std::cerr << segment_term.str() << endl;

<sstream>库定义了三种类：istringstream、ostringstream和stringstream，分别用来进行流的输入、输出和输入输出操作。

　　1.stringstream::str(); returns a string object with a copy of the current contents of the stream.

　　2.stringstream::str (const string& s); sets s as the contents of the stream, discarding any previous contents.

　　3.stringstream清空，stringstream s; s.str("");

stringstream segment_term;
segment_term <<"";
for (int j = 0; j< jbterms.size(); ++j)
{
if(j < jbterms.size()-1)
{
//cout<< jbterms[j].word <<endl;
segment_term << jbterms[j].word<< " ";
}
else
{
segment_term << jbterms[j].word;
}
}
std::cerr << segment_term.str() << endl;   !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!    .str()  就是得到流的字符串。