gsl :: not_null< T *> vs. std :: reference_wrapper< T>与T& [英] gsl::not_null<T*> vs. std::reference_wrapper<T> vs. T&

问题描述

最近，我提出了 C ++核心指南（congrats！），我非常关心 gsl :: not_null 类型。如 I.12：声明一个不能为null的指针 not_null ：

帮助避免解除引用nullptr错误。要通过
提高性能，避免对nullptr进行冗余检查。

...

通过说明
源中的意图，实现者和工具可以提供更好的诊断，例如
通过静态分析找到一些类错误，并执行
优化，例如删除分支和空测试。

目的很清楚。但是，我们已经有了一个语言功能。不能为空的指针称为引用。虽然引用创建后不能被反弹，但是这个问题可以通过 std :: reference_wrapper 来解决。

gsl :: not_null 与 std :: reference_wrapper之间的主要区别我看到后者只能用来代替指针，而前者在任何 nullptr -assignable（quote from F.17：使用not_null表示null不是有效值）：

<$> / code>不只是用于内置指针。它适用于
array_view ， string_view ， unique_ptr ， shared_ptr 和其他
类指针类型。

我想象功能比较表如下：

T& ：

• 无法存储 nullptr ？ - 是


• 可重新绑定？ - 否


• 可以使用指针之外的其他方式吗？ - 否

std :: reference_wrapper< T> ：

• 无法存储 nullptr ？ - 是


• 可重新绑定？ - 是


• 可以使用指针以外的其他方式吗？ - 否

gsl :: not_null< T *> c $ c>：

• 无法存储 nullptr ？ - 是


• 可重新绑定？ - 是


• 可以使用指针以外的其他方式吗？ - 是

现在是问题，最后是：

1. 这些概念之间的区别是否正确？


2. 这是否意味着 std :: reference_wrapper 我创建了标签 cpp-c $ c>现在没有用了？





<核心指南和 guideline-support-library 为此，我希望正确。

解决方案

我认为 std :: reference_wrapper 还有用例:: not_null 。基本上， std :: reference_wrapper 镜像一个引用并且具有运算符T& 转换，而 not_null 有一个具有 operator-> 的指针接口。一个使用情况立即到我的脑海里是创建一个线程：

  void funcWithReference（int& x）{x = 42; } 
 int i = 0; 
 auto t = std :: thread（funcWithReference，std :: ref（x））; 
  

如果我不能控制 funcWithReference ，我不能使用 not_null 。

这同样适用于算法的函子，我不得不使用它 boost :: signals 。

C++ Core Guidelines has been presented recently (congrats!) and I am concerned about gsl::not_null type. As stated in I.12: Declare a pointer that must not be null as not_null:

To help avoid dereferencing nullptr errors. To improve performance by avoiding redundant checks for nullptr.

...

By stating the intent in source, implementers and tools can provide better diagnostics, such as finding some classes of errors through static analysis, and perform optimizations, such as removing branches and null tests.

The intent is clear. However, we already have a language feature for that. Pointers that cannot be null are called references. And while references cannot be rebound once they are created, this problem is solved by std::reference_wrapper.

The main difference between gsl::not_null and std::reference_wrapper I see in that the latter can be used only instead of pointers, while the former works on anything nullptr-assignable (quote from F.17: Use a not_null to indicate that "null" is not a valid value):

not_null is not just for built-in pointers. It works for array_view, string_view, unique_ptr, shared_ptr, and other pointer-like types.

I imagine the feature comparison table like the following:

T&:

• Cannot store nullptr? - Yes
• Rebindable? - No
• Can be used instead of something other than pointers? - No

std::reference_wrapper<T>:

• Cannot store nullptr? - Yes
• Rebindable? - Yes
• Can be used instead of something other than pointers? - No

gsl::not_null<T*>:

• Cannot store nullptr? - Yes
• Rebindable? - Yes
• Can be used instead of something other than pointers? - Yes

Now here are the questions, finally:

1. Is my understanding of differences between these concepts correct?
2. Does that mean that std::reference_wrapper is now useless?

PS I created tags cpp-core-guidelines and guideline-support-library for this, I hope properly.

解决方案

I think that there are still use-cases for std::reference_wrapper which are not covered by gsl::not_null. Basically, std::reference_wrapper mirrors a reference and has a operator T& conversion, while not_nullhas a pointer interface with operator->. One use-case that comes to my mind immediatly is when creating a thread:

void funcWithReference(int& x) { x = 42; }
int i=0;
auto t = std::thread( funcWithReference, std::ref(x) );

If I don't have control over funcWithReference, I cannot use not_null.

The same applies to functors for algorithms, and I had to use it for binding boost::signals too.

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