Support for const_string in std::ostream operator <<

Question

I'm currently using the very clever package boost::const_string until http://libcxx.llvm.org/ is available pre-packaged on Ubuntu or GCC make its __versa_string (in header ext/vstring.h) its default string implementation. libcxx's std::string aswell as __versa_string uses _small-string optimization (SSO) by default. Default support for outputting to an std::ostream is lacking however. The code

#include <iostream>
#include <boost/const_string.hpp>

const_string<char> x;
std::cout << x << endl;

does not work unless we force x into a c-string via c_str() which becomes

std::cout << x.c_str() << endl;

which compiles and works as expected. I added the following line to const_string.hpp

template <typename T>
inline std::ostream & operator << (std::ostream & os, const boost::const_string<T> & a)
{
    return os.write(a.data(), a.size());
}

This should improve performance over x.c_str() because size() is already known and does not need to be calculated by searching for NULL as in c_str(). I works for me but I am uncertain whether it works all cases. Have I missed something?

Answer 1

Have I missed something?

Yes, just include const_string/io.hpp. All it does however is:

return o << std::basic_string<char_type, traits_type>(s.data(), s.size());

Answer 2

It seems that this could have implications based on the locale and/or facets applied to the stream for strings vs just writing the straight data as you're doing.

It would be less performant, but what about creating a std::string from the const_string and using << to insert that into the stream?

Answer 3

No (you have not missed anything, afaik). If your aim is not to copy over content, str.data() is the way to go.