How to print out a 2D vector of objects in C++?

Question

I'm writing a program which includes a 2D vector of objects:

class Obj{
public:
    int x;
    string y;
};

vector<Obj> a(100);

vector< vector<Obj> > b(10);

I've stored some values of vector a in vector b.

I get an error when I try to print it out like this:

for(int i=0; i<b.size(); i++){
    for(int j=0; j<b[i].size(); j++)
      cout << b[i][j];
    cout << endl;
}

error info:

D:\main.cpp:91: error: no match for 'operator<<' (operand types are 'std::ostream {aka std::basic_ostream}' and '__gnu_cxx::__alloc_traits >::value_type {aka Obj}') cout << b[i][j]; ^

There is no such thing as a 2D vector of objects. You have a vector of vectors of objects. — Lightness Races in Orbit, Feb 17 '18 at 16:11
How would you print the object if it weren't inside a "2D vector"? Being inside a vector doesn't magically transform a type. — juanchopanza, Feb 17 '18 at 16:12
Learn more about [range `for` loops](http://en.cppreference.com/w/cpp/language/range-for) — Basile Starynkevitch, Feb 17 '18 at 16:45
In future, please provide a complete program all in one piece. [MCVE] — Jive Dadson, Feb 17 '18 at 18:50

Ron · Accepted Answer · 2018-02-17T19:24:41.063

Your problem is not related to vectors, it is related to sending object(s) of some user defined type Obj to the standard output. When you send an object to the output stream using the operator<< as you do with:

cout << b[i][j];

the stream does not know what to do with it as none of the 12 overloads accepts a user defined type Obj. You need to overload the operator<< for your class Obj:

std::ostream& operator<<(std::ostream& os, const Obj& obj) {
    os << obj.x  << ' ' << obj.y;
    return os;
}

or even a vector of Objs:

std::ostream& operator<<(std::ostream& os, const std::vector<Obj>& vec) {
    for (auto& el : vec) {
        os << el.x << ' ' << el.y << "  ";
    }
    return os;
}

For more info on the subject check out this SO post:
What are the basic rules and idioms for operator overloading?

score 6 · Answer 2 · answered Feb 17 '18 at 16:12

6

This has nothing to do with your vectors.

You are trying to print an Obj, but you didn't tell your computer how you would like it to do that.

Either print b[i][j].x and b[i][j].y individually, or overload operator<< for Obj.

answered Feb 17 '18 at 16:12

Lightness Races in Orbit

378,754
76
643
1,055

Jive Dadson · Answer 3 · 2018-02-17T19:17:30.573

There is no cout::operator<< that takes a class Obj as a right hand side. You could define one. Simplest solution is to send x and y to cout separately. But use range-based for-loops!

#include <string>
#include <vector>
#include <iostream>

using namespace std;

class Obj {
public:
    int x;
    string y;
};

vector<vector<Obj>> b(10);

void store_some_values_in_b() {
    for (auto& row : b) {
        row.resize(10);
        for (auto& val : row) {
            val.x = 42; val.y = " (Ans.) ";
        }
    }
}

int main() { 

    store_some_values_in_b();

    for (auto row: b) {
        for (auto val : row) {
            cout << val.x << " " << val.y;
        }
        cout << endl;
    }
}

score 1 · Answer 4 · answered Feb 17 '18 at 16:12

1

Maybe like below

for(int i=0; i<b.size(); i++){
    for(int j=0; j<b[i].size(); j++)
        cout << "(" << b[i][j].x << ", " << b[i][j].y << ") ";
    cout << endl;
}

answered Feb 17 '18 at 16:12

273K

29,503
10
41
64

This answer is missing an explanation. – Robert Andrzejuk Feb 17 '18 at 21:59

How to print out a 2D vector of objects in C++?

4 Answers4