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Re: Compile error in GCC but not other compilers

On May 15, 2006, at 5:57 PM, Duane Murphy wrote:

--- At Mon, 15 May 2006 17:15:55 -0400, Howard Hinnant wrote:

On May 15, 2006, at 3:08 PM, James W. Walker wrote:

One of my problems in porting from CodeWarrior to Xcode boils down
to the code below:

namespace
{
	class xxx
	{
		xxx();
	};
}

class yyy
{
	friend class xxx;
};

xxx::xxx()
{
}


Xcode says:

error: 'xxx' has not been declared
error: ISO C++ forbids declaration of 'xxx' with no type

CodeWarrior has no problem with this code, nor does the Comeau
online compiler test page.  So, is this a bug in GCC?


This looks like a bug to me.  You could pop "::" unto xxx in the
friend statement to workaround:

friend class ::xxx;

Unfortunately it isn't a very good workaround because it is now
illegal C++ code. :-(


I've hit this myself and scratched my head.

Can you explain why

friend class ::xxx;

is also illegal?

I thought I knew, but the more I looked into it, the more puzzled I became! ;-)

So I went to the committee on this one.  Here's the reply:

It is not possible to befriend a class in an unnamed namespace in C+ +03. So this behavior of gcc isn't a bug after all, and one can consider the:

friend class ::xxx;

an extension for now.

In C++0X it will be possible to befriend such a class, but the syntax will be slightly different:

friend xxx;

I'm not going to pretend to understand the name lookup rules in enough detail to explain the rationale here. But for the truly interested, the twisty tale starts here:

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#138

This link will further link to other core issues, and to a paper (which introduces the friend xxx syntax). Sorry for the initial false direction.

-Howard

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References: 
 >Compile error in GCC but not other compilers (From: "James W. Walker" <email@hidden>)
 >Re: Compile error in GCC but not other compilers (From: Howard Hinnant <email@hidden>)
 >Re: Compile error in GCC but not other compilers (From: "Duane Murphy" <email@hidden>)



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