How to override cout in C++?

Question

I have a requirement, I need to use printf and cout to display the data into console and file as well. For printf I have

Answer 1

There's already a Boost class for this: tee

Answer 2

cout is normally implemented as an object instance so you can't override it in the way that you would overload / override a function or a class.

Your best bet is not to fight that - yes you could build a my_cout and #define cout my_cout but that would make your code obtuse.

For readability I'd leave cout as it is. It's a standard and everyone knows what it can and can't do.

Answer 3

Overriding the behaviour of std::cout is a really bad idea as other developers will have a hard time understanding that the use of std::cout doesn't behave as usual.

Make your intention clear with a simple class

#include <fstream>
#include <iostream>

class DualStream
{
   std::ofstream file_stream;
   bool valid_state;
   public:
      DualStream(const char* filename) // the ofstream needs a path
      :
         file_stream(filename),  // open the file stream
         valid_state(file_stream) // set the state of the DualStream according to the state of the ofstream
      {
      }
      explicit operator bool() const
      {
         return valid_state;
      }
      template <typename T>
      DualStream& operator<<(T&& t) // provide a generic operator<<
      {
         if ( !valid_state ) // if it previously was in a bad state, don't try anything
         {
            return *this;
         }
         if ( !(std::cout << t) ) // to console!
         {
            valid_state = false;
            return *this;
         }
         if ( !(file_stream << t) ) // to file!
         {
            valid_state = false;
            return *this;
         }
         return *this;
      }
};
// let's test it:
int main()
{
   DualStream ds("testfile");
   if ( (ds << 1 << "\n" << 2 << "\n") )
   {
      std::cerr << "all went fine\n";
   }
   else
   {
      std::cerr << "bad bad stream\n";
   }
}

This provides a clean interface and outputs the same for both the console and the file. You may want to add a flush method or open the file in append mode.