有关外部链接和调用约定的C ++标准 [英] C++ Standard regarding external linkage and calling conventions

问题描述

我已经阅读了最新的C ++ 11草案(n3337-它是最后一个吗?)，我对正在研究的可能实现方案有疑问.

I've read the last C++11 draft (n3337 - is it the last one?), and I got a question for a possible implementation I've been working on.

假设我们有以下代码:

extern "Objective C" {
  class Object {
    public:
      static Object *alloc();
      Object *init();
  };
};

然后打电话

Object *x = Object::alloc()->init();

问题是，我不知道编译器是否可以控制外部"X"块的调用约定:这个想法是将调用"翻译"到objc_msgSend(blablabla)－符合该标准，还是将其视为扩展(因为它不仅会修改符号名称，而且还会修改此处的复杂调用约定")?当然，我可以通过创建一个称为__thiscall的弱函数来实现此功能，然后调用该方法并返回该方法本身-但问题仍然存在，是否会保持一致?

The question is that I didn't understand if it is allowed for the compiler to control the calling convention of extern "X" blocks: the idea would be to "translate" the calls to objc_msgSend(blablabla) - would that be conformant to the standard, or would it be considered an extension (since it wouldn't just modify the symbol name, but as well as a complex "calling convention" here)? Of course I could implement this by making a weak function that is called as __thiscall and then calls and returns the method itself - but the question continues, would it be conformant?

谢谢！

推荐答案

您描述的所有声音都符合语言链接功能的意图.

Everything you describe sounds to be in line with the intent of the language linkage feature.

班级成员被明确排除在"C"语言链接之外.根据标准(C ++ 11§7.5/4)中的示例，类成员声明，​​任何上下文中的extern声明以及所有其他函数声明中的函数指针类型均继承了封闭的extern "C" {}块.您正在定义除"C"之外的语言链接，但是遵循它的示例可能并不奇怪，也许是在this和self之间进行了额外的映射.

Class members are specifically excluded from "C" language linkage. According to examples in the Standard (C++11 §7.5/4), function pointer types within class member declarations, extern declarations in any context, and all other function declarations do inherit the enclosing extern "C" {} block. You're defining a language linkage besides "C" but it would be least surprising to follow its example, perhaps with extra mapping between this and self.

根据§7.5/2，要求是开放的:

The requirements are open, according to §7.5/2:

通过实现定义的语义有条件地支持使用除"C"或"C ++"之外的 string-literal . [注意:因此，实现未知的带有字符串文字的链接规范需要诊断. —尾注] [注:建议从定义该语言的文档中获取字符串字面量的拼写.例如，取决于年份，Ada(不是ADA)和Fortran或FORTRAN. —尾注]

Use of a string-literal other than "C" or "C++" is conditionally-supported, with implementation-defined semantics. [ Note: Therefore, a linkage-specification with a string- literal that is unknown to the implementation requires a diagnostic. — end note ] [ Note: It is recommended that the spelling of the string-literal be taken from the document defining that language. For example, Ada (not ADA) and Fortran or FORTRAN, depending on the vintage. — end note ]

您可以在花括号内切换到完全不同的语言，这样就可以了. (尽管从语法上讲，我认为所有内容都应解析为C ++ 声明或 declaration-seq .)

You could switch to a completely different language within the braces an it would be alright. (Although, grammatically, I suppose everything should parse as a C++ declaration or declaration-seq.)

但是，因此，您应该将extern "Objective-C"与连字符一起使用，因为最终文档的标题是"The Objective-C Programming Language".

According to that, though, you should use extern "Objective-C" with a hyphen since the definitive document's title is "The Objective-C Programming Language."

编辑:可以肯定的是，通过objc_msgSend调用不会影响标准指定的任何内容.它无处说明如何调用C ++函数.添加的中间函数只是机器级别的指令，超出了语言的语义范围，与用于调用Pascal，Fortran等的特殊指令没有什么不同，例如.通过更改堆栈上参数的顺序.我并不是要将您的方案与在花括号内切换为完全不同的语言"进行比较，只是要强调还有足够的余量.

To be sure, calling through objc_msgSend doesn't affect anything that the Standard specifies. Nowhere does it say how C++ functions are called. The added intermediate function is just machine-level instructions, beyond language semantics, and no different from the special instructions used to call into Pascal, Fortran, etc., e.g. by altering the order of parameters on the stack. I don't mean to compare your scheme to "switching to a completely different language within the braces," just to emphasize that there's plenty of headroom.

以Standard中的extern "C"规范为例，它从根本上更改了ODR规则，这打破了很多事情，因为不需要C支持任何处理.因此，您的方案至少可以破坏 这么多.

Taking the extern "C" specification in the Standard as an example, it breaks a lot of things by fundamentally changing the ODR rule because C isn't required to support any mangling. So your scheme is allowed to break at least that much.

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