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我正在尝试重新学习 C++，使用 Sams Teach Yourself C++ in 21 Days（第 6 版）。我正在尝试非常彻底地完成它，确保我理解每一章（尽管我已经熟悉 C 语法语言）。

在第 5 章（清单 5.2）的开头附近，有一点是关于无符号整数溢出的。根据他们的例子，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

如果我更改程序以便声明的行bignum改为读取，unsigned int bignum = 3000000000;则输出改为

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

其中第一个显然很好。这个数字1294967346是由一个事实来解释1294967346的2^32 - 3000000000。我不明白为什么第二行没有读取1294967396，因为 50 贡献了smallnum.

第三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 - 它是否仅通过找到2^32适合有符号 int 允许的值范围的解模来给出此结果？

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1 回答 1

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2^32 - 3000000000 = 1294967296 (!)

于 2010-10-20T22:25:35.837 回答