific operations when it was created or destroyed, but my plans appear to be foiled by the realization that shared_ptr's destructor is non virtual, meaning when I override it, my pointers never get cleaned up when the last instance of them are destroyed.

想到的唯一替代方法是将这个行为构建到我想要与我的假设自定义 shared_ptr 一起使用的每个类中，这是不可行的或在某些情况下可能）。

The only alternative that comes to mind is to build in this behavior into every class that I want to use with my hypothetical custom shared_ptr, and that's not feasible (or possible in some cases).

编辑：

我想要的原因是因为我想在lua中使用一些类作为userdata对象，我想要我的每个对象，我使用这种方式有一个fenv表是唯一的，将被清理时，对该对象的所有引用已删除。我计划使用指针的地址作为他们的钥匙到一个表中保存fenv表。

The reason I want this is because I want to use some classes as userdata objects in lua, and I want each one of my objects that I use this way to have a fenv table unique to it that will be cleaned up when all references to the object have been removed. I plan on using the address of the pointer as they key into a table that holds the fenv table.

让我说我有一个小部件，可以有其他小部件作为孩子。我在Lua中创建两个小部件，然后将其中一个设置为另一个的子对象，并删除对子小部件的所有lua引用（事实上，它是一个使用C ++处理的子代）。 GC现在可以随时运行并删除孩子。我不一定希望孩子有它的析构函数运行，所以我想做一个shared_ptr。这样，C ++对象仍然可以使用它，在Lua清理它。如果我已经为它的fenv分配了值或函数，我仍然希望能够访问它们。

Lets say I have a widget that can have other widgets as children. I create two widgets in Lua, then set one as the child of the other and remove all lua references to the child widget (the fact that it's a child is handled in C++). The GC can now run at any time and remove the child. I don't necessarily want the child to have it's destructor run though, so I want to make it a shared_ptr. That way, C++ objects can still use it after Lua has cleaned it up. If I've assigned values or functions to it's fenv I still want to be able to access them. Only when the final reference to my child widget is removed do I want the fenv tabled to be removed totally.

推荐答案

它已经有了这种能力建立在不需要让人们做危险的事情，如从中得到：

It already has this ability built in without the need to let people do dangerous things like derive from it:

#include <boost/shared_ptr.hpp>
#include <iostream>

/*
 * Done as a function for simplicity.
 * But this can be done in so many ways
 */
void MyCleanup(int* x)
{
    std::cout << "DONE\n";
    delete x;
}

int main()
{
    boost::shared_ptr<int>  x(new int(5), MyCleanup);

}

导出问题：

Problem with deriving:
Just off the top of my head.

class X: public shared_ptr<int> { /* STUFF. With a special destructor. */ };

int main()
{
    /* what happens now? Similar to slicing but not quite */
    X                data1(new int(5));
    shared_ptr<int>  data2;
    shared_ptr<int>  data3(data);

    data2 = data1;
}

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