英文:
How to declare a constant from a variable?
问题
问题是什么：
我想从一个变量声明一个常量。以下是我想要做的非常简单的版本：
someVar := 1
const someConst = someVar // 错误：常量初始化器someVar不是一个常量
fmt.Printf("%d %d", someVar, someConst)
我如何将someConst声明为常量？在Go语言中是否不可能实现这一点？
我为什么想要这样做：
someVar是一个全局变量。它可以改变是可以接受的。someConst是函数作用域的。在该函数的作用域内，它不应该改变。
以数据库术语来说：someConst是someVar的一个不可变快照。
英文:
What is the problem:
I would like to declare a constant from a variable. Here is a very simple version of what I'd like to do:
someVar := 1
const someConst = someVar // error: const initializer someVar is not a constant
fmt.Printf("%d %d", someVar, someConst)
How can I make someConst a constant? Is this impossible in go?
Why do I want this?
someVar is a global Variable. It is fine that this can change. someConst is function scoped. For the scope of this function it should not change.
In DB terms: someConst is an immutable snapshot of someVar
答案1
得分: 6
你不能这样做。Go语言的常量必须是编译时常量。
常量声明将一组标识符（常量的名称）绑定到一组常量表达式的值。
常量表达式只能包含常量操作数，并在编译时进行求值。
你示例中的someVar是一个变量，它不符合"常量操作数"的要求。
如果包级别的变量在函数执行过程中可能会发生变化，而你不想观察到这种变化，可以创建该变量的本地副本，并使用本地副本。
还要注意，如果变量的值可能因为（并发的）goroutine而发生变化，在制作副本时必须进行同步访问（就像在其他goroutine中对其进行写入一样）。
例如：
var (
someVarMu sync.RWMutex
someVar = 1
)
func foo() {
someVarMu.RLock()
myVar := someVar
someVarMu.RUnlock()
// 使用myVar
fmt.Println(myVar)
}
英文:
You can't. Go's constants must be compile-time constants.
> A constant declaration binds a list of identifiers (the names of the constants) to the values of a list of constant expressions.
And
> Constant expressions may contain only constant operands and are evaluated at compile time.
someVar in your example is a variable, it does not qualify for the "constant operands".
If the package level variable may change during the execution of your function which change you don't want to observe, make a local copy of the variable, and use the local copy.
Also note that if the variable's value may change due to a (concurrent) goroutine, access to it (when making the copy) must be synchronized (just like the write to it in other goroutines).
For example:
var (
someVarMu sync.RWMutex
someVar = 1
)
func foo() {
someVarMu.RLock()
myVar := someVar
someVarMu.RUnlock()
// Use myVar
fmt.Println(myVar)
}
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