I tried :
ostringstream oss; read a string from file and put to oss; string str; str << oss.str();// error here "error: no match for ‘operator>>’ in 'oss >> str' " If I use str = oss.str(); Instead of printing the value of the string, it prints out "....0xbfad75c40xbfad75c40xbf...." likes memory address.
Can anybody tell me why? Thank you.
string str = oss.str(); // this should do the trick 
If you're trying to copy the whole file to a stringstream, then this:
oss << ifs; is wrong. All that does is prints the address of ifs. What you want to do is this:
oss << ifs.rdbuf(); And then of course, to copy that to a string, like the others are saying:
str = oss.str(); If you just want to get a single line, then skip the stringstream, and just use getline:
std::getline(ifs,str); 
<< is an operator defined on streams, which a string is not. You just want to use = here.
That doesn't make any sense. oss.str() returns a std::string. You can't stream a string into a string. You either need str = oss.str(), or use a standard stringstream instead, and do ss >> str.
the operator "<<" is usable for ostringstream and you are using it for a string. for string I think you can use append function:
str.append(oss.str());
