是否允许实现将公共成员添加到标准类型？ [英] Are implementations allowed to add public members to standard types?

问题描述

是否允许C ++标准库实现向标准类型的接口添加public（和protected）成员？ N3797 17.6.5.5 [member.functions] / 2说：

一个实现可以在类中声明额外的非虚函数成员函数签名：

- 通过向成员函数签名中添加具有默认值的参数; [注意：实施可能不会向虚拟，全局或非成员函数添加具有默认值的参数。 - end note ]

- 通过两个或更多成员函数签名替换具有默认值的成员函数签名，

p>这是否意味着在任何情况下（包括保留的标识符），标准库不能添加任何标准中没有提及的公共成员？

一点点解释：这是关于添加签名的文本（我假设谈论新的签名只是为已经定义的功能，因此没有新的名称）我设法找到标准。还有脚注189，它说：

一个有效的C ++程序总是调用预期的库成员函数或具有相同行为的库成员函数。一个实现也可以定义另外的成员函数，否则不能被有效的C ++程序调用。

.functions]，所以它显然只是关于成员函数。我的问题是更通用，并要求任何参考我可能错过了：是允许添加新的名称到公共（和/或受保护的）标准类型的接口的标准库实现，无论是数据

我相信你有你需要的脚注组合 189 / code>，它说：

一个有效的C ++程序总是调用预期的库成员函数，一个实现还可以定义另外的成员函数，否则不能被有效的C ++程序调用。

和 17.6.5.11 的派生类：

，但不添加任何限制，即它不让我们说限制访问限定符等...

，我们可以看到 libstdc ++ 相当有效地使用派生类，例如在 stl_vector.h 。虽然我可以看到 libstdc ++ 似乎避免了添加公共数据成员，但这可能更多的是干净的设计。

至少，这看起来低于指定，但如果你坚持类似于 libstdc ++ 实现风格，你应该是好的。

Are C++ standard library implementations allowed to add public (and protected) members to standard types' interfaces? N3797 17.6.5.5 [member.functions]/2 says:

An implementation may declare additional non-virtual member function signatures within a class:

— by adding arguments with default values to a member function signature; [ Note: An implementation may not add arguments with default values to virtual, global, or non-member functions. — end note ]

— by replacing a member function signature with default values by two or more member function signatures with equivalent behavior; and

— by adding a member function signature for a member function name.

Does this mean that a standard library cannot add any additional public members with names not mentioned in the standard under any circumstances (that include, for example, reserved identifiers)?

A tiny bit of explanation: this is the text about adding signatures (which I assume talks about new signatures just for functions that are already defined to be there, so no new names) I managed to find in the standard. There is also the footnote 189, which says:

A valid C++ program always calls the expected library member function, or one with equivalent behavior. An implementation may also define additional member functions that would otherwise not be called by a valid C++ program.

All this text originates from [member.functions], so it is clearly about member functions only. My question is more generic and asks for any references I could've missed: is a standard library implementation allowed to add new names to public (and/or protected) interfaces of a standard type, be it data or function members?

解决方案

I believe you have what you need with a combination of foot note 189 which says:

A valid C++ program always calls the expected library member function, or one with equivalent behavior. An implementation may also define additional member functions that would otherwise not be called by a valid C++ program.

and section 17.6.5.11 Derived classes which says:

An implementation may derive any class in the C++ standard library from a class with a name reserved to the implementation.

but does not add any restrictions, i.e. it does not let's say restrict the access qualifiers etc...

and we can see libstdc++ uses derived classes pretty effectively, for example in stl_vector.h. Although as far as I can see libstdc++ does seem to eschew adding public data members but that is probably more for clean design.

At minimum, this looks under-specified but if you stick to something similar to libstdc++ implementation style you should be good.

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