Re: [boost] numeric_limits<Abstract> or how to fix 1358600?

From: Andrey Semashev (andysem_at_[hidden])
Date: 2007-01-20 06:28:12

Hello Alexander,

Saturday, January 20, 2007, 12:22:35 AM, you wrote:

> I commited the fix for 1358600 - lexical_cast & pure virtual functions
> & VC 8 STL (http://tinyurl.com/yvowz6) for VC++ only but later I
> found that Intel 8.1 and Comeau online don't compile as well.

> The problem appears in numeric_limits functions. In this usecase,
> they return an abstract class by value. Although lexical_cast never
> uses those functions, it seems that some compilers do more strick
> checking than I thought they should.

> For example,

> class A
> {
> public:
>     virtual void foo() = 0;
> };

> template<class T>
> struct limits
> {
>     static int const value = true;
>     static T declared_only();
> };

> int main()
> {
>     limits<A>::value;
> }

> fails to compile on VC++ 8, Intel 8.1 and Comeau online at the point
> of the declared_only declaration.

> However, if I replace a body of A with a declaration (class A;),
> those compilers don't complain anymore.
> What is this, a compiler bug or my misunderstanding? I remember
> that some freedom compiling template code is allowed (and therefore,
> some compiler compiles while some others don't) but I don't know
> where to search in the standard.

According to 14.7.1 (namely 14.7.1/2 and 14.7.1/3) the declared_only
member should not be instantiated unless it is used somewhere in the
code. So I see no reason a compiler to want T to be not abstract.
Looks like compiler bug to me.

-- 
Best regards,
 Andrey                            mailto:andysem_at_[hidden]