How to write user-defined manipulators for streaming self-written class

Question

How do I write user-defined stream manipulators in C++ that control the format of streaming a self-written class?

Specifically, how would I write simple manipulators verbose and terse to control the amount of output streamed?

My environment is GCC, versions 4.5.1 and up.

Example:

class A
{
 ...
};

A a;

// definition of manipulators verbose and terse

cout << verbose << a << endl; // outputs a verbosely
cout << terse << a << endl; // outputs a tersely

PS: What follows is just a side question, feel free to ignore it: Can this portably be extended to manipulators taking arguments? Josuttis writes in "The C++ Standard Library" near the end of section 13.6.1 that writing manipulators taking argument is implementation dependent. Is this still true?

Not a duplicate but a related (interesting) topic : [How do the stream manipulators work?](http://stackoverflow.com/questions/4633864/how-do-the-stream-manipulators-work) — Nawaz, Apr 28 '11 at 12:24

Šimon Tóth · Answer 1 · 2011-04-28T10:00:01.967

I don't see any reason for them to be implementation dependent.

This is comething that I use, for the actual manipulator, create a function that returns an instance of the following helper. If you need to store the data, just store it inside the helper, some global variable, singleton, etc...

    /// One argument manipulators helper
    template < typename ParType >
    class OneArgManip
    {
        ParType par;
        std::ostream& (*impl)(std::ostream&, const ParType&);

        public:
            OneArgManip(std::ostream& (*code)(std::ostream&, const ParType&), ParType p) 
                : par(p), impl(code) {}

            // calls the implementation
            void operator()(std::ostream& stream) const
            { impl(stream,par); }

            // a wrapper that allows us to use the manipulator directly
            friend std::ostream& operator << (std::ostream& stream, 
                            const OneArgManip<ParType>& manip)
            { manip(stream); return stream; }
    };

Example of a manipulator based on this:

OneArgManip<unsigned> cursorMoveUp(unsigned c) 
{ return OneArgManip<unsigned>(cursorMoveUpI,c); }

std::ostream& cursorMoveUpI(std::ostream& stream, const unsigned& c)
{ stream << "\033[" << c << "A"; return stream; }

For some explanation:

u use the manipulator, that returns a new instance of the helper bound to an implementation of the helper
stream tries to process the helper, that invokes the << overload on the helper
that invokes the () operator on the helper
that invokes the real implementation of the helper with the parameter passed from the original manipulator call

If you want I can post 2 param and 3 param helpers as well. The principle is the same though.

Thanks, however I was thinking of manipulators that affect how later items are streamed. — Peter G., Apr 28 '11 at 20:59
@Peter it's the same thing. You just store the setting somewhere. Instead of outputting something into the stream you set `YourClass::outputtype` or something similar. — Šimon Tóth, Apr 28 '11 at 21:30

How to write user-defined manipulators for streaming self-written class

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