CODEWITHSUNDEEP
c++c++11
waqar
waqar

Reputation: 21

Cout does not work - (error: no match for ‘operator<<’)

I am trying to learn c++. I have understood few things.

I was trying to implement program using vector in c++ for dynamic array, while everything seems to work but one thing in particular cout statement throws some weird error .

#include <iostream> 
#include <vector> 

using namespace std; 

int main() 
{ 
    vector<int> g1; 

    for (int i = 1; i <= 5; i++) {
        g1.push_back(i); 
    }

    cout << "This works"; // this cout works

    cout << g1; // but this does not why ?

    return 0;
}

Error that I am getting after running.

main.cpp: In function ‘int main()’:
main.cpp:18:7: error: no match for ‘operator<<’ (operand types are ‘std::ostream {aka std::basic_ostream<char>}’ and ‘std::vector<int>’)
  cout << g1;
  ~~~~~^~~~~

Here is the program. I was trying to debug on hackerrank and I came across this problem. Why does not cout work for vector variable only? What am I missing?

Upvotes: 1

Views: 528

Answers (3)

P0W
P0W

Reputation: 47794

There is no overload for operator<< for std::vector<T>. If you really wish to use operator<< for std::vector<T>, you will have to provide the implementation for it yourself, eg:

template <typename T>
std::ostream& operator<< (std::ostream& out, const std::vector<T>& vec) {
  if ( !vec.empty() ) {
    out << '{';
    std::copy (std::cbegin(vec), 
               std::cend(vec), 
               std::ostream_iterator<T>(out, ", "));
    out << "}";
  }
  return out;
}

Demo Here

Upvotes: 2

CoderCharmander
CoderCharmander

Reputation: 1910

cout is a stream. You can write all the integral types (int, long, char, short, pointers etc.) and string to a stream, but vector is unsupported. If you want to print it, you may want to iterate through elements like this:

for (int i: g1) {
    cout << i << ' ';
}

Or you can also implement operator<< for vector:

template <typename T> ostream& operator<<(ostream& lhs, vector<T> rhs) {
    for (T i: rhs) {
         lhs << i;
    }
    return lhs;
}

Then you will be able to use cout << g1;.

Upvotes: 0

Rafaqat Ali
Rafaqat Ali

Reputation: 718

You need to print vector like below code snippet:

#include <iostream> 
#include <vector> 

using namespace std; 

int main() 
{ 
    vector<int> g1; 

    for (int i = 1; i <= 5; i++) {
        g1.push_back(i); 
    }

    cout << "This works"; // this cout works

    for (int i = 0; i < g1.size(); ++i)
        cout << g1[i] << ' ';

    return 0;
}

Upvotes: 0

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