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Subject: Re: [boost] safe-bool CRTP class
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-05-27 12:28:55

Krzysztof Czainski wrote:
> Vladimir Batov prepared a safebool helper class as part of his
> Pimpl library.
> In the review of Pimpl Joej Falcou suggested to make safebool a
> CRTP base class.
> I had prepared such a class some time ego, and I've been using
> it for a while now. Is there interest in adding it to boost?
> Perhaps as part of the operators library?


> template < class Derived, class Base = empty_t >

I like that you're allowing for a base class.

> class boolable : public Base

Why not s/boolable/safe_bool/?

> {
> void unspecified_bool_t_f() {}
> typedef void (boolable::*unspecified_bool_t)();
> public:
> operator unspecified_bool_t() const
> {
> return ! static_cast<Derived const&>( *this ) ? 0 :
> &boolable::unspecified_bool_t_f;

This doesn't support types that have no other reason to define operator !() or that have a ready-made is_valid() sort of function. IOW, the "valid" or "invalid" function on which to rely should be configurable. (It should be possible to provide either one.)

I understand that you're assuming that if one can write if (x), one might assume that if (!x) should also work, but that should work anyway with the safe-bool operator, right?

Rob Stewart robert.stewart_at_[hidden]
Software Engineer using std::disclaimer;
Dev Tools & Components
Susquehanna International Group, LLP

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