C++ << overloading in queue template

Question

from building the source code I get this error, I am not sure what it means from my previous post I got helped and I got further and this is what I have until now, thanks for any help,

main.cpp:31:2: error: no match for 'operator<<' in 'os << (& q)->Queue::PrintQ()'

template<class T> class Queue;
template<class T> ostream& operator<<(ostream& os, Queue<T>& q);

template<class T>
class Queue{

protected:
    vector<T> theQ;
public:
    void Push(T item);
    T pop();
    void ReadAnItem();
    void PrintQ();
    friend ostream& operator<< <T>(ostream& os, Queue<T>& q);
};

template<class T>
ostream& operator<< (ostream& os, Queue<T>& q){
    os << q.PrintQ();
    return os;
}

template<class T>
void Queue<T>::Push(T item){
    theQ.push_back(item);
}

template<class T>
T pop(){
}

template<class T>
void Queue<T>::ReadAnItem(){
    T item;
    cout << "Enter the data please: " << endl;
    cin >> item;
    Push(item);
}

template<class T>
void Queue<T>::PrintQ(){
    cout << "The content of the array is as follows: " << endl;
        for (int i=0; i < theQ.size(); i++){
            cout << theQ[i];
            cout<< endl;
        }

}

class Employee{

protected:
    long empId;
    string empName;
    string email;
public:
    Employee(){}
    Employee(long i, string n){
        empName = n,
        empId =i;
        email = "Unknown";
    }
    friend ostream& operator<<(ostream& os, Employee& e){
        os << e;
        return os;
    }
};

class Student{

protected:
    long stId;
    int year;
    string email;
    string schoolName;
public:
    Student(){}
    Student(long i, int y, string sn){
        stId = i;
        year =y;
        email = "Unknown";
        schoolName=sn;
    }
    friend ostream& operator<<(ostream& os, Student& s){
        os << s;
        return os;
    }
};

int main(){

        Queue<Student> s;
        s.Push(Student(300982, 21, "Andrey"));
        cout << s;
        return 0;
}

`void PrintQ();` How do you want it to print a **void**? – Adam Nov 25 '13 at 21:30 — Adam, Nov 25 '13 at 21:30

score 1 · Accepted Answer · answered Nov 25 '13 at 21:47

1

This is a problem:

os << q.PrintQ();

PrintQ() is void. You are trying to stream the result of a call to a void function to std::cout. A possible solution would be the following:

template<class T>
void Queue<T>::PrintQ(std::ostream& o) const {
  o << "The content of the array is as follows: " << endl;
    for (int i=0; i < theQ.size(); i++){
        o << theQ[i];
        o << endl;
  }
}

and then

template<class T>
ostream& operator<< (ostream& os, const Queue<T>& q){
    q.PrintQ(os);
    return os;
}

answered Nov 25 '13 at 21:47

juanchopanza

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now I get this error, I edited the code with your suggestion and when I tried to print ion main I get an error...how should I call the function I tried cout << s; and s.PrintQ(cout); main.cpp:107: undefined reference to `std::ostream& operator<< (std::ostream&, Queue&) – Andrey Arias Nov 25 '13 at 22:12
@AndreyArias You need to fix the implementation of `ostream& operator<<(ostream& os, Student& s)`. Currently it is calling itself. – juanchopanza Nov 26 '13 at 06:40

score 0 · Answer 2 · answered Nov 25 '13 at 23:18

Some small points:

theQ.size() returns an unsigned integer, so technically it should be unsigned int.
Testing theQ.size() on every loop is inefficient, Initialise a variable to theQ.size() and use that to test.
Better still is to use iterators to run through your vector.
Best is to you the STL and use an algorithm:

copy( theQ.begin(), theQ.end(), ostream_iterator<T&>(os, "\n"));
Your operator<< functions have a problem: when you write os << s you are telling a student object to stream itself. As a Student object has absolutely no idea how to do that the program will create a segment violation. Solve this by replacing os << s with something like os << "Id: " << s.stId << "\tYear: " << s.year << "\temail: " << s.email << "\tschoolname: " << s.schoolName; You can also write a stream member function and keep the os << s; line.

Hope this was of any help.

Good luck

Alwin

C++ << overloading in queue template

2 Answers2