Saving string to overloaded brackets not changing field of object - c++

Question

I have a problem in my code which i can't understand. I want to make a list of lists and use it like a two-dimensional associative array.
Something like this:
token["window"]["title"] = "Amazing!";
cout << token["window"]["title"]; //Amazing!
The second line works good. Data is readed from file. Problem is with the first instruction.
This is how i overload the second square brakcets:
TokenPair Token::operator[](string keyName){
for( list<TokenPair>::iterator pair=keys.begin(); pair != keys.end();++pair){
if(pair->key == keyName){
return *pair;
}
}
}
As you see I return object of class TokenPair. To properly get the value from object (field TokenPair::value) i overload streaming and casting on string().
TokenPair::operator string() const{
return value;
}
ostream & operator<< (ostream &stream, const TokenPair &pair){
return stream << pair.value;
}
And as i say before getting value works great. Problem is with overloading operator of attribution:
TokenPair TokenPair::operator=( const string newValue ){
value = newValue;
return *this;
}
This method assing the value but it not remember that! For example:
token["window"]["title"] = "Ok";
will cause that inside method TokenPair::operator= variable newValue=="Ok" and after the first line even value is set to "Ok"; But when i later do:
cout << token["window"]["title"] ;
the field in TokenPair is still not changed. I want to ask: Why? Maybe iterators return a copy of that object? I don't know. Please help.

Answer 1

Your problem is that the operator[] returns a TokenPair by value, so when assigning you assign the new value to a temporary, not to the object stored in the list.
For this to work the operator[] should return a reference to the object you want to modify.

Answer 2

Here is an example of how to use a map-of-maps:
#include <map>
#include <string>
#include <iostream>
#include <sstream>
const char inputString[] =
"President 16 Lincoln "
"President 1 Washington "
"State Best Illinois "
"State Biggest Alaska ";
int main () {
std::map<std::string, std::map<std::string, std::string> > token;
std::string primary, secondary, result;
std::istringstream input(inputString);
while( input >> primary >> secondary >> result )
token[primary][secondary] = result;
std::cout << "Abe " << token["President"]["16"] << "\n";
std::cout << "Springfield, " << token["State"]["Best"] << "\n";
std::cout << "Blank: " << token["President"]["43"] << "\n";
}