英文:
Pointer to a struct (or lack thereof)
问题
假设我定义了这个结构体：
type Vertex struct {
X, Y float64
}
现在，在Go语言中，可以这样使用它：
func (v *Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func main() {
v := &Vertex{3, 4}
fmt.Println(v.Abs())
}
但是也可以不使用指针：
func main() {
v := Vertex{3, 4}
fmt.Println(v.Abs())
}
两种情况下的结果是相同的，但是它们在内部上有什么不同呢？使用指针会使程序运行更快吗？
PS. 我明白Abs()函数需要一个指针作为接收器。这解释了为什么在main函数中后面使用了指针。但是为什么当我不使用指针直接在结构体实例上调用Abs()时，程序不会报错呢？
英文:
Let's say I have defined this struct:
type Vertex struct {
X, Y float64
}
now it's perfectly legal Go to use it like this:
func (v *Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func main() {
v := &Vertex{3, 4}
fmt.Println(v.Abs())
}
but it's also ok not to use a pointer:
func main() {
v := Vertex{3, 4}
fmt.Println(v.Abs())
}
The results in both cases is the same, but how are they different, internally? Does the use of pointer makes the program run faster?
PS. I get it that the Abs() function needs a pointer as a receiver. That explains the reason why a pointer has been used later in the main function. But why doesn't the program spit out an error when I don't use a pointer and directly call Abs() on a struct instance?
答案1
得分: 3
为什么在我不使用指针直接在结构体实例上调用Abs()时，程序没有报错？
因为你可以获取结构体实例的指针（地址）。
正如在《Golang中的指针接收器和值接收器是什么意思？》中所提到的：
Go会自动对指针进行取址和自动解引用（在大多数情况下），所以
m := MyStruct{}; m.DoOtherStuff()仍然可以工作，因为Go会自动为你执行(&m).DoOtherStuff()。
正如《在Golang中不要被指针和非指针方法接收器搞混》或《Go 101: 指针方法和值方法》所示，使用指针接收器(v *Vertex)可以避免拷贝，因为Go通过值传递一切。
与方法调用一样，使用指针接收器的非接口方法的引用将自动获取该值的地址：
t.Mp等同于(&t).Mp。
英文:
> why doesn't the program spit out an error when I don't use a pointer and directly call Abs() on a struct instance?
Because you can get the pointer to (address of) a struct instance.
As mentioned in "What do the terms pointer receiver and value receiver mean in Golang?"
> Go will auto address and auto-dereference pointers (in most cases) so m := MyStruct{}; m.DoOtherStuff() still works since Go automatically does (&m).DoOtherStuff() for you.
As illustrated by "Don't Get Bitten by Pointer vs Non-Pointer Method Receivers in Golang" or "Go 101: Methods on Pointers vs. Values", using a pointer receiver (v *Vertex) is great to avoid copy, since Go passes everything by value.
The spec mentions (Method values):
> As with method calls, a reference to a non-interface method with a pointer receiver using an addressable value will automatically take the address of that value: t.Mp is equivalent to (&t).Mp.
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