C++核心准则C.166:​ 重载的单目运算符&只能用于智能指针和引用

C.166: Overload unary & only as part of a system of smart pointers and references

C.166: 重载的单目运算符&只能用于智能指针和引用

Reason(原因)

The & operator is fundamental in C++. Many parts of the C++ semantics assumes its default meaning.

取地址运算符&是C++的基本要素，C++语义的很多地方为它设定了默认含义。

Example(示例)

class Ptr { // a somewhat smart pointer
    Ptr(X* pp) :p(pp) { /* check */ }
    X* operator->() { /* check */ return p; }
    X operator[](int i);
    X operator*();
private:
    T* p;
};

class X {
    Ptr operator&() { return Ptr{this}; }
    // ...
};

Note(注意)

If you "mess with" operator & be sure that its definition has matching meanings for ->, [], *, and . on the result type. Note that operator . currently cannot be overloaded so a perfect system is impossible. We hope to remedy that: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4477.pdf. Note that std::addressof() always yields a built-in pointer.

如果你要"招惹"&运算符，一定要确保它的结果类型和->，[]，*和 . 相匹配。注意 . 运算符现在不能被重载，因此不可能实现完美系统。我们可以希望可以改进这一点：

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4477.pdf。

注意std::addressof()总是返回一个内置类型的指针。

Enforcement(实施建议)

Tricky. Warn if & is user-defined without also defining -> for the result type.

不好办。如果定制了&运算符却没有为结果定义->运算符，报警。

原文链接：

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c166-overload-unary--only-as-part-of-a-system-of-smart-pointers-and-references

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