bind(std::plus&lt;std::string&gt;(), boost::ref(s1), boost::ref(s2))</strong> ];
</pre>
<!-- @example.prepend('''
#include <boost/bind.hpp>
#include <boost/ref.hpp>
#include <boost/parameter.hpp>
#include <string>
#include <functional>

namespace parameter = boost::parameter;

BOOST_PARAMETER_NAME(s1)
BOOST_PARAMETER_NAME(s2)
BOOST_PARAMETER_NAME(s3)

template <class ArgumentPack>
std::string f(ArgumentPack const& args)
{
    std::string const& s1 = args[_s1];
    std::string const& s2 = args[_s2];''') -->
