On Apr 16, 7:43 am, thant.tess...@[EMAIL PROTECTED]
wrote:
> I'm using an instance of the boost::variant type as the token value of
> a non-trivial grammar. The type list is 24 items long. Apparently
> there is a type list limit of 20 items. Is this correct?
Yes, the limit is 20 by default. Look at "boost/variant/
variant_fwd.hpp" and search for the macro BOOST_VARIANT_LIMIT_TYPES
which defines this limit. See also the macro
BOOST_VARIANT_VISITATION_UNROLLING_LIMIT in "boost/variant/detail/
visitation_impl.hpp", which defines the limit specifically for the
visitation mechanism (also 20 by default).
> Is this just
> a matter of building the cases, or is it dictated by some platform
> limit on template type arguments? (Is it actually dictated by the
> apply_visitor template?)
20 is an arbitrary choice that had to be made for putting a max limit
on :
- the number of template arguments a variant can handle, since C++
doesn't sup****t variadic templates yet.
- the number of cases in the switch-case statement of the
apply_visitor function. Note that, no matter the actual number of
types given when instantiating a variant, that switch-case will always
have the maximal number of cases (20 cases or, more accurately
BOOST_VARIANT_VISITATION_UNROLLING_LIMIT cases) , which is a pity
since there are techniques to adapt the number of cases depending on
the number of template arguments.
Alexandre Courpron.
--
[ See http://www.gotw.ca/resources/clcm.htm
for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]
|
