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

Joel de Guzman wrote:

Tobias Schwinger wrote:

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Joel de Guzman wrote:

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Tobias Schwinger wrote:

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Stjepan Rajko wrote:

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

Actually, the latter is more severe...

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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).

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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. I haven't been following the review (yet, though I'll be in the next few days) and I haven't read the docs yet. But from intuition, having implemented at least 2 switch implementations (spirit and phoenix), I'm guessing that there are N functions with N return types, right? If so, it follows (to my mind) that the result should be a boost::variant of all the possible return types given the arguments (through application of result_of to all the functions). What are we going to do with that Variant? Switch again and go back where we're coming from?

Use the variant visitor. That may amount to the same thing, but in this case, we have a value, not a function that returns a value.

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It might but isn't necessarily what we want.

Why not?

Because we're just coming from inside the switch and if we wanted to do something conditionally we could have done it there. Variant, Any, Optional or some non-union type can all make sense, depending on what we're up to.

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Now in order to get our Variant we have to o check result_of::type for every function, o figure out unique types, and o make a variant from them, possibly handling special cases like using Boost.Optional or the bare type where more appropriate.

It's surely useful for certain use cases but takes a lot of template instantiations, so I think it should be an extra metafunction.

Sorry, perhaps I missed the "extra metafunction" solution. What is it again?

Ooops -- the English word "extra" might not work quite the way I expected (see below for clarification).

Anyway, you have good points. Yes, an explicitly specified return type would be ok too. In the standpoint of optimization, yes, a variant return is suboptimal at both compile time and at runtime.

Exactly. The mechanism you propose could still be useful and provided by the library as a utilty metafunction (which is what I was trying to say with "should be an extra metafunction" :-)). Regards, Tobias