PS:c++ clear（）在流中时标识清空所有标记！ 而不是像在string中时清空字符串.
在STL容器里clear方法的含义均为清空容器，但在STL的所有流中clear的含义均为清空错误标记！
那么问题来了。。。如何清除呢？
oss.str("");
string str ( ) const; // returns a copy of the string object currently associated with the string stream buffer.
void str ( const string & s ); // copies the content of string s to the string object associated with the string stream buffer. The function effectivelly calls rdbuf()->str(). Notice that setting a new string does not clear the error flags currently set in the stream object unless the member function clear is explicitly called.
ostringstream.str()方法，尽量控制不必要的重复调用，这可能会带来性能问题。