ios - 编译器如何证明内存安全？

Question

当我阅读 The Swift Programming Language: Memory Safety 时，我对 Conflicting Access to Properties 部分感到困惑。 :

The code below shows that the same error appears for overlapping writeaccesses to the properties of a structure that’s stored in a globalvariable.

var holly = Player(name: "Holly", health: 10, energy: 10)
balance(&holly.health, &holly.energy)  // Error 

In practice,most access to the properties of a structure can overlap safely. Forexample, if the variable holly in the example above is changed to alocal variable instead of a global variable, the compiler can provethat overlapping access to stored properties of the structure issafe:

func someFunction() {
    var oscar = Player(name: "Oscar", health: 10, energy: 10)
    balance(&oscar.health, &oscar.energy)  // OK 
}

In the example above, Oscar’s health and energy are passed as the two in-out parameters to balance(_:_:). The compiler can prove that memorysafety is preserved because the two stored properties don’t interactin any way.

编译器如何证明内存安全？

Answer 1

在函数范围内使编译器确定将在 struct 上执行哪些操作以及何时执行。编译器知道 structs 是如何工作的，以及函数内的代码如何以及何时(相对于调用函数的时间)执行。

在全局或更大的范围内，编译器无法看到可能修改内存的内容以及修改时间，因此它无法确保安全。