如何在Go中存储对操作结果的引用？ [英] How can I store reference to the result of an operation in Go?

问题描述

好吧，很难用单词来描述它，但是让我们说我有一个存储 int 指针的地图，并希望将操作的结果存储在我的另一个键上哈希：

  m：= make（map [string] * int）
 
m [d ] =&（* m [x] + * m [y]）
  

这不行，给我错误：不能取* m [x]& * m [y]

想法？

解决方案

指针是内存地址。例如，变量在内存中有一个地址。

像 3 + 4 这样的操作的结果没有一个地址，因为没有为它分配特定的内存。结果可能只是在处理器寄存器中。

您必须分配可以放置到地图中的地址的内存。最简单和最简单的就是为它创建一个局部变量。

看到这个例子：

  x，y：= 1，2 
m：= map [string] * int {x：& x，y：& y} 
 
d：= * m [x] + * m [y] 
m [d] =& d 
 
 fmt.Println（m [d] ，* m [d]）
  

输出（尝试在 Go Playground ）：

  0x10438300 3 
  

注意：如果上面的代码在一个函数中，我们刚刚放入地图的局部变量（ d ）的地址将继续生效，即使我们从函数返回（即如果 map 在外部返回或创建 - 例如全局变量）。在Go中，取出并返回局部变量的地址是非常安全的。编译器将分析代码，如果地址（指针）转义函数，它将自动分配到堆上（而不是在堆栈上）。有关详细信息，请参阅常见问题：如何知道变量是否在堆或堆栈上分配？

注意＃2：还有其他方法可以创建一个指向值的指针（在此答案中详细说明：如何在Go中做一个文字* int64？），但它们只是技巧，而不是更好或更有效率。使用局部变量是最干净和推荐的方法。

例如，这也可以在不创建局部变量的情况下工作，但显然不直观：

  m [d] =& [] int {* m [x] + * m [y]} [0 ] 
  

输出是一样的。尝试在 Go Playground 。

Okay it's hard to describe it in words but let's say I have a map that stores int pointers, and want to store the result of an operation as another key in my hash:

m := make(map[string]*int)

m["d"] = &(*m["x"] + *m["y"])

This doesn't work and gives me the error: cannot take the address of *m["x"] & *m["y"]

Thoughts?

解决方案

A pointer is a memory address. For example a variable has an address in memory.

The result of an operation like 3 + 4 does not have an address because there is no specific memory allocated for it. The result may just live in processor registers.

You have to allocate memory whose address you can put into the map. The easiest and most straightforward is to create a local variable for it.

See this example:

x, y := 1, 2
m := map[string]*int{"x": &x, "y": &y}

d := *m["x"] + *m["y"]
m["d"] = &d

fmt.Println(m["d"], *m["d"])

Output (try it on the Go Playground):

0x10438300 3

Note: If the code above is in a function, the address of the local variable (d) that we just put into the map will continue to live even if we return from the function (that is if the map is returned or created outside - e.g. a global variable). In Go it is perfectly safe to take and return the address of a local variable. The compiler will analyze the code and if the address (pointer) escapes the function, it will automatically be allocated on the heap (and not on the stack). For details see FAQ: How do I know whether a variable is allocated on the heap or the stack?

Note #2: There are other ways to create a pointer to a value (as detailed in this answer: How do I do a literal *int64 in Go?), but they are just "tricks" and are not nicer or more efficient. Using a local variable is the cleanest and recommended way.

For example this also works without creating a local variable, but it's obviously not intuitive at all:

m["d"] = &[]int{*m["x"] + *m["y"]}[0]

Output is the same. Try it on the Go Playground.

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