[boost] Re: base/derived conversions (Was: Any good reason not to have == for shared_ptr?)

From: Dave Abrahams (abrahams_at_[hidden])
Date: 1999-11-11 00:46:17

> ivan66_at_[hidden] wrote:
> 
>> I assume that it would be legal to compare a shared_ptr<Derived> with a
>> shared_ptr<Base>?  Also, that a shared_array<Base> could be compared to
>> a shared_array<Base>,
>
> It depends on the declaration of op== (member or non member).
>
> 1) member: conversions allowed on rhs but not lhs
>    => base == derived passes
>    but derived == base fails
>    => very strange dissymmetric effect
>
> 2) non member: no conversions at all
>    => symmetry in that base == derived and derived == base
>       both fail the same way

They could also be defined to succeed (symmetrically) in the same way, using
either approach. All that's needed is a member template op==().

-Dave