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From: Joel de Guzman (joel_at_[hidden])
Date: 2003-11-25 18:46:51

Peter Dimov <pdimov_at_[hidden]> wrote:
> Joel de Guzman wrote:
>> IIUC, what you mean is that we do not have to review something like
>> that. Ok, but it would still be good to have a single implementation.
>> Spirit uses:
>> struct safe_bool_impl
>> {
>> int* dummy;
>> };
>> typedef int* safe_bool_impl::*safe_bool;
>> inline safe_bool
>> make_safe_bool(bool cond)
>> {
>> return cond ? &safe_bool_impl::dummy : 0;
>> }
>> and it is used all throughout.
> Then you should call it unsafe_bool since having a conversion to it permits
> comparisons between unrelated types.

You are right! How could I have missed that?

Second take:

    template <typename T>
    struct safe_bool
        int* dummy;
        typedef int* safe_bool::*type;

    template <typename T>
    inline typename safe_bool<T>::type
    make_safe_bool(bool cond)
        return cond ? &safe_bool<T>::dummy : 0;

Can you spot some problems with this too? The point is that it would be
nice to have one *correct* and generic implementation that everyone
can use. It's easy to get it wrong (I did). There are some subtle issues
that make it an ideal candidate for a small library component (e.g.
Borland does not like the pointer to member stuff, maybe there's a
better formulation for Borland? e.g. While the code above is fine with
g++3.2 and Comeau, VC7.1 does not like it in some cases, etc...).


Joel de Guzman

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