变量的作用域与生命周期
变量的作用域和生命周期有什么关系?如果一个变量超出范围，它的内存是否允许被另一个变量覆盖，或者是否保留空间直到函数离开.
What is the relation between the scope and the lifetime of a variable? If a variable is out of scope, is the memory of it allowed to be overwritten by another variable, or is the space reserved until the function is left.
我在 aksing 是因为我想知道下面的代码是否真的有效，或者 *p 是否可能未定义
I am aksing because I want to know whether the code below actually works, or whether it can be that *p might be undefined
foo() {
int *p;
{
int x = 5;
p = &x;
}
int y = *p;
}
推荐答案
什么是范围?
范围是可以访问变量的区域或代码段.
Scope is the region or section of code where a variable can be accessed.
什么是一生?
Lifetime 是对象/变量处于有效状态的持续时间.
Lifetime is the time duration where an object/variable is in a valid state.
对于，自动/局部非静态变量Lifetime仅限于它们的Scope.
换句话说，一旦创建它们的范围({,})结束，自动变量就会自动销毁.因此，名称自动开始.
For, Automatic/Local non-static variables Lifetime is limited to their Scope.
In other words, automatic variables are automagically destroyed once the scope({,}) in which they are created ends. Hence the name automatic to begin with.
您的代码示例有什么问题?
所以是的，您的代码有一个未定义行为.
So Yes your code has an Undefined Behavior.
在您的示例中，*p 的范围是创建后的整个函数体.
然而，x 是一个非静态的局部/自动变量，因此 x 的生命周期以它的作用域结束，即 } 的右括号它被创建，一旦范围结束 x 不存在.*p 指向不再存在的东西.
In your example scope of *p is entire function body after it was created.
However, x is a non-static local/automatic variable and hence the lifetime of x ends with it's scope i.e the closing brace of } in which it was created, once the scope ends x does not exist. *p points to something that doesn't exist anymore.
请注意，从技术上讲，x 并不存在于其范围之外，但是可能会发生编译器没有删除 x 的内容，并且人们可能能够访问其内容的情况x 通过指针超出其范围(正如您所做的).但是，执行此操作的代码不是有效的 C++ 代码.这是一个调用未定义行为的代码.这意味着任何事情都可能发生(您甚至可能会看到 x 的价值是完整的)并且人们不应该期望从这样的代码中可以观察到行为.
Note that technically x does not exist beyond its scope however it might happen that the compiler did not remove the contents of x and one might be able to access contents of x beyond its scope through a pointer(as you do).However, a code which does this is not a valid C++ code. It is a code which invokes Undefined Behaviour. Which means anything can happen(you might even see value of x being intact) and one should not expect observable behaviors from such a code.
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