Struggling to understand a piece of code in C++

template <class _Key, class _Tp, class _Compare=less<_Key> >
class IsMap : public map<_Key, _Tp, _Compare>
{
public:
_Tp& operator[](const _Key& __k) throw (ExptAppl)
{
for (IsMap<_Key, _Tp, _Compare>::iterator it = this->begin();
it != this->end(); it++)
if ((*it).first == __k)
return (*it).second;
string errMsg = "Element <" + __k + "> does not exist";
throw ExptAppl("", 0, 3003, "", (char*)errMsg.c_str());
}
};

Quote:

What I understood?

IsMap is a class template which inherits the class map. So here the generic Data type used here are:
_key -> This Denotes Key.
_Tp -> This Denotes Type.
_Compare=less<_key> -> A comparison function.
Now we need to understand how Map works?
std::map <key_type,data_type,[comparison>
The key comparison function, an ordering whose argument type is key_type; it returns true if its first argument is less than its second argument, and false otherwise. This is also defined as map::key_compare.This means that if you want to retrieve every key, value pair stored in the map, you can retrieve them in the order of the keys.

What I did not?
just the line : _Tp& operator[](const _Key& __k) throw (ExptAppl)
The Rest I have guessed.

Can you please help me understanding this whole piece of code by identifying this missing piece of logic?

Thanks
Deb