Can I have your feedback on this general mix-in pattern?

From:
DeMarcus <use_my_alias_here@hotmail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Sun, 15 Aug 2010 17:39:58 CST
Message-ID:
<4c67fea0$0$276$14726298@news.sunsite.dk>
Hi,

I needed to implement cloning and saw that I ended up with copy and
paste all over the code. I thought that this should be able to be solved
with mix-ins. After research and help from you (Is the Mixin pattern
accepted in all camps? 6 July) I thought that I probably would find a
good solution to this.

I found a nice first approach at Alf P. Steinbach's blog.

http://alfps.wordpress.com/2010/06/12/cppx-3-ways-to-mix-in-a-generic-cloning-implementation/

The approach that caught my interest was Way #2: Mix-in via a middleman
base class.

However, I thought the syntax would be slightly verbose using it
straight out of the box, and since I'm a perfectionist I tried to find a
solution where I could pack mix-ins together.

In Andrei Alexandrescu's library Loki he presents type list as a way to
pack types.

http://www.drdobbs.com/184403813

The problem, though, is that I don't want to pack types, but templates.

I have now worked on the problem and think I've found something quite
powerful.

The general mix-in pattern idea builds on C++0x and looks like this.

All mix-ins must conform to the following interface.

template<class T, class Base>
class SomeCustomMixin : public Base
{
public:
    // Pass on all constructor arguments to the Base.
    // If you don't want to be able to pass arguments to the Base you
    // can remove this.
    template<typename... Arguments>
    inline SomeCustomMixin( Arguments&&... arguments )
       : Base( std::forward<Arguments>( arguments )... ) {}

    /* Implement your mix-in here. */
};

-------------------------------------------------

This is what the mix-in list looks like. This is not a /type/ list but
rather a /template/ list.

//file NullType.h
#ifndef NULL_TYPE_H_
#define NULL_TYPE_H_

class NullType {};

#endif

-----------------------

// file MixinList.h
#ifndef MIXIN_LIST_H_
#define MIXIN_LIST_H_

#include "NullType.h"

// This is needed to circumvent unimplemented parts in gcc.
template<template<class, class> class... Rest>
struct MixinList;

template
<
    template<class, class> class Mixin,
    template<class, class> class... Rest

struct MixinList<Mixin, Rest...>
{
    // This is a trick to simulate 'using Head = Mixin;'
    template<class T, class Base>
    class Head : public Mixin<T, Base>
    {
    public:
       // Pass on all constructor arguments to the Base.
       // If you don't want to be able to pass arguments to the Base you
       // can remove this.
       template<typename... Argument>
       inline Head( Argument&&... arguments )
          : Mixin<T, Base>( std::forward<Arguments>( arguments )... )
       {
       }
    };
    // When template aliasing comes to gcc we can use
    // using Head = Mixin;

    typedef MixinList<Rest...> Tail;
};

// Cap the MixinList recursion with a specialization using
// NullType as Tail.
template
<
    template<class, class> class Mixin

struct MixinList<Mixin>
{
    template<class T, class Base>
    class Head : public Mixin<T, Base>
    {
    public:
       // Pass on all constructor arguments to the Base.
       // If you don't want to be able to pass arguments to the Base you
       // can remove this.
       template<typename... Arguments>
       inline Head( Arguments&&... arguments )
          : Mixin<T, Base>( std::forward<Arguments>( arguments )... )
       {
       }
    };

    typedef NullType Tail;
};
#endif

----------------

Now comes the actual Mixins template that applies all the mix-ins to a
class.

//file EmptyType.h
#ifndef EMPTY_TYPE_H_
#define EMPTY_TYPE_H_

struct EmptyType {};

#endif

-----------------------

// file Mixins.h
#ifndef MIXINS_H_
#define MIXINS_H_

#include "EmptyType.h"
#include "NullType.h"

template<class T, class MixinList, class Base = EmptyType>
class Mixins : public MixinList::template Head<T,
    Mixins<T, typename MixinList::Tail, Base>>
{
public:
    // Pass on all constructor arguments to the Base.
    // If you don't want to be able to pass arguments to the Base you
    // can remove this.
    template<typename... Arguments>
    inline Mixins( Arguments&&... arguments )
       : MixinList::template Head<T,
          Mixins<T, typename MixinList::Tail, Base>>(
             std::forward<Arguments>( arguments )... )
    {
    }
};

