Re: boost variant template type argument count

Carl Barron <>
Wed, 16 Apr 2008 04:13:20 CST
In article
<> 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? 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?)

Thanks for any info. (I briefly wandered around the boost headers, but
their organization is not transparent to the uninitiated.)


    It is defined as BOOST_MPL_LIMIT_LIST_SIZE so if you need more than
20 items, you will need to reconfigure mpl and not use its preprocessed
headers which is the default. Perhaps you can split the variant into
two variants and have a variant of variants.

can four or more be split off.

typedef boost::variant< T1,T2,T3,T4,T5,...,T12> varaint_1;
typedef boost::variant<T12,T13,....,T24> variant_2;

typedef boost::variant<variant_1,variant_2> my_variant;

struct my_visitor:public boost::static_visitor<T>
    struct visitor_1:boost::static_visitor<T>
       T operator () (T1 x);
       // handle T1 - T12;

    struct visitor_2:boost::static_visitor<T>
       // handle T13-T24

    T operator () (variant_1 const &x)
       return boost::apply_visitor(visitor_1(),x);
    T operator () (variant_2 const &x)
       return boost::apply_visitor(visitor_2(),x);

should be able to visit your 24 types.

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