C++ Check if stream reference is NULL

I am compiling an outdated project with my latest gcc g++ compilers, (version > 6)

There is a class CodeWriter with an ostream reference variable.

class CodeWriter
{
  //private:
protected:
  ostream &m_stream;
public:
  CodeWriter(ostream &stream):m_stream(stream){}
  ~CodeWriter(){
    if(m_stream != NULL){
      m_stream.flush();
    }
  }
};

The class is quite large so i included only the relevant variables and functions.

As you can see the destructor seems to be comparing the reference to NULL. This project compiled i when i used it long back with old gnu toolchain.

But now it is throwing an error saying that there is no matching operator != to compare ostream and long int.

Can anyone explain the rationale behind the comparison, and how i can fix this.

I would be happy to provide additional information/ include the whole class if required.