ES.46: Avoid lossy (narrowing, truncating) arithmetic conversions
ES.46:避免有損（窄化，截短）算數轉換
Reason（原因）
A narrowing conversion destroys information, often unexpectedly so.
窄化轉換破壞信息，通常不是期待的動作。
Example, bad（反面示例）
A key example is basic narrowing:
主要的示例說明窄化的基本情況：
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:
準則支持庫提供了一個narrow_cast操作，可以用來表明窄化是可接受的；一個narrow（“如果發生窄化轉換”）操作，它可以在丟失了任何信息時拋出異常。
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:
這兩個操作也可以處理有損算數轉換，例如從負浮點數轉換爲無符號整數的情況。
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（實施建議）
A good analyzer can detect all narrowing conversions. However, flagging all narrowing conversions will lead to a lot of false positives. Suggestions:
實現良好的代碼分析器可以檢出所有的窄化轉換。但是標識所有的窄化轉換會導致大量的假陽性結果。建議：
-
Flag all floating-point to integer conversions (maybe only float->char and double->int. Here be dragons! we need data).
-
標記所有浮點數到整數的轉換（或許只需要標記float到char和double到int。 都有可能! 我們需要數據）
-
Flag all long->char (I suspect int->char is very common. Here be dragons! we need data).
-
標記所有long到char的轉換（我懷疑int到char的轉換很普遍。都有可能! 我們需要數據）
-
Consider narrowing conversions for function arguments especially suspect.
-
函數參數的窄化轉換尤其可疑。
原文鏈接
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es46-avoid-lossy-narrowing-truncating-arithmetic-conversions
覺得本文有幫助？歡迎點贊並分享給更多的人。
閱讀更多更新文章，請關注微信公衆號【面向對象思考】