英文:
Does loading a map element in Go imply copying?
问题
假设有这样的代码片段。
mapper := make(map[int]SomeStructType)
mapper[0] = SomeStructType{}
somestruct := mapper[0] // 将 mapper[0] 加载到 'somestruct' 中
在最后一行，这是否意味着 mapper[0] 在所有情况下都被复制到 somestruct 中，即使 somestruct 之后被用作只读常量？
如果是这样，是否有办法创建一个对映射元素（这里是 mapper[0]）的 引用，就像在 C/C++ 中一样，这样我就可以通过别名引用它，同时避免不必要的对象复制？我尝试创建一个指向映射元素的指针，但显然，Go 不允许我这样做。
英文:
Suppose there is a code snippet like this.
mapper := make(map[int]SomeStructType)
mapper[0] = SomeStructType{}
somestruct := mapper[0] // load mapper[0] to 'somestruct'
At the last line, does that mean mapper[0] is copied to somestruct in all situations, like even if somestruct is ever used as a read-only constant afterward?
If so, is there any way to make a reference to a map element (mapper[0] here) like in C/C++, so that I can reference it through an alias while avoiding unnecessary object copy? I tried to make a pointer to a map element, but apparently, Go does not allow me to do so.
答案1
得分: 2
简单的答案是不可以。在Go语言中，map的实现可能会移动数据，因此引用会失效，这是不安全的。在C语言中，你可以定义自己的数据结构，所以如何处理这个问题取决于你；而在Go语言中，map是在Go运行时实现的，它无法猜测你的意图。
我认为你正在寻找的解决方案是在map中保留指针，例如：
mapper := make(map[int]*SomeStructType)
现在访问mapper的元素只会“复制”一个指针（通常是一个单词），这非常廉价。
somestruct := mapper[0] // 复制一个指针
在Go语言中使用指针类型非常常见，所以通过像这样定义mapper并没有什么神奇或异常的地方。
英文:
The simple answer is NO. In Go, the map implementation may move data around, so references would get invalidated and it wouldn't be safe. In C you define your own data structure, so it's up to you how this is done; in Go, maps are implemented in the Go runtime, and it can't guess at your intent.
The solution you're looking for, I think, is keep pointers in the map, i.e.:
mapper := make(map[int]*SomeStructType)
Now accessing mapper elements will just "copy" a pointer (typically a single word), which is very cheap.
somestruct := mapper[0] // copies a pointer
It's very common to use pointer types in Go, so you wouldn't be doing anything too magical or unusual by defining mapper like this.
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