什么时候返回一个指向struct的指针是一个好主意？ [英] When is it a good idea to return a pointer to a struct?

问题描述

我正在学Go，我对使用指针的时间有点困惑。特别是，当从一个函数返回一个 struct c>时，什么时候返回结构实例本身是合适的，何时适合返回一个指向结构的指针？

示例代码：

  type Car结构{
 make string 
型号字符串
} 
 
 func无论（）{
 var car汽车
 
汽车：=汽车{honda，civic } 
 
 // ... 
 
返回汽车
} 
  

什么情况下我想返回一个指针，以及我不想 的地方？是否有一个很好的经验法则？

解决方案

有两件事情你要记住，性能和API。 p>

汽车如何使用？它是具有状态的对象吗？它是一个大型结构？不幸的是，当我不知道汽车是什么时，不可能回答。实际上，最好的办法是看看其他人做什么并复制它们。最终，你会对这种事情有一种感觉。我现在将描述来自标准库的三个示例，并解释为什么我认为他们使用了他们所做的。

1. hash / crc32 ： crc32。 NewIEEE（） 函数返回一个指针类型（实际上，一个接口，但底层类型是一个指针）。哈希函数的一个实例具有状态。当你将信息写入散列函数时，它会汇总数据，所以当你调用 Sum（）方法时，它会给你这个实例的状态。




2. 时间： time.Date 函数返回一个 Time 结构。为什么？时间是一个时间。它没有国家。它就像一个整数，你可以比较它们，对它们进行数学运算等。API设计者决定修改时间不会改变当前的一个，而是创建一个新的。作为图书馆的用户，如果我想从现在起一个月的时间，我会想要一个新的时间对象，而不是改变我现有的一个。一段时间也只有3个字的长度。换句话说，它很小，并且在使用指针时不会有任何性能提升。

$ c>： big.NewInt（）非常有趣。我们几乎同意，当你修改一个 big.Int 时，你通常会想要一个新的。一个 big.Int 没有内部状态，为什么它是一个指针？答案只是表现。程序员意识到，大整数是...大。每次进行数学运算时不断分配可能不切合实际。因此，他们决定使用指针，并允许程序员决定何时分配新空间。 题？可能不会。这是一项设计决定，您需要根据具体情况进行判断。当我设计我自己的库时，我使用标准库作为指导。这一切都归结为判断以及您期望客户端代码如何使用您的类型。



I'm learning Go, and I'm a little confused about when to use pointers. Specifically, when returning a struct from a function, when is it appropriate to return the struct instance itself, and when is it appropriate to return a pointer to the struct?

Example code:

type Car struct {
  make string
  model string
}

func Whatever() {
  var car Car

  car := Car{"honda", "civic"}

  // ...

  return car
}

What are the situations where I would want to return a pointer, and where I would not want to? Is there a good rule of thumb?

解决方案

There are two things you want to keep in mind, performance and API.

How is a Car used? Is it an object which has state? Is it a large struct? Unfortunately, it is impossible to answer when I have no idea what a Car is. Truthfully, the best way is to see what others do and copy them. Eventually, you get a feeling for this sort of thing. I will now describe three examples from the standard library and explain why I think they used what they did.

1. hash/crc32: The crc32.NewIEEE() function returns a pointer type (actually, an interface, but the underlying type is a pointer). An instance of a hash function has state. As you write information to a hash, it sums up the data so when you call the Sum() method, it will give you the state of that one instance.

2. time: The time.Date function returns a Time struct. Why? A time is a time. It has no state. It is like an integer where you can compare them, preform maths on them, etc. The API designer decided that a modification to a time would not change the current one but make a new one. As a user of the library, if I want the time one month from now, I would want a new time object, not to change the current one I have. A time is also only 3 words in length. In other words, it is small and there would be no performance gain in using a pointer.

3. math/big: big.NewInt() is an interesting one. We can pretty much agree that when you modify a big.Int, you will often want a new one. A big.Int has no internal state, so why is it a pointer? The answer is simply performance. The programmers realized that big ints are … big. Constantly allocating each time you do a mathematical operation may not be practical. So, they decided to use pointers and allow the programmer to decide when to allocate new space.

Have I answered your question? Probably not. It is a design decision and you need to figure it out on a case by case basis. I use the standard library as a guide when I am designing my own libraries. It really all comes down to judgement and how you expect client code to use your types.

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