Re: [boost] attempting sandbox svg_plot templates

Subject: Re: [boost] attempting sandbox svg_plot templates
From: Jan Hudec (bulb_at_[hidden])
Date: 2012-08-24 14:03:41

On Tue, Aug 21, 2012 at 15:09:02 -0400, Roger Martin wrote:
> Run into many template and friend issues. Anybody getting this lib to work?
> 
> Much coming from https://svn.boost.org/svn/boost/sandbox/SOC/2007/visualization/boost/svg_plot/uncertain.hpp.
> First two:
> ..............
> In file included from ../../testing-grounds/divcon-support_libs/local/r.linux-x86_64.gcc-shared/include/boost/svg_plot/svg_2d_plot.hpp:32:0,
> from tests/plotting_tests.cpp:21:
> ../../testing-grounds/divcon-support_libs/local/r.linux-x86_64.gcc-shared/include/boost/svg_plot/uncertain.hpp:241:11:
> required from here
> ../../testing-grounds/divcon-support_libs/local/r.linux-x86_64.gcc-shared/include/boost/svg_plot/uncertain.hpp:97:24:
> error: template-id ‘operator<< <>’ for ‘std::ostream&
> boost::svg::operator<<(std::ostream&, const
> boost::svg::unc<false>&)’ does not match any template declaration
> ../../testing-grounds/divcon-support_libs/local/r.linux-x86_64.gcc-shared/include/boost/svg_plot/uncertain.hpp:102:24:
> error: template-id ‘operator<< <>’ for ‘std::ostream&
> boost::svg::operator<<(std::ostream&, const
> std::pair<boost::svg::unc<false>, boost::svg::unc<false> >&)’ does
> not match any template declaration
> ..............

It's making friends with templates that don't exist. I don't think the
involved templates can work in any compler, ever.

There is however big difference between Gcc and Visual C++ in treating of
templates. Gcc parses everything as it comes across it and insists that it's
well-formed, while VC++ delays lot of it until instantiation, apparently in
an attempt to relieve programmers from having to write `typename` and
`template` in many cases. That means Gcc will complain about things like this
while VC++ won't.

However the templates seem pretty bogus. There is a 

    template <bool correlated> class unc;

followed up by

    template<typename correlated> std::ostream& operator<< (std::ostream& os, const unc<false>& u);

followed up by

    friend std::ostream& operator<< <> (std::ostream& os, const unc<correlated>& u);

and that simply does not add up under any compiler. The middle template is
not using it's template argument in it's signature, so it's neither
possible to give it's specialization as the friend declaration is trying to
do, nor is it actually useful, because you'd need to give the template
parameter explicitly, so the only possible call syntax is

   operator<< <whatever> (out, u)

and that certainly does not look like what was intended.

So I suggest you just try to comment the offending declarations out and see
if it gets you somewhere.

-- 
						 Jan 'Bulb' Hudec <bulb_at_[hidden]>