operator< for user defined key of map - C2679 - visual-c++

Question

I'm trying to create a map like
std::map<std::map <std::string,std::string>, MyDataTyp*>,
therefore I put
std::map<std::string, std::string>
in a separate class like:
class TagList {
public:
std::map<std::string, std::string> _map;
TagList() {}
~TagList() {}
void addTag(const std::string tag, const std::string value) { if (tag != "") _map[tag] = value; }
std::string getValue(const std::string tag) {
std::map<std::string, std::string>::iterator it = _map.find(tag);
if (it == _map.end()) return ("");
else return (it->second);
}
};
inline bool operator< (const TagList &a, const TagList &b) {
std::map<std::string, std::string>::iterator it ;
for (it = a._map.begin(); it != a._map.end(); it++) {
std::string myVal1 = it->second;
std::string myVal2 = b._map.find(it->first);
if (myVal2 == "") return false;
if (strcmp(myVal1.c_str(),myVal2.c_str()) > 0) return true;
}
return false;
}
Hope someone can explain me the error message
Error 4 error C2679: binary '=' : no operator found which takes a right-hand operand of type 'std::_Tree_const_iterator<_Mytree>' (or there is no acceptable conversion) (...)
...and what is the reason for my confision....

Answer 1

Sorry, my fault...
found the answer by myself:
inline bool operator< (const TagList &a, const TagList &b) {
std::map<std::string, std::string>::const_iterator it ;
std::map<std::string, std::string>::const_iterator it2 ;
for (it = a._map.begin(); it != a._map.end(); it++) {
std::string myVal1 = it->second;
it2 = b._map.find(it->first);
std::string myVal2 = it2->second;
if (myVal2 == "") return false;
if (strcmp(myVal1.c_str(),myVal2.c_str()) > 0) return true;
}
return false;
}
it wasn't const
Thx
George