(C++) Stream out operator <<

include <iostream>


int main()
{
  std::cout << "Hello world" << std::endl;
  return 0;
}

Compile error 'operator<<' not implemented in type 'ostream' for arguments of type 'std::string'

include <iostream>

include <vector>


int main()
{
  std::string s = "Hello";
  std::cout << s;
}

include <iostream>

include <vector>

#include <string>
int main()
{
  std::string s = "Hello";
  std::cout << s;
}

Questions

• How do I overload this operator?