Why doesn`t operator<< work successfully?

Question

While compiling the below code, I am getting an error:

Expression.h

class Expression{
    ...
protected:
    std::ostream Os;
};

Expression.c

Expression::Expression() : Os(std::cout)
{
    ...
} 

Expression::Expression(std::ofstream &os) : Os(os)
{
    ...
}

Expression::Dump()
{
     Os << "=============================================================" << std::endl;
     Os << "Os:               " << Os << std::endl;
}

error: no match for 'operator<<' (operand types are 'std::basic_ostream<char>' and 'std::ostream {aka std::basic_ostream<char>}')

What is my mistake? What should I do to fix it?

And while giving initial value to parameter like this Os(std::cout), what does it mean?

Answer 1

The error is because there is no standard operator<< that writes an ostream to another ostream.

Also, ostream can't be copy-constructed from another ostream, so if you are trying to specify an output ostream for Expression to write to then your Os member needs to be declared as a reference instead.

class Expression {
    ...
protected:
    std::ostream &Os;
};

Expression::Expression() : Os(std::cout)
{
    ...
}

Expression::Expression(std::ofstream &os) : Os(os)
{
    ...
}

Expression::Dump()
{
    Os << "=============================================================" << std::endl;
    Os << "Os: " << SomethingThatIsNotOs << std::endl;
}

Answer 2

Consider

    Os << "Os:               " << Os << std::endl;

Let us unpack it into its full functional glory:

std::ostream& t1 = Os.operator <<("Os:               ");
std::ostream& t2 = t1.operator <<(Os);                   // << Here
std::ostream& t3 = t2.operator <<(std::endl);

The problem is the line marked "Here". You are streaming an ostream to itself. This is nonsensical. What are you trying to do?

If you are trying to output an identifier for the stream, you should use the address:

    Os << "Os:               " << &Os << std::endl;

If you are trying to output the contents of the stream to stdout, you need to make Os be an ostringstream, and then output the string buffer:

    std::ostringstream Os;
    ...
    std::stdout << "Os:               " << Os.str() << std::endl;

---

Note for the pedantic: I believe my breakdown of the original line into function calls and initializations is actually identical in this particular case (because there are no temporary objects - only references); in general, the lifetime of temporaries might be subtly different.