C++中的指针解释

Question

我正在自学 C++，我在 C++ 的指针部分找到了自己。

据我理解，指针将变量的值分配给内存。

但是我遇到了这个问题，答案是588。

我不明白这个数字是怎么来的。

有人可以一步步解释一下588是如何产生的吗？

提前致谢。

#include <iostream>

int main() {
    int *p, *q, *t;

    // Allocate memory and initialize pointers and values
    p = new int;
    q = new int;
    t = new int;
    *p = 17;
    *q = 7;
    *t = 42;

    p = t; // Make pointer p point to the same location as pointer t
    t = q; // Make pointer t point to the same location as pointer q

    *t = *p * *q;

    *q = *q + *t;

    std::cout << *t << std::endl;


    return 0;
}

Answer 1

如果在每次更改后输出所有三个指针及其值，或者使用调试器，您可以轻松了解正在发生的情况。事情是这样的：

*p = 17;
*q = 7;
*t = 42;

// p points to an int with value 17
// q points to an int with value 7
// t points to an int with value 42

p = t; // Make pointer p point to the same location as pointer t

// p points to an int with value 42 (the same as t)
// q points to an int with value 7
// t points to an int with value 42 (the same as p)
    
t = q; // Make pointer t point to the same location as pointer q

// p points to an int with value 42 (now different than t)
// q points to an int with value 7 (the same as t)
// t points to an int with value 7 (the same as q)

*t = *p * *q;   // *t = (*p) * (*q) = 42 * 7 = 294

// p points to an int with value 42
// q points to an int with value 294 (still the same as t)
// t points to an int with value 294 (still the same as q)

*q = *q + *t;   // *q = (*q) + (*t) = 294 + 294 = 588

// p points to an int with value 42
// q points to an int with value 588 (still the same as t)
// t points to an int with value 588 (still the same as q)

std::cout << *t << std::endl;   // prints 588 (the same as q)

Answer 2

程序的注释里有写

p = t; // Make pointer p point to the same location as pointer t
t = q; // Make pointer t point to the same location as pointer q

也就是说，在这些语句之后，

*p

等于

42

并且

*t

等于

7

，就像

*q

等于

7

一样，因为现在两个指针

t

和

q

指向相同的内存。

因此你有这样的说法

*t = *p * *q;

相当于

*t = 42 * 7;

那就是指针

t

指向的对象，

q

弓包含

294

。

本声明

*q = *q + *t;

与

相同

*t = *q + *t;

因为两个指针都指向同一个对象并且也相同

*t = *t + *t;

或

*q = *q + *q;

所以

294

加上

294

会产生

588

。