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Subject: Re: [boost] [fusion] improving compile times
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2009-06-02 15:51:56


Eric Niebler wrote:
> Cool! I wonder how that's possible. I have it from Walter Bright
> (Zortech, Symantec, Digital Mars) that instantiating a template is
> inherently expensive, and certain features of the C++ language (ADL,
> partial specialization, etc.) force that to be the case. If Clang has
> found a way to solve these problems, that's good news indeed. I read
> form the Wikipedia entry that Clang's C++ support is 2-3 years from
> being usable, though.

It should be possible to reduce the cost by using fancier data structures.
With appropriate data structures, you don't necessarily have to
look at every specialization or overload to know which is the
most specialized. In most cases with class template specialization,
there are only a few cases that can lead to ambiguities. Most
specializations are disjoint, so it should be possible to prune
most of them quickly. (I haven't thought this through fully--it's
probably pretty difficult to get right). Also, except for the templates
used for the various extension mechanisms, class templates tend
to have only a few specializations.

In Christ,
Steven Watanabe

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