I usually implement the output operator for a class, operator<<().
I find it useful for debugging and regression testing purposes.
Occasionally when defining a class hierarchy with polymorphism in mind
I’ll instead define a virtual print() method.

Base  base;
std::cout << "base = " << base << std::endl;

Here’s the typical implementation:

class Base {
// ...
public:
    /// print Base with flags indicating hw much extra to include
    virtual void print (std::ostream &out, int flags);
};
std::ostream &
operator<< (std::ostream &out, const Base &base)
{
    base.print (out, 0);
}

So everything is great except… I now want to pass that extra
argument. I could write the following ugly code:

std::cout << "base = ";
base.print (std::cout, 1);
std::cout << std::endl;

Instead, I’m going to follow the example from <iomanip> and define a struct
that saves the arguments and then define an output operator for that
struct. The following code makes it so you can write the above as:

std::cout << "base = " << printstream (base, 1) << std::endl;

Here is the implementation of that. This expects the class to implement a method print() that takes a std::ostream and a second argument. It’s pretty easy to extend this to a print() that takes a third argument.

#ifndef INCLUDED_PRINTSTREAM_H
#define INCLUDED_PRINTSTREAM_H
#include 
namespace printstream_impl {
template
struct PrintStream1
{
    const ObjectType &  m_object;
    const Arg1Type &    m_arg1;
    PrintStream1 (const ObjectType &object, const Arg1Type &arg1)
        : m_object (object),
          m_arg1 (arg1)
    {
    }
    std::ostream &print (std::ostream &out) const
    {
        m_object.print (out, m_arg1);
        return out;
    }
};
template
std::ostream &operator<<(std::ostream &out, const PrintStream1 &ps)
{
    ps.print (out);
    return out;
}
} // End namespace printstream
template
printstream_impl::PrintStream1
printstream (const ObjectType &object,
             const Arg1Type &arg1)
{
    return printstream_impl::PrintStream1 (object, arg1);
}
#endif /*  INCLUDED_PRINTSTREAM_H */