C.168: Define overloaded operators in the namespace of their operands
C.168: 將重載的運算符定義在操作對象的命名空間內
Reason(原因)
Readability. Ability for find operators using ADL. Avoiding inconsistent definition in different namespaces
可讀性。提供使用ADL發現操作符的能力。避免不同命名空間中的不一致。
ADL,Argument-dependent lookup.詳細請參照以下鏈接：
https://en.cppreference.com/w/cpp/language/adl
--譯者注
Example(示例)
<code>struct S { };
bool operator==(S, S); // OK: in the same namespace as S, and even next to S
S s;
bool x = (s == s);/<code>This is what a default == would do, if we had such defaults.
這正是默認相等比較運算符做的事情，如果存在這麼一個默認的話。
Example(示例)
<code>namespace N {
struct S { };
bool operator==(S, S); // OK: in the same namespace as S, and even next to S
}
N::S s;
bool x = (s == s); // finds N::operator==() by ADL/<code>Example, bad(反面示例)
<code>struct S { };
S s;
namespace N {
S::operator!(S a) { return true; }
S not_s = !s;
}
namespace M {
S::operator!(S a) { return false; }
S not_s = !s;
}/<code>Here, the meaning of !s differs in N and M. This can be most confusing. Remove the definition of namespace M and the confusion is replaced by an opportunity to make the mistake.
代碼中N和M兩個命名空間中!s的含義不一樣。這會非常混亂。如果去掉命名空間M的定義又會增加出錯的可能。
Note(注意)
If a binary operator is defined for two types that are defined in different namespaces, you cannot follow this rule. For example:
如果為不同命名空間內的兩個不同的類型定義二目運算符，你無法遵守本準則。例如：
<code>Vec::Vector operator*(const Vec::Vector&, const Mat::Matrix&);/<code>
This may be something best avoided.
這可能是最好狀態了。
See also(參照)
This is a special case of the rule that helper functions should be defined in the same namespace as their class.
這可以說是【幫助函數應該和它幫助的類定義在一個命名空間內】規則的特例。
Enforcement(實施建議)
- Flag operator definitions that are not it the namespace of their operands
- 標記沒有和操作對象定義在同一個命名空間中的運算符。
原文鏈接：
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c168-define-overloaded-operators-in-the-namespace-of-their-operands
覺得本文有幫助？請分享給更多人。
關注【面向對象思考】輕鬆學習每一天！
面向對象開發，面向對象思考！
閱讀更多 面向對象思考 的文章
