C ++中的空传播运算符 [英] Null propagation operator in C++

问题描述

是否有人在C ++中实现了空传播运算符，类似于功能语言中使用的传播运算符？我正在考虑一些聪明的模板解决方案，可能类似于operator->的传播行为。

Has anyone implemented a null propagation operator in C++, similar to that used in functional languages? I'm thinking of some clever template solution, possibly akin to the propagating behavior of operator->.

让我们假设我们有一个来自图的对象链，例如 foo-> bar-&b; baz （对不起，得墨meter耳法则）。假设其中任何一个可以为null，并且应在取消引用之前进行测试。然后，代码突然变得更加复杂：

Let's suppose we have a chain of objects from a graph, like foo->bar->baz (sorry, Law of Demeter). Suppose any one of these could be null, and should be tested before dereferencing. Then the code suddenly becomes much more complicated:

if( !foo )
  return nullptr;
if( !foo->bar )
  return nullptr;
return foo->bar->baz;

我想使用某种紧凑的语法分解空检查：

I'd like to "factor out" the null checks with some kind of compact syntax like this:

foo?->bar?->baz   // imaginary null-propagation syntax

现在，当然不必看起来像那样，只需紧凑即可。我想需要的是C ++中的monad，可以启用null测试和 continuation。最好避免使用宏和lambda，但这可能是不可能的。我可以想象如果 this 为null，则会在每个步骤中重载operator->（）并短路。

Now, of course it doesn't have to look like that, just be nearly as compact. I suppose what's needed is a monad in C++, enabling the null test and a 'continuation'. It would be great to avoid macros and lambdas, but that's probably not possible. I could imagine overloading operator->() at each step and short-circuiting out if this was null.

但这很令人讨厌。理想的解决方案是将每个对象包装在链中。

But that's very intrusive. An ideal solution would wrap each object in the chain.

推荐答案

在设计的这个阶段，这可能是一个根本性的偏离，但也许可以考虑使用空对象模式。然后，您根本不需要任何null检查。

At this stage in your design it might be a radical departure, but perhaps consider using the Null Object Pattern. Then you don't need any null checks at all.

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