Re: [Boost-users] [boost] upcoming Switch library review Jan 5th - Jan 9th

Stjepan Rajko wrote:

On Jan 6, 2008 7:34 AM, Tobias Schwinger wrote:

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Tobias Schwinger writes:

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Of course there are no pointers to templates, so using a function pointer for anything but the default is pretty pointless. So is trying to handle varying result types -- maybe the result type should be passed in with another template parameter? I'd rather leave it as result_of::type. Actually 'result_of::type' determines the result of the nullary call to F. I don't think I like this sort of "result_of abuse"... The correct usage would be 'result_of::type' but it's

Steven Watanabe wrote: pointless since 'MPLConstant' varies and so may the whole type expression.

So, if you insist on deducing the result type from the function object (instead of having it specified explicitly) my vote is 'F::result_type', however, I still find another template parameter more appropriate for the following good reasons:

o The function object can work fine with result_of in a non-'switch_' context. The cases can return different things as long as they are convertible to the result of 'switch_', and

o there is no way to determine this type with 'result_of.

Actually, the latter is more severe...

The current implementation seems to use result_type - is it planned to change to use result_of?

I agree that result_of::type is slightly abusive, since that's not what actually gets called.

Would using result_of be an option for a non-empty case sequence?

I think it's too complicated: We can't use 'result_of' to determine the result of 'switch_', so it should be as simple to determine as possible (ideally without deduction at all).

As long as the order of the cases doesn't matter (btw, does it?), the user could put the desired type in the front of the Cases sequence if the return type differs for different MPLConstant types.

Further, we still need a special-engineered function object; one of the cases will have a special role. It might work, but it feels inelegant to me: The function object's result type should be convertible to whatever 'switch_' wants to return. So what will deducing that type from the function object buy us? The only answer I can currently see is "nothing but trouble" :-). Please tell me if I'm missing something. Regards, Tobias