c++ - *restrict/*__restrict__ 在 C/C++ 中如何工作？

Question

这是我写的一些代码(使用 GCC 的 __restrict__ 扩展到 C++):

#include <iostream>

using namespace std;

int main(void) {
    int i = 7;
    int *__restrict__ a = &i;
    *a = 5;
    int *b = &i, *c = &i;
    *b = 8;
    *c = 9;

    cout << **&a << endl; // *a - which prints 9 in this case

    return 0;
}

或者，C 版本(如果由于使用了每个流行的 C++ 编译器都支持的扩展而导致 C++ 版本不清楚)，使用 GCC:

#include <stdio.h>

int main(void) {
    int i = 7;
    int *restrict a = &i;
    *a = 5;
    int *b = &i, *c = &i;
    *b = 8;
    *c = 9;

    printf("%d \n", **&a); // *a - which prints 9 in this case

    return 0;
}

据我所读，如果我执行 *a = 5，它会更改他 a 指向的内存的值；之后，他指向的内存不应该被除a以外的任何人修改，这意味着这些程序是错误的，因为b和c 之后修改它。 或者，即使 b 首先修改 i，之后只有 a 应该可以访问该内存 (i) . 我理解正确了吗？

P.S:在此程序中进行限制不会改变任何内容。无论有没有限制，编译器都会产生相同的汇编代码。我写这个程序只是为了澄清事情，它不是 restrict 用法的好例子。您可以在此处查看 restrict 用法的一个很好的示例:http://cellperformance.beyond3d.com/articles/2006/05/demystifying-the-restrict-keyword.html

Answer 1

没有。

声明

*b = 8;
*c = 9;

会导致未定义的行为。

来自文档:

A pointer is the address of a location in memory. More than one pointer can access the same chunk of memory and modify it during the course of a program. The restrict type qualifier is an indication to the compiler that, if the memory addressed by the restrict-qualified pointer is modified, no other pointer will access that same memory. The compiler may choose to optimize code involving restrict-qualified pointers in a way that might otherwise result in incorrect behavior. It is the responsibility of the programmer to ensure that restrict-qualified pointers are used as they were intended to be used. Otherwise, undefined behavior may result.