Re: [boost] A C++ puzzler

From: Juraj Ivančić (juraj.ivancic_at_[hidden])
Date: 2008-04-12 18:21:55

Eric Niebler wrote:
> I recently had a C++ problem and found an answer that tickled my brain. 
> In the spirit of Car Talk on NPR, I thought I'd share it in the form of 
> a puzzler.

Here is my solution. Not sure that it satisfies the conditions though.

Regards,
        Juraj Ivančić


#include "boost/static_assert.hpp"
#include "boost/type_traits/is_same.hpp"
#include "boost/mpl/if.hpp"

class ErrorFoundTwoNonDefaultTypes {};
class Default {};

template<typename T1, typename T2>
struct FindNonDefault
{
    typedef
    typename boost::mpl::if_
    <
        boost::is_same<T1, Default>,
        T2,
        typename boost::mpl::if_
        <
            boost::is_same<T2, Default>,
            T1,
            ErrorFoundTwoNonDefaultTypes
        >::type
    >::type type;
};

template<typename T1, typename T2>
struct DetermineCommonBetweenTwo
{
    typedef
    typename boost::mpl::if_
    <
        boost::is_same<T1, T2>,
        T1,
        typename FindNonDefault<T1, T2>::type
    >::type
    type;
};

template
<
    typename T1          ,
    typename T2 = Default,
    typename T3 = Default,
    typename T4 = Default,
    typename T5 = Default,
    typename T6 = Default,
    typename T7 = Default
>
struct DetermineCommon
{
    typedef
    typename DetermineCommonBetweenTwo
    <
        T7,
        typename DetermineCommonBetweenTwo
        <
            T6,
            typename DetermineCommonBetweenTwo
            <
                T5,
                typename DetermineCommonBetweenTwo
                <
                    T4,
                    typename DetermineCommonBetweenTwo
                    <
                        T3,
                        typename DetermineCommonBetweenTwo
                        <
                            T2,
                            T1
                        >::type
                    >::type
                >::type
            >::type
        >::type
    >::type
    type;
};


//--------------------

class A {};
class B {};

BOOST_STATIC_ASSERT(( boost::is_same<typename DetermineCommon<Default, Default, Default, Default>::type, Default                     >::value ));
BOOST_STATIC_ASSERT(( boost::is_same<typename DetermineCommon<A      , A      , Default, A      >::type, A                           >::value ));
BOOST_STATIC_ASSERT(( boost::is_same<typename DetermineCommon<A      , Default, Default, A      >::type, A                           >::value ));
BOOST_STATIC_ASSERT(( boost::is_same<typename DetermineCommon<Default, A                        >::type, A                           >::value ));
BOOST_STATIC_ASSERT(( boost::is_same<typename DetermineCommon<Default, A      , Default, A      >::type, A                           >::value ));
BOOST_STATIC_ASSERT(( boost::is_same<typename DetermineCommon<A      , B                        >::type, ErrorFoundTwoNonDefaultTypes>::value ));
BOOST_STATIC_ASSERT(( boost::is_same<typename DetermineCommon<A      , Default, B               >::type, ErrorFoundTwoNonDefaultTypes>::value ));
BOOST_STATIC_ASSERT(( boost::is_same<typename DetermineCommon<A      , Default, Default, B      >::type, ErrorFoundTwoNonDefaultTypes>::value ));
BOOST_STATIC_ASSERT(( boost::is_same<typename DetermineCommon<A      , B      , Default         >::type, ErrorFoundTwoNonDefaultTypes>::value ));

//--------------------

int main()
{
    return 0;
}