指针运算:++*ptr 还是 *ptr++? [英] Pointer Arithmetic: ++*ptr or *ptr++?

问题描述

我正在学习 C 语言，对 ++*ptr 和 *ptr++ 之间的区别感到很困惑.

I am learning C language and quite confused the differences between ++*ptr and *ptr++.

例如:

int x = 19;
int *ptr = &x;

我知道 ++*ptr 和 *ptr++ 产生不同的结果，但我不知道为什么会这样?

I know ++*ptr and *ptr++ produce different results but I am not sure why is that?

推荐答案

由于运算符绑定的方式不同，这些语句会产生不同的结果.特别地，前缀++ 运算符与* 具有相同的优先级，并且它们从右到左关联.于是

These statements produce different results because of the way in which the operators bind. In particular, the prefix ++ operator has the same precedence as *, and they associate right-to-left. Thus

++*ptr

被解析为

++(*ptr)

意思是增加ptr指向的值，".另一方面，后缀 ++ 运算符比解引用运算符 * 具有更高的优先级.之前

meaning "increment the value pointed at by ptr,". On the other hand, the postfix ++ operator has higher precedence than the dereferrence operator *. Thefore

*ptr++

意思

*(ptr++)

这意味着增加 ptr 以转到它指向的元素之后的元素，然后取消引用它的旧值"(因为后缀 ++ 将值交还给以前有指针).

which means "increment ptr to go to the element after the one it points at, then dereference its old value" (since postfix ++ hands back the value the pointer used to have).

在您描述的上下文中，您可能想要编写 ++*ptr，它会通过 ptr 间接增加 x.编写 *ptr++ 会很危险，因为它会将 ptr 向前推进 x，并且因为 x 不是一部分在数组中，指针将悬空在内存中的某个位置(可能在其自身之上！)

In the context you described, you probably want to write ++*ptr, which would increment x indirectly through ptr. Writing *ptr++ would be dangerous because it would march ptr forward past x, and since x isn't part of an array the pointer would be dangling somewhere in memory (perhaps on top of itself!)

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