英文:
How to declare an immutable variable in Go?
问题
我正在解决一些练习题，以便在Go语言中进行一些实践，并遇到了以下问题。
考虑这个函数：
func foo(s string) {
length := len(s);
// 只是从length中读取一些内容
}
在这里，由于在其他语言中编程的经验，我立刻想到"length不会被修改，所以应该标记为const"。
所以我这样做了：
const length = len(s);
但是它给我返回了以下错误：
const initializer len(s) is not a constant
我猜这是因为len(s)不能在"编译时"计算出来（关于Go语言是否完全正确，我不太确定）。
有没有办法指示length不应该被修改？我在Google上搜索了一下，但没有找到有用的信息。我找到了一些关于不可变结构体的东西，但我觉得对于我想做的事情来说太复杂了。
英文:
I was solving some exercises to get some practice in go and came across the
following problem.
Take this function:
func foo(s string) {
length := len(s);
// Do something just reading from lenght
}
Here, thanks to programing in other languages, my mind immediately went "length
will not be modified, so it should be marked const".
So I did this:
const length = len(s);
But it gives me the following error:
const initializer len(s) is not a constant
I suppose that is because len(s) cant be calculated "at compile time" (Not
sure if this is 100% correct talking about go).
Is there any way to indicate that length should not be modified? I searched
in Google, but I found nothing usefull. I got some thins about immutable structs, but
I think its too complex for what I want to do.
答案1
得分: 8
你不能将变量指定为不可变的。只有常量具有这个属性。
在Go语言中，常量也是编译时常量，这意味着你不能将其初始化为依赖于某个变量的特定值，比如你示例中的s。
此外，除了一些罕见的例外，函数不能返回常量。即使可以返回常量，它仍然必须是编译时常量。len函数可以返回一个常量值，但前提是参数具有固定大小，比如字符串常量或数组类型（在Go中具有固定长度）。
英文:
You cannot specify a variable to be immutable. Only constants have this property.
Constants in Go are also compile-time constant, meaning you can't initialize it to something that depends on the particular value of some variable like s in your example.
Also, with rare exceptions, functions cannot return constants. Even when they can, it must still be a compile-time constant. len can return a constant value, but only if the parameter is constantly sized, such as a string constant or of an array type (which has fixed length in Go).
答案2
得分: 2
对于具有类似结果的情况，您可以使用私有字段。创建一个类似下面的模块：
package foo
type Length struct { value int }
func NewLength(s string) Length {
return Length{
len(s),
}
}
func (l Length) Int() int { return l.value }
然后像这样使用：
package main
import "foo"
func main() {
l := foo.NewLength("north")
println(l.Int() == 5)
// l.value undefined (cannot refer to unexported field or method value)
l.value = 6
}
英文:
For something with similar result, you could use a private field. Make a module
like this:
package foo
type Length struct { value int }
func NewLength(s string) Length {
return Length{
len(s),
}
}
func (l Length) Int() int { return l.value }
then use like this:
package main
import "foo"
func main() {
l := foo.NewLength("north")
println(l.Int() == 5)
// l.value undefined (cannot refer to unexported field or method value)
l.value = 6
}
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