override operator

19.override operator <<
            struct topic_descript
            {
                btstring topic_url;     //RuleEngine/LineDetector/Crossed
                btstring name_space;
                btvector<field_item> source_items;
                btvector<field_item> key_items;      
                btvector<field_item> data_items;

                friend std::ostream& operator <<(std::ostream& ou,topic_descript& obj){
                    ou<<"topic_url: "<<obj.topic_url<<"\n";
                    ou<<"name_space: "<<obj.name_space<<"\n";
                    btvector<field_item>::iterator it;
                    for(it=obj.source_items.begin(); it!=obj.source_items.end(); ++it){
                        ou<<*it<<"\n";
                    }
                    for(it=obj.key_items.begin(); it!=obj.key_items.end(); ++it){
                        ou<<*it<<"\n";
                    }
                    for(it=obj.data_items.begin(); it!=obj.data_items.end(); ++it){
                        ou<<*it<<"\n";
                    }
                    return ou;
                }               
            };

注意： 找不到ostream 是由於 
1.沒包含 #include <iostream> 2.或者命名空間 std的 緣故。
C++ 就是比 C麻煩的多，  不會有 namespace， 找到就是找到，  找不到就是 undefined！
3. the order:
    friend std::ostream& operator <<(std::ostream& ou,topic_descript& obj)
    not like this
    friend std::ostream& operator <<(topic_descript& obj,std::ostream& ou),
    like that errors  happened!