Re: [Boost-users] [fusion] adapted std::pair and ostream

Subject: Re: [Boost-users] [fusion] adapted std::pair and ostream
From: Joel de Guzman (joel_at_[hidden])
Date: 2011-07-19 19:16:43

On 7/20/2011 3:40 AM, Nathan Ridge wrote:
>> The docs:
>>
>> http://www.boost.org/doc/libs/1_47_0/libs/fusion/doc/html/fusion/adapted/std__pair.html
>>
>> seem to suggest that one can pipe an adapted std::pair 'directly' to cout.
>>
>> I find the following though:
>>
>> #include <iostream>
>> #include <boost/fusion/adapted.hpp> // adapt pair.
>> #include <boost/fusion/sequence.hpp> // operator<<() ?
>> #include <boost/fusion/container.hpp> // as_vector
>>
>> int main(int argc, char** argv)
>> {
>> std::pair<std::string, int> p("foo", 7);
>>
>> std::cout << p << std::endl; // does not compile although docs
>> suggest it should.
>> std::cout << boost::fusion::as_vector(p) << std::endl; //
>> compiles and works as expected.
>>
>> return 0;
>> }
>>
>> Am I missing an include? Is sending to cout without first using
>> as_vector actually supported?
> 
> Hmm... well boost/fusion/sequence/io/out.hpp (which is included
> by boost/fusion/sequence/io.hpp which is in turn included by
> boost/fusion/sequence.hpp) does define the correct operator<<
> for all sequences, but it's in the boost::fusion namespace, and I
> don't see it being exported into the global namespace anywhere.
> 
> If you add "using boost::fusion::operator<<;" to your code, it
> works fine.
> 
> But this seems to be in contradiction with the docs, which say [1]
> (emphasis mine):
> 
> The **global** |operator<<| has been overloaded for generic
> output streams such that Sequence
> <http://www.boost.org/doc/libs/1_47_0/libs/fusion/doc/html/fusion/sequence.html>(s) are
> output by recursively
> calling |operator<<| for each element.
> 
> Perhaps the authors were relying on ADL to find operator<<,
> but overlooking the fact that not all sequences would be in the
> boost::fusion namespace?

Thanks. That's certainly not correct. Adapted sequences in other
namespaces can't avail of that. I just fixed the docs to avoid
further confusion.

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://boost-spirit.com