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From: Joel de Guzman (djowel_at_[hidden])
Date: 2003-04-20 00:44:15

Fernando Cacciola wrote:

> Currently, optional<> is using its own aligned_storage
> template, but this is because I've found that the
> aligned_storage as it was designed for both optional
> and variant ICEed BCB5 on some particular cases,
> so I had to provide a custom template; but I intend
> to find a 'general' version of aligned_storage that
> can be used by optional and variant at the same time.
>> There will be a specialization
>> for the case where the padding is non-zero and the flag is placed
>> in the padding and another specialization for the case where padding
>> is zero and the flag is a data member. Generic accessor member
>> functions can be provided: bool is_initialized() const, void
>> is_initialized(bool f);
> We might come to conclude that the 'initialized' flag is approptiate
> even
> for a general 'aligned_storage' facility. In this case,
> I think your technique will work.

In fact, it might be tempting to use the other bits as well.
For example, I'd imagine a variant implementation that uses
8 bits to flag the currently active type. I imagine this interface:

    template <class T>
    class aligned_storage
        void const* address() const;
        void* address();
        unsigned char tag() const;
        void tag(unsigned char data);

Joel de Guzman
joel at

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