[boost] Re: compressed_pair requirements?

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2005-05-28 08:46:33

On May 28, 2005, at 6:37 AM, John Maddock wrote:

>> I am getting the error from Intel C++ at the point of declaration of
>> the compressed_pair specialization.
>
> I've experimented some more, and found one bug in the current 
> implementation:
>
> If one of the templates arguments is both const and an empty class, 
> then compressed_pair attempts to inherit from the const-class type 
> which generates an error for Intel C++, but not strangely for VC++.

Interesting.  I'm not seeing that problem here (on CW) either.

> BTW, at the risk of stating the obvious, compiler error messages 
> really help when reporting issues (JM runs and ducks for cover!).

Umm... yeah!  And what was the error on Intel with the const empty 
type? ;-)

This is a pretty interesting point.  I whipped this test up to explore:

struct Base {};

typedef const Base ConstBase;

struct A
     : ConstBase
{
     A() {}
};

It compiles on CW.  But it doesn't on Comeau C++ Online (thanks Greg).  
Normally when CW and EDG disagree, the decision usually goes to EDG.  
So I started searching the standard for where it says that cv-qualified 
classes can't be used as base specifiers so that I could send our 
compiler team the bug report.

Couldn't find it.  So I started digging into the core issues list and 
found:

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_closed.html#484

Subject:  Can a base-specifier name a cv-qualified class type?

Ah, perfect.

> The resolution of issue 298 added new text to 9.1   class.name 
> paragraph 5 making it clear that a typedef that names a cv-qualified 
> class type is a class-name. Because the definition of base-specifier 
> simply refers to class-name, it is already the case that cv-qualified 
> class types are permitted as base-specifiers.

No kidding.  And check out the date on that:

> Rationale (April, 2005):

Fresh from the Lillehammer presses! :-)

-Howard