Re: [boost] boost::mpl::vector : too many template arguments

From: David Abrahams (dave_at_[hidden])
Date: 2002-11-07 07:49:46

Aleksey Gurtovoy <agurtovoy_at_[hidden]> writes:

> Li Lirong wrote:
>> I'm using msvc 7.0 and boost_1_29_0.  I got this error,
>> "boost::mpl::vector : too many template arguments", when the 
>> number of the template arguments of mpl::vector exceeds 10 no 
>> matter how I define the BOOST_MPL_LIMIT_VECTOR_SIZE value.  
>> Is this a bug of the library, limitation of the library or my 
>> misuse of the library?
>
> It's a documentation omission. By default the library is configured to use
> preprocessed headers, and in this mode changing the values of (most of)
> configuration macros will have no effect. If you switch the preprocessor
> headers off (by commenting out BOOST_MPL_USE_PREPROCESSED_HEADERS define in
> "mpl/aux_/config/use_preprocessed.hpp" header), everything will work as
> expected (at the price of slightly longer compile times).

Aleksey, don't you want this to be configurable on the command-line,
so users aren't forced to modify boost sources?

> Note that usually if you need a longer 'vector' or 'list', it's a better
> idea to use the numbered forms, e.g. 
>
>     #include "boost/mpl/vector/vector40.hpp"
>
>     typedef mpl::vector37<.....> types;
>
> Increasing BOOST_MPL_LIMIT_VECTOR_SIZE value will work, but you should
> realize that if you do that, then _every other_ translation unit (besides
> the one you're making the change for) that includes "mpl/vector.hpp" will
> pay for what it most probably doesn't use (in full). When the numbers are
> huge (e.g. 50), the difference in compilation times is quite noticeable.

Hmm, maybe that's a reason you don't want to do this...

-- 
                    David Abrahams
dave_at_[hidden] * http://www.boost-consulting.com