声明朋友时必须声明类键
当我这样声明友元时，g++ 编译器会抱怨此错误：
friend MyClass;
而不是
friend class MyClass;
Why should the class keywords be required? （Borland C++ 编译器，顺便说一句，不需要它。）
编译器不能简单地在符号表中查找 MyClass 并告诉它被声明为一个类吗？ （显然它无论如何都会进行查找，因为当 MyClass 未声明时它会抱怨）
它不像是在对类进行前向声明：我仍然必须声明上面的类，或者至少要前向声明它。
如果需要的话进行前向声明，这对我来说是有意义的（实际上会很棒）
friend class MyClass;
，否则对我来说它就像语法盐。
近 20 年来，我一直愉快地使用不带 class 或 struct 关键字的 friend 语句，没有编译器抱怨。 这是相当新的东西吗？
The g++ compiler complains with this error when I declare a friend thusly:
friend MyClass;
instead of
friend class MyClass;
Why should the class keyword be required?
(the Borland C++ compiler, BTW, does not require it.)
Couldn't the compiler simply look-up MyClass in the symbol table and tell it was declared as a class? (it is obviously doing the look-up anyway because it complains when MyClass it not declared)
It is not like it is making a forward declaration of the class: I still have to have either declared the class above or at least have forward declared it.
It would make sense to me (would be great actually) if
friend class MyClass;
makes a forward declaration if needed, otherwise it just seems like syntactic salt to me.
I have been merrily using friend statements without the class or struct keyword with no compiler complaints for almost 20 years.
Is this something fairly new?
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我对此感到惊讶（因此删除了之前的错误答案）。 C++03标准在11.4中说：
然后为了确保没有误解，它在脚注中添加了以下内容：
GCC 是我拥有的唯一一个抱怨缺少类键的编译器，但看起来其他编译器让我们摆脱了一些非标准的东西......
现在至于理由 - 你必须问某人关于编译器（或标准）的了解比我多。
I was surprised about this (and as a result deleted a previous incorrect answer). The C++03 standard says in 11.4:
Then to make sure there's no misunderstanding, it footnotes that with:
GCC is the only compiler that I have that complains about the missing class-key, but it looks like other compilers are letting us get away with something non-standard...
Now as for the rationale - you'd have to ask someone who knows more about compilers (or standards) than I do.
对于您的问题，因为这是 ISO/IEC 14882:2003 指定的方式（第 7.1.4 节）。
friend构造本质上指定为：其中
因此，
不是有效的声明，而：
或：
是。
同上，相应地：
或
To the point of your question, because it is the way ISO/IEC 14882:2003 specifies it (section 7.1.4). The
friendconstruct is essentially specified as:where
<declaration>is the valid declaration of a class, struct, template, or function.Thus,
is not a valid declaration, whereas:
or:
are.
Idem for, correspondingly:
or