From: Joel de Guzman (joel_at_[hidden])
Date: 2006-06-10 13:04:34
Oleg Abrosimov wrote:
> Hello dear authors of boost tuple libraries!
>
> I've a need to implement a function that gets a tuple of manipulators
> and applies them to stream:
>
> template <typename T, typename Tuple>
> std::string string_from(T const& t, Tuple const& manip)
> {
> std::stringstream ss;
> ss << manip << t;
> return ss.str();
> }
>
> usage of this function would be:
>
> std::string s = string_from(1.2, boost::make_tuple(std::setprecision(2),
> std::scientific));
>
> The problem is: how I can efficiently suppress output of
> left/right/delimiter strings?
> Of cause, with fusion::tuple I can do the following (manipulator names
> are taken from boost::tuple docs, though):
>
> template <typename T, typename Tuple>
> std::string string_from(T const& t, Tuple const& manip)
> {
> std::stringstream ss;
> ss << set_open("") << set_close("") << set_delimiter("") << manip << t;
> return ss.str();
> }
>
> but it has a lot of overhead in tuple's output iterator that should be
> avoided for me.
>
> I feel that the best solution would be to have a function
> "print_tuple(stream, tuple)" in tuples library that just do what I want.
With Fusion, this is easy. Using lambda/phoenix:
fusion::for_each(tup, std::cout << _1);
If you want it to be a function (using plain-ole functors):
namespace detail
{
template <class Out>
struct my_printer
{
printer(Out& out) : out(out) {}
template <class T>
void operator()(T const& val) const
{
out << val;
}
Out& out;
};
}
template <class Out, class Tuple>
Out& print_tuple(Out& out, Tuple const& tup)
{
fusion::for_each(tup, detail::my_printer<Out>(out));
}
> One more issue with fusion::tuple manipulators as they are in boost
> 1.33.1 (unofficially): I've discovered that only narrow strings and
> characters are supported. am I right? If so, I believe that wide
> versions should be added too.
Hmmm... Have you tried? Those are templated on Char type AFICT.
I have to admit not testing that though. The Fusion I/O tests
are simply those from Boost.Tuples. Added in my TODO list.
Regards,
-- Joel de Guzman http://www.boost-consulting.com http://spirit.sf.net