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

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";
}