C ++中的空和std :: shared_ptr之间的区别是什么？ [英] What is the difference between an empty and a null std::shared_ptr in C++?

问题描述

cplusplus.com shared_ptr 页面通话区分空 std :: shared_ptr 和 null shared_ptr 。 cppreference.com页面未显式调用区分，但使用空和比较 std :: shared_ptr 行为

中 nullptr

空和 shared_ptr 之间是否有区别？这样的混合行为指针有什么用例吗？是否非空的null shared_ptr 甚至有意义？在正常使用中是否会出现一个情况（即如果你没有明确构造一个），你可以结束一个空但是非空 shared_ptr ？

如果您使用的是Boost版本而不是C ++ 11版本，那么这些答案是否会改变？

这是 shared_ptr 行为的一个奇怪的角落。它有一个构造函数，允许你使拥有某事的 shared_ptr 和指向 >

 模板<类Y> 
 shared_ptr（const shared_ptr< Y& r，T * ptr）; 
  

使用此构造函数构建的 shared_ptr >分享所有权与 r ，但指向， ptr （即调用 get（）或 operator->（）将返回 ptr ）。这对于 ptr 指向 r 拥有的对象的子对象（例如，数据成员）

您链接的页面调用了不含任何内容的 shared_ptr shared_ptr 指向无（ get（）== nullptr ） null 。 （ ） shared_ptr ，但它不会很有用。一个空的但不是null null shared_ptr 本质上是一个非拥有的指针，可以用来做一些奇怪的事情，如传递一个指向堆栈上分配的某个指针的函数，期望一个 shared_ptr （但是我建议你先在API里面加入 shared_ptr ）。

boost :: shared_ptr 也有这个构造函数，它们称为别名构造函数

。

The cplusplus.com shared_ptr page calls out a distinction between an empty std::shared_ptr and a null shared_ptr. The cppreference.com page doesn't explicitly call out the distinction, but uses both "empty" and comparison to nullptr in its description of std::shared_ptr behavior.

Is there a difference between an empty and a null shared_ptr? Is there any use case for such mixed-behavior pointers? Does a non-empty null shared_ptr even make sense? Would there ever be a case in normal usage (i.e. if you didn't explicitly construct one) where you could end up with an empty-but-non-null shared_ptr?

And do any of these answers change if you're using the Boost version instead of the C++11 version?

解决方案

It's a weird corner of shared_ptr behavior. It has a constructor that allows you to make a shared_ptr that owns something and points to something else:

template< class Y > 
shared_ptr( const shared_ptr<Y>& r, T *ptr );

The shared_ptr constructed using this constructor shares ownership with r, but points to whatever ptr points to (i.e., calling get() or operator->() will return ptr). This is handy for cases where ptr points to a subobject (e.g., a data member) of the object owned by r.

The page you linked calls a shared_ptr that owns nothing empty, and a shared_ptr that points to nothing (i.e., whose get() == nullptr) null. (Empty is used in this sense by the standard; null isn't.) You can construct a null-but-not-empty shared_ptr, but it won't be very useful. An empty-but-not-null shared_ptr is essentially a non-owning pointer, which can be used to do some weird things like passing a pointer to something allocated on the stack to a function expecting a shared_ptr (but I'd suggest punching whoever put shared_ptr inside the API first).

boost::shared_ptr also has this constructor, which they call the aliasing constructor.

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