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From: Larry Evans (cppljevans_at_[hidden])
Date: 2005-10-22 06:58:30

On 10/21/2005 04:37 PM, Korcan Hussein wrote:
> "Larry Evans" <cppljevans_at_[hidden]> wrote in message
>>I'm afraid I don't know what:
>> type deduction of intermediate results from expression templates
>>means. Could you explain more with examples?
> I'll make a quote from the boost MPL book :-)
That helps a lot (BTW, it's on p. 236).
> better alternative to type erasor (unless it's already in use or have been
> discussed about). I think it would be very useful to libraries such as uBlas
> and Spirit.
> Boost.Any is one example where type erasing is used, basically you keep
> richful type info to the very last possible moment and then lose most of it
> in a polymophic type and use dynamic dispatching etc. So it's like your
> erasing the type, would you like a code example? ;)
No. "type erasure" is in index of the MPL book; so, I'll just consult
>>>parse tree of an intermediate expression and create an exact runtime
>>>of the expression tree using recursive variant types (using Boost.Variant
>>>C++). The creation of the expression tree can be completely automated.
>>Why would you want a runtime replica? I suppose to solve the "type
>>deduction of intermediate results" problem mentioned above, but, as
>>I've said, I don't know what this is :(
> I think that should make a little more sense now, if not i can explain some
> more if need be ;)

Please do. I'm guessing that the replica is encoded in the variant's
which member function. IOW, just like the expression type in:

encodes the type of expression as:

   variant.which() expression type
   1 int
   2 binary_op<add>
   3 binary_op<sub>

your proposal would do the same for each possible value of expression
type. IOW, instead of dynamic dispatching as done by type erasure,
the dispatching would be done on variant.which()?

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