如何enable_shared_from_this既parend和衍生 [英] How to enable_shared_from_this of both parend and derived

问题描述

我有简单的基类和派生类，我想都有 shared_from_this（）。

I have simple base and derived class that I want both have shared_from_this().

这个简单的解决方案：

class foo : public enable_shared_from_this<foo> {
	void foo_do_it()
	{
		cout<<"foo::do_it\n";
	}
public:
	virtual function<void()> get_callback()
	{
		return boost::bind(&foo::foo_do_it,shared_from_this());
	}
	virtual ~foo() {};
};

class bar1 : public foo , public enable_shared_from_this<bar1> {
	using enable_shared_from_this<bar1>::shared_from_this;
	void bar1_do_it()
	{
		cout<<"foo::do_it\n";
	}
public:
	virtual function<void()> get_callback()
	{
		return boost::bind(&bar1::bar1_do_it,shared_from_this());
	}
};

导致异常 TR1 :: bad_weak_ptr 在以下code：

shared_ptr<foo> ptr(shared_ptr<foo>(new bar1));
function<void()> f=ptr->get_callback();
f();

所以，谷歌搜索后，我发现了以下解决方案：

So after "googling" I have found following solution:

class bar2 : public foo {
	void bar2_do_it()
	{
		cout<<"foo::do_it\n";
	}
	shared_ptr<bar2> shared_from_this()
	{
		return boost::static_pointer_cast<bar2>(foo::shared_from_this());
	}
public:
	virtual function<void()> get_callback()
	{
		return boost::bind(&bar2::bar2_do_it,shared_from_this());
	}
};

和现在的作品。

有没有为父母和孩子到 enable_shared_from_this 的更好，更方便易和正确的方式？

Is there any better and more convinient and correct way to enable_shared_from_this for both parent and child?

感谢

推荐答案

很抱歉，但没有。

问题是，的shared_ptr＆LT;富＆GT; 和的shared_ptr＆LT; BAR1＆GT; 是不同的类型。我不明白的引擎盖下发生的一切，但我的认为的，当构造函数返回并分配给的shared_ptr＆LT;富＆GT; ，内部的weak_ptr＆LT; BAR1＆GT; 认为没有什么指向它（因为只有的shared_ptr＆LT; BAR1＆GT; 将增加柜台），并自动复位。当你调用 BAR1 :: shared_from_this 在 get_callback ，你得到的异常，因为内部的的weak_ptr 不指向任何东西。

The problem is that shared_ptr<foo> and shared_ptr<bar1> are different types. I don't understand everything that's going on under the hood, but I think that when the constructor returns and is assigned to a shared_ptr<foo>, the internal weak_ptr<bar1> sees that nothing is pointing to it (because only a shared_ptr<bar1> would increment the counter) and resets itself. When you call bar1::shared_from_this in get_callback, you get the exception because the internal weak_ptr isn't pointing to anything.

从本质上讲， enable_shared_from_this 似乎只能从一个类层次结构中透明地工作。如果您尝试<一个href=\"http://www.boost.org/doc/libs/1%5F38%5F0/libs/smart%5Fptr/sp%5Ftechniques.html#from%5Fthis\">implementing它手动的，这个问题应该成为显而易见的。

Essentially, enable_shared_from_this only seems to work transparently from a single class in a hierarchy. If you try implementing it manually, the problem should become obvious.

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