Mike Conley <[EMAIL PROTECTED]> writes:
> David Abrahams <[EMAIL PROTECTED]> wrote in
> news:[EMAIL PROTECTED]:
>
>> That completely scuttles the ODR, as far as I can tell.
>
> Naturally, you wouldn't want to use a built in is_convertible this way.
> Better to pass it as a template parameter directly, rather than wrapping
> it: and_, is_convertible >.
>
> If you know that the result of is_convertible depends on context, and you
> know about the ODR, then you know not to use is_convertible to as part of a
> metafunction's return value without also making it part of the
> metafunction's type (ie, using it as a template parameter).
It doesn't matter if it's a template parameter. It's an ODR violation
to have:
template <>
struct is_whatever
{
static bool const value = false;
};
and
template <>
struct is_whatever
{
static bool const value = true;
};
in the same program, which is essentially the effect if is_whatever
uses a context-dependent is_convertible in its body.
--
Dave Abrahams
Boost Consulting
www.boost-consulting.com
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