C++核心准则ES.46:避免有损（窄化，截短）算数转换

ES.46: Avoid lossy (narrowing, truncating) arithmetic conversions

ES.46:避免有损（窄化，截短）算数转换

Reason（原因）

A narrowing conversion destroys information, often unexpectedly so.

窄化转换破坏信息，通常不是期待的动作。

Example, bad（反面示例）

A key example is basic narrowing:

主要的示例说明窄化的基本情况：

double d = 7.9;
int i = d;    // bad: narrowing: i becomes 7
i = (int) d;  // bad: we're going to claim this is still not explicit enough

void f(int x, long y, double d)
{
    char c1 = x;   // bad: narrowing
    char c2 = y;   // bad: narrowing
    char c3 = d;   // bad: narrowing
}

Note（注意）

The guidelines support library offers a narrow_cast operation for specifying that narrowing is acceptable and a narrow ("narrow if") that throws an exception if a narrowing would throw away information:

准则支持库提供了一个narrow_cast操作，可以用来表明窄化是可接受的；一个narrow（“如果发生窄化转换”）操作，它可以在丢失了任何信息时抛出异常。

i = narrow_cast<int>(d);   // OK (you asked for it): narrowing: i becomes 7
i = narrow<int>(d);        // OK: throws narrowing_error

We also include lossy arithmetic casts, such as from a negative floating point type to an unsigned integral type:

这两个操作也可以处理有损算数转换，例如从负浮点数转换为无符号整数的情况。

double d = -7.9;
unsigned u = 0;

u = d;                          // BAD
u = narrow_cast<unsigned>(d);   // OK (you asked for it): u becomes 4294967289
u = narrow<unsigned>(d);        // OK: throws narrowing_error

Enforcement（实施建议）

A good analyzer can detect all narrowing conversions. However, flagging all narrowing conversions will lead to a lot of false positives. Suggestions:

实现良好的代码分析器可以检出所有的窄化转换。但是标识所有的窄化转换会导致大量的假阳性结果。建议：

• Flag all floating-point to integer conversions (maybe only float->char and double->int. Here be dragons! we need data).
• 标记所有浮点数到整数的转换（或许只需要标记float到char和double到int。 都有可能! 我们需要数据）
• Flag all long->char (I suspect int->char is very common. Here be dragons! we need data).
• 标记所有long到char的转换（我怀疑int到char的转换很普遍。都有可能! 我们需要数据）
• Consider narrowing conversions for function arguments especially suspect.
• 函数参数的窄化转换尤其可疑。

链接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es46-avoid-lossy-narrowing-truncating-arithmetic-conversions