英文:
Pointers (GoLang)
问题
这是一个初学者的问题，请耐心等待。疑问是为什么f()函数指向不同的地址。我的理解是变量v必须覆盖旧值。
package main
import "fmt"
var p = f()
func f() *int {
v := 1
return &v
}
func main() {
fmt.Println(f())
fmt.Println(f())
fmt.Println(f())
fmt.Println(p)
}
//0xc0000140b0
//0xc0000140b8
//0xc0000140e0
//0xc000014098
这段代码中，函数f()每次被调用时，都会创建一个新的局部变量v，并返回其地址。因此，每次调用f()时，返回的地址都是不同的。变量p在全局作用域中调用f()，因此它只会在程序启动时被初始化一次，所以它的值是固定的。
英文:
This is a rookie question, please bear with me. So the doubt is why f() function points different address. My understanding is the variable v must overwrite the old value.
package main
import "fmt"
var p = f()
func f() *int {
v := 1
return &v
}
func main() {
fmt.Println(f())
fmt.Println(f())
fmt.Println(f())
fmt.Println(p)
}
//0xc0000140b0
//0xc0000140b8
//0xc0000140e0
//0xc000014098
答案1
得分: 5
编译器检测到v在函数f中逃逸，所以它在堆上分配。每次调用f都会返回v的一个新实例，这就是为什么每次调用都会看到不同的地址。
英文:
The compiler detects that v escapes the function f, so it is allocated on the heap. Each call to f returns a new instance of v, that's why you see a different address for each call.
答案2
得分: 1
简单回答一下：
> Go语言会寻找超出当前栈帧的变量，并将它们分配到堆上。
基本上，变量v逃逸了函数f的栈帧，并在堆上分配内存，这就是为什么每次打印时看到不同的地址。
阅读这篇关于逃逸分析的好文章：https://medium.com/a-journey-with-go/go-introduction-to-the-escape-analysis-f7610174e890
尝试运行逃逸分析，查看所有逃逸的变量。
go build -gcflags="-m" main.go:
./main.go:7:2: moved to heap: v //指向v := 1
./main.go:12:15: moved to heap: v //指向fmt.Println(f())
./main.go:13:15: moved to heap: v //指向fmt.Println(f())
./main.go:14:15: moved to heap: v //指向fmt.Println(f())
请注意，最后一个fmt.Println(f())语句不被认为是逃逸的，因为传递给Println的值是全局变量p，它已经在堆上，因此不需要逃逸。
英文:
To give a simple answer to this
> Go looks for variables that outlive the current stack frame and
> then heap-allocates them
Basically, the variable v escapes the function f stack frame and gets allocated in heap which is why you see different addresses printed everytime.
Read this nice introduction to escape analysis. https://medium.com/a-journey-with-go/go-introduction-to-the-escape-analysis-f7610174e890
Try running escape analysis to see all the variables that got escaped.
go build -gcflags="-m" main.go:
./main.go:7:2: moved to heap: v //points to v := 1
./main.go:12:15: moved to heap: v //points to fmt.Println(f())
./main.go:13:15: moved to heap: v //points to fmt.Println(f())
./main.go:14:15: moved to heap: v //points to fmt.Println(f())
Note that the last fmt.Println(f()) statement is not considered for escaping as value passed to Println is p which is a global variable so its already in heap and thus doesn't need to escape.
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