C++核心准则ES.102:使用有符号数进行数学运算-轻识

ES.102: Use signed types for arithmetic

ES.102：使用有符号数进行数学运算

Reason（原因）

Because most arithmetic is assumed to be signed; x - y yields a negative number when y > x except in the rare cases where you really want modulo arithmetic.

因为大部分数学运算都是有符号的。当x>y时，x-y会返回一个负数，极少情况实际需要的是按模运算。

Example（示例）

Unsigned arithmetic can yield surprising results if you are not expecting it. This is even more true for mixed signed and unsigned arithmetic.

如果不是你有意为之，无符号运算可能产生意外的结果。如果混用有符号数和无符号数，问题会更明显。

template
T subtract(T x, T2 y)
{
    return x - y;
}

void test()
{
    int s = 5;
    unsigned int us = 5;
    cout << subtract(s, 7) << '\n';       // -2
    cout << subtract(us, 7u) << '\n';     // 4294967294
    cout << subtract(s, 7u) << '\n';      // -2
    cout << subtract(us, 7) << '\n';      // 4294967294
    cout << subtract(s, us + 2) << '\n';  // -2
    cout << subtract(us, s + 2) << '\n';  // 4294967294
}

Here we have been very explicit about what's happening, but if you had seen us - (s + 2) or s += 2; ...; us - s, would you reliably have suspected that the result would print as 4294967294?

代码中我们已经很明确地知道发生了什么。但是如果你看到us - (s + 2) or s += 2; ...; us - s，你真的可以想象结果是4294967294么？

Exception（例外）

Use unsigned types if you really want modulo arithmetic - add comments as necessary noting the reliance on overflow behavior, as such code is going to be surprising for many programmers.

如果你真的需要按模运算-增加必要的注释提示对溢出行为的依赖，这样的代码会令很多程序员疑惑。

Example（示例）

The standard library uses unsigned types for subscripts. The built-in array uses signed types for subscripts. This makes surprises (and bugs) inevitable.

标准库使用无符号类型作为下标。内置数组使用有符号数作为下标。这会导致代码难于理解并不可避免地带来错误。

int a[10];
for (int i = 0; i < 10; ++i) a[i] = i;
vector v(10);
// compares signed to unsigned; some compilers warn, but we should not
for (gsl::index i = 0; i < v.size(); ++i) v[i] = i;

int a2[-2];         // error: negative size

// OK, but the number of ints (4294967294) is so large that we should get an exception
vector v2(-2);

Use gsl::index for subscripts; see ES.107.

使用ES.107中介绍的gsl::index作为下标。

Enforcement（实施建议）

Flag mixed signed and unsigned arithmetic
标记有符号数和无符号数混用的数学运算。
Flag results of unsigned arithmetic assigned to or printed as signed.
标记将无符号数学运算的结果赋值给有符号数或者作为有符号数print输出的情况。
Flag negative literals (e.g. -2) used as container subscripts.
标记使用负值作为容器下标的情况。
(To avoid noise) Do not flag on a mixed signed/unsigned comparison where one of the arguments is sizeof or a call to container .size() and the other is ptrdiff_t.
（为了避免误判）当一个参数是sizeof或者container.size()的返回值，而另一个参数是ptrdiff_t的时候，不要标记有符号数/无符号数混合的比较操作。

原文链接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es102-use-signed-types-for-arithmetic

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