问题描述
我注意到函数 扔
I notice there is one line *(*int)(nil) = 0 in function throw
//go:nosplit
func throw(s string) {
// Everything throw does should be recursively nosplit so it
// can be called even when it's unsafe to grow the stack.
systemstack(func() {
print("fatal error: ", s, "\n")
})
gp := getg()
if gp.m.throwing == 0 {
gp.m.throwing = 1
}
fatalthrow()
*(*int)(nil) = 0 // not reached
}
*(* int)(nil)= 0 是什么意思?并且由于无法到达此行 *(* int)(nil)= 0 ，为什么在这里?有什么特殊用途吗?
What does *(*int)(nil) = 0 means? and since this line *(*int)(nil) = 0 could NOT be reached, why it is here? any special usage?
推荐答案
该行:
*(*int)(nil) = 0
尝试取消引用 nil 指针并为其分配值，这始终是运行时的恐慌.该代码永远都不会到达此行，但是如果仍然会发生(例如，将来发生错误的代码更改)，它将惊慌失措，因此可以检测到该错误，并且不会被忽略.
Tries to dereference a nil pointer and assign a value to it, which is always a runtime panic. The code should not reach this line, ever, but if it would anyhow (e.g. a faulty code change in the future), it would panic so the error could be detected and wouldn't go unnoticed.
在代码中也做类似的事情是常识，但是在代码中有一个更明显的"construct".例如 panic("unreachable").例如:
It's common sense to do something similar in your code too, but with a more obvious "construct" such as panic("unreachable"). For example:
func sign(a int) string {
switch {
case a > 0:
return "Positive"
case a < 0:
return "Negative"
case a == 0:
return "Zero"
default:
panic("unreachable")
}
}
请注意，在此示例中，这不仅是要及早发现错误，也是一个要求，因为对于编译器而言，无法保证将返回return语句.您还可以将 panic("unreachable"))语句移至 switch (而不是 default 分支)之后，
Note that in this example this is not just to detect errors early, it's also a requirement because to the compiler there would be no guarantee that a return statement would be reached. You could also move the panic("unreachable") statement after the switch (instead of the default branch), it's a matter of taste.
如果您将上述功能更改为不返回而是打印符号，则将 default 分支置于紧急状态仍然是一个好习惯，尽管在此变体中这不是必需的:
If you would change the above function to not return but print the sign, it would still be a good practice to leave the default branch to panic, although it's not a requirement in this variant:
func printSign(a int) {
switch {
case a > 0:
fmt.Println("Positive")
case a < 0:
fmt.Println("Negative")
case a == 0:
fmt.Println("Zero")
default:
panic("unreachable")
}
}
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