CODEWITHSUNDEEP
c++11dictionaryoperator-overloadingostreamistream
andres
andres

Reputation: 321

ostream operator overload for struct with map

I have created a grades struct for students and am attempting to overload '<<' operator. 

// Sample output:
a12345678
2             //number of elements in map
COMP3512 87  
COMP3760 68

struct Grades {
  string             id;       // student ID, e.g,, a12345678
  map<string, int> scores;     // course, score, e.g. COMP3512, 86
};

I have previously overloaded the operator<< to obtain the information independently.

ostream& operator<<(ostream& os, const Grades g) { 
  return os << g.id << '\n' ... 

  // return os << g.id << '\n' << g.scores;   produces an error
}

I suspect it has something to do with the overload not having correct syntax for a map, like the one below. 

ostream& operator<<(ostream& os, const map<string, int>& s) {
  for (auto it = s.begin(); it != s.end(); ++it) 
    os << (*it).first << ' ' << (*it).second << endl;

  return os;
}

Is there a way to produce the sample output through one overload, or do I require both of the current implementations: one for the map:grades.scores and the other one for the string: grades.id

Thanks for the assistance.

Upvotes: 3

Views: 6131

Answers (2)

Rudolfs Bundulis
Rudolfs Bundulis

Reputation: 11934

It seems weird that you weren't able to sort it out yourself, since if I understood the question correctly you just need to merge the two overloads into one so you iterate the map from the const Grades g like so:

#include <iostream>
#include <map>
#include <string>

using namespace std;

struct Grades {
    string             id;       // student ID, e.g,, a12345678
    map<string, int> scores;     // course, score, e.g. COMP3512, 86
};


ostream& operator<<(ostream& os, const Grades g) { 
    os << g.id << endl << g.scores.size() << endl;
    for (auto it = g.scores.begin(); it != g.scores.end(); ++it) 
        os << (*it).first << ' ' << (*it).second << endl;
    return os;
}

int main(int argc, char** argv)
{
    Grades g;
    g.id = "a12345678";
    g.scores["COMP3512"] = 87;
    g.scores["COMP3760 "] = 68;
    cout << g;
    return 0;
}

Upvotes: 3

Angew is no longer proud of SO
Angew is no longer proud of SO

Reputation: 171107

There is no standard << provided for std::map, so you have to output it yourself. But nothing prevents you from concatentating your implementations in one function:

std::ostream& operator<< (std::ostream &os, const Grades &g)
{
  os << g.id << '\n';
  os << g.scores.size() << '\n';
  for (const auto &s : g.scores) {
    os << s.first << ' ' << s.second << '\n';
  }
  return os;
}

Upvotes: 1

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