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