// When MixinList is NullType, end the recursion with a specialization.
template<class T, class Base>
class Mixins<T, NullType, Base> : public Base
{
public:
    // Pass on all constructor arguments to the Base.
    // If you don't want to be able to pass arguments to the Base you
    // can remove this.
    template<typename... Arguments>
    inline Mixins( Arguments&&... arguments )
       : Base( std::forward<Arguments>( arguments )... )
    {
    }
};

#endif

------------

Or, when template aliasing comes to gcc we may be able to use this file
instead.

// file Mixins.h
#ifndef MIXINS_H_
#define MIXINS_H_

#include "EmptyType.h"
#include "NullType.h"

template<class T, class MixinList, class Base = EmptyType>
using Mixins = MixinList::Head<T,
    Mixins<T, typename MixinList::Tail, Base>>;

// End the recursion with a specialization using NullType.
template<class T, class Base = EmptyType>
using Mixins<T, NullType, Base> = Base;

#endif

-----------------

That's it! Let's use it. All code here is written using gcc 4.5.0.

First we create a mix-in called CloneMixin.

// file CloneMixin.h
#ifndef CLONE_MIXIN_H_
#define CLONE_MIXIN_H_

#include <memory>
#include <assert.h>

template<class T, class Base>
class CloneMixin : public Base
{
public:
    typedef std::unique_ptr<T> CloneUPtr;

    // Pass on all constructor arguments to the Base.
    // If you don't want to be able to pass arguments to the Base you
    // can remove this.
    template<typename... Arguments>
    inline CloneMixin( Arguments&&... arguments )
       : Base( std::forward<Arguments>( arguments )... ) {}

    virtual ~CloneMixin() {}

    // Use NVI to be able to do an important assert.
    inline CloneUPtr clone() const
    {
       CloneUPtr c( static_cast<T*>(clone_()) );
       assert( typeid(*c.get()) == typeid(*this) );
       return c;
    }

protected:
    virtual CloneMixin* clone_() const
    {
       return new T( *static_cast<const T*>(this) );
    }
};

#endif

-------------------------------------

Let's make another mix-in.

// file TypeNameMixin.h
#ifndef TYPE_NAME_MIXIN_H_
#define TYPE_NAME_MIXIN_H_

template<class T, class Base>
class TypeNameMixin : public Base
{
public:

    // Pass on all constructor arguments to the Base.
    // If you don't want to be able to pass arguments to the Base you
    // can remove this.
    template<typename... Arguments>
    inline TypeNameMixin( Arguments&&... arguments )
       : Base( std::forward<Arguments>( arguments )... ) {}

    virtual ~TypeNameMixin() {}

    virtual std::string getTypeName() const
    {
       // This could be changed to something of your own taste.
       // Maybe the mix-in could be initialized with a more convenient
       // type name.
       return typeid(T).name();
    }
};

#endif

----------------------------------------

And here's the main program.

class MyBaseClass : public Mixins<MyBaseClass,
    MixinList<CloneMixin, TypeNameMixin>>
{
};

class MyDerivedClass : public Mixins<MyDerivedClass,
    MixinList<CloneMixin, TypeNameMixin>,
    MyBaseClass>
{
};

int main()
{
    MyDerivedClass dc;
    std::unique_ptr<MyBaseClass> bc = dc->clone();

    assert( typeid(*bc.get()) == typeid(dc) );

    std::cout << dc.getTypeName() << std::endl;
    std::cout << bc->getTypeName() << std::endl;

    return 0;
}

I would really appreciate your feedback, both if you find flaws, but
also if you do not find flaws (or even find it usable).

I have two questions myself though:

1. The clone() function will be overridden in each derived class using
the CloneMixin. In C++ FAQ Lite they mention something about this, but
what's your opinion about overriding non-virtual for mix-in purposes?

http://www.parashift.com/c++-faq-lite/strange-inheritance.html#faq-23.8

2. To be able to store templates in the template list MixinList without
template aliasing that doesn't exist yet for gcc, I use a special inner
template trick for Head. Do you see any problems with that trick?

Thanks,
Daniel

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