Custom manipulator for class

Question

I\'m trying to write a stream manipulator with arguments. I have class with 3 int\'s CDate(Year, Month, Day). So I need to make manipulator date_format(const char*)

Answer 1

Like Dietmar said you can push the params into the iword() but i find this kind of solution to be tedious and annoying..

I prefer to just install lambda functions as a iomanips and use them to directly call into the various classes methods or otherwise build the custom print in place. For that purpose I have created a simple 100 line template installer/helper class for mainpulators which can add a lambda function as the manipulator of any class..

So for your CDate you might define you manip as

std::ostream& dummy_date_format_manipulator (std::ostream& os)
{
    CustomManip::install(os,
                        [](std::ostream& oos, const CDate& a)
                        {
                            os << a.year() 
                                << "-hello-" 
                                << a.day()
                                << "-world-" 
                                << a.month() 
                                << "-something-"
                                << a.day() << a.day();
                        });
    return os;
}

Then just direct the << op to use the manip installers print helper:

std::ostream& operator<<(std::ostream& os, const CDate& a)
{
    CustomManip::print(os, a);
    return os;
}

And your basically done..

The mainp installer code and a fully working example is in my blog post at: http://code-slim-jim.blogspot.jp/2015/04/creating-iomanip-for-class-easy-way.html

But to be nice.. here is the key part you want to put in a .h somewhere less all the printouts to demonstrate how it works:

//g++ -g --std=c++11 custom_class_manip.cpp

#include 
#include 
#include 
#include 

template 
class CustomManip
{
private:
    typedef std::function ManipFunc;

    struct CustomManipHandle
    {
        ManipFunc func_;
    };

    static int handleIndex()
    {
        // the id for this Custommaniputors params
        // in the os_base parameter maps
        static int index = std::ios_base::xalloc();
        return index;
    }

public:
    static void install(std::ostream& os, ManipFunc func)
    {
        CustomManipHandle* handle =
            static_cast(os.pword(handleIndex()));

        // check if its installed on this ostream
        if (handle == NULL)
        {
            // install it
            handle = new CustomManipHandle();
            os.pword(handleIndex()) = handle;

            // install the callback so we can destroy it
            os.register_callback (CustomManip::streamEvent,0);
        }

        handle->func_ = func;
    }

    static void uninstall(std::ios_base& os)
    {
        CustomManipHandle* handle =
            static_cast(os.pword(handleIndex()));

        //delete the installed Custommanipulator handle
        if (handle != NULL)
        {
            os.pword(handleIndex()) = NULL;
            delete handle;
        }
    }

    static void streamEvent (std::ios::event ev,
                             std::ios_base& os,
                             int id)
    {
        switch (ev)
        {
            case os.erase_event:
                uninstall(os);
                break;
            case os.copyfmt_event:
            case os.imbue_event:
                break;
        }
    }

    static void print(std::ostream& os, const TYPE& data)
    {
        CustomManipHandle* handle
            = static_cast(os.pword(handleIndex()));

        if (handle != NULL)
        {
            handle->func_(os, data);
            return;
        }

        data.printDefault(os);
    }
};

Of course if you really do need the parameters then use the CustomManip::make_installer(...) function to get it done but for that you will have to visit the blog..