Question

我正在尝试再次学习C ++，在21天内使用Sams Teach Yourself C ++（第6版）。我正在努力完成它，确保我理解每一章（尽管我已经熟悉了C语法语言）。

在第5章（代码清单5.2）的开头附近，有一个关于无符号整数溢出的观点。基于他们的例子，我写了这个：

#include <iostream>

int main () {
    unsigned int bignum = 100;
    unsigned int smallnum = 50;
    unsigned int udiff;
    int diff;
    udiff = bignum - smallnum;
    std::cout << "Difference (1) is " << udiff << "\n";
    udiff = smallnum - bignum;
    std::cout << "Difference (2) is " << udiff << "\n";
    diff = bignum - smallnum;
    std::cout << "Difference (3) is " << diff << "\n";
    diff = smallnum - bignum;
    std::cout << "Difference (4) is " << diff << "\n";
    return 0;
}

这给出了以下输出，这对我来说并不奇怪：

Difference (1) is 50
Difference (2) is 4294967246
Difference (3) is 50
Difference (4) is -50

如果我更改程序以使声明bignum的行改为unsigned int bignum = 3000000000;，那么输出就是

Difference (1) is 2999999950
Difference (2) is 1294967346
Difference (3) is -1294967346
Difference (4) is 1294967346

第一个显然很好。 ~~数字1294967346的解释是1294967346正是2^32 - 3000000000。我不明白为什么第二行没有读1294967396，因为smallnum贡献了50。~~

第三次~~和第四行~~我无法解释。这些结果是如何产生的？

编辑：对于第三行 - 它是否只是通过找到符合有符合条件的int允许的值范围的解模mod 2^32来提供此结果？

Answer 1

2 ^ 32 - 3000000000 = 1294967296（！）

减法时整数溢出的种类

1 个答案: