如何打印出包含对象的向量？

Question

This question stems from a larger code I am working on but hopefully I can break it down to simplify:

Say I have two classes, one which holds 2 integer variables, and another which contains a vector that holds the first class

class TwoInts {
     int num1;
     int num2;
}

class NumHold {
    std::vector<TwoInts> numVector;
}

How would I go about printing my numHold class exactly? I know I would have to provide an overloaded stream operator

std::ostream &operator<< (std::ostream &out, const NumHold &myNumHold){;}

But I'm a little confused as to the syntax regarding how I would print out these integers from the vector. I also understand that in order for the stream operator to work I would have to add it to my class as a friend, I just wanted to keep my NumHold class as simplified as possible for the examples sake. Hopefully this was clear enough of an explanation, any help is appreciated

Answer 1

Treat out how you would treat std::cout , then return it at the end of the function:

std::ostream& operator<<(std::ostream& out, const NumHold &myNumHold){
    //one possible output format
    for(const auto& twoInt : myNumHold.numVector){
        out << twoInt.num1 << ", " << twoInt.num2 << '\n'; //pretend out is std::cout
    }

    return out; //don't forget to return it at the end
}

If appropriate, declare the overload as a friend function within the NumHold class:

#include<iostream>

class NumHold {
    private:
        std::vector<TwoInts> numVector;

    public:
        //friend operator overload declaration
        friend std::ostream& operator<<(std::ostream& out, const NumHold& myNumHold);
};

In your example, it is appropriate, as the overload function accesses private variables.

Answer 2

You need to be able to access the members of each TwoInts in order to print them. One way to do this is to make them public.

class TwoInts {
public: //lets NumHold access variables to print
    int num1;
    int num2;
};

class NumHold {
public:
    std::vector<TwoInts> numVector;

    friend std::ostream& operator << (std::ostream& out, const NumHold& myNumHold) {
        for (const TwoInts& ti : myNumHold.numVector) {
            out << "(" << ti.num1 << ", " << ti.num2 <<  ")" << std::endl;
        }
        return out;
    }
};

Answer 3

You can also consider adding printing methods to your classes:

#include<iostream>

class TwoInts {
    private:
        int num1;
        int num2;
    public:
        void printnum1(){
            std::cout << num1;
        }
        void printnum2(){
            std::cout << num2;
        } 
}


class NumHold {
    private:
         std::vector<TwoInts> numVector;

    public:
         //this function prints the private attributes of the class
         void printnumVector(){
             for (int i = 0; i < numVector.size(); i++){
                 std::cout << numVector[i].printnum1() << " " << numVector[i].printnum2() << std::endl;
             }    
         }
};