C++核心準(zhǔn)則C.168: 將重載的運(yùn)算符定義在操作對(duì)象的命名空間內(nèi)

C.168: Define overloaded operators in the namespace of their operands

C.168: 將重載的運(yùn)算符定義在操作對(duì)象的命名空間內(nèi)

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(fā)現(xiàn)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(xì)請(qǐng)參照以下鏈接：

https://en.cppreference.com/w/cpp/language/adl

--譯者注

Example(示例)

struct S { };
bool operator==(S, S);   // OK: in the same namespace as S, and even next to S
S s;

bool x = (s == s);

This is what a default?==?would do, if we had such defaults.

這正是默認(rèn)相等比較運(yùn)算符做的事情，如果存在這么一個(gè)默認(rèn)的話。

Example(示例)

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

Example, bad(反面示例)

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;
}

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(gè)命名空間中!s的含義不一樣。這會(huì)非常混亂。如果去掉命名空間M的定義又會(huì)增加出錯(cuò)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(nèi)的兩個(gè)不同的類型定義二目運(yùn)算符，你無(wú)法遵守本準(zhǔn)則。例如：

Vec::Vector operator*(const Vec::Vector&, const Mat::Matrix&);

This may be something best avoided.

這可能是最好狀態(tài)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(shuō)是【幫助函數(shù)應(yīng)該和它幫助的類定義在一個(gè)命名空間內(nèi)】規(guī)則的特例。

Enforcement(實(shí)施建議)

• Flag operator definitions that are not it the namespace of their operands

• 標(biāo)記沒(méi)有和操作對(duì)象定義在同一個(gè)命名空間中的運(yùn)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

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