The following code crashes in for_each when trying to output the 1st pair. What's wrong with it?

 

Thanks!

 

namespace bl = boost::lambda;

std::map<std::string, std::string> temp;
temp["a"] = "1";
temp["b"] = "2";
const std::map<std::string, std::string> attrs(temp);
typedef std::pair<std::string, std::string> string_pair;
std::stringstream stream;
std::for_each(attrs.begin(), attrs.end(),
   stream << bl::bind(&string_pair::first, bl::_1) << bl::bind(&string_pair::second, bl::_1));