Question

Suppose that I have following class

template<unsigned char B, unsigned char F>
class Foo
{
   .....
}

I hope to overload the operator+ so that if the two inputs are of

Foo<B1, F1> 

and

Foo<B2, F2>, 

respectively, I would like the return value is type of

Foo<max(B1, B2), max(F1, F2)>. 

or something like

Foo<max(B1-F1, B2-F2)+max(F1, F2), max(F1, F2)>. 

Any tips?

Answer 1

Just compute your new type in the operator+ return type, and use MPL for comparisons
Moreover, you don’t need friend for that neither your operator+ need to be mutable.

Simple example with simple classes:

#include <boost/mpl/max.hpp>

template<unsigned char B, unsigned char F>
class Foo
{};

template< unsigned char B1, unsigned char F1
        , unsigned char B2, unsigned char F2
        >
Foo< boost::mpl::max_<boost::mpl_::char_<B1>, boost::mpl_::char_<B2> >::value
   , boost::mpl::max_<boost::mpl_::char_<F1>, boost::mpl_::char_<F2> >::value
   >
operator+(Foo<B1,F1> const& a, Foo<B2,F2> const& b)
{
  Foo< boost::mpl::max_<boost::mpl_::char_<B1>, boost::mpl_::char_<B2> >::value
   , boost::mpl::max_<boost::mpl_::char_<F1>, boost::mpl_::char_<F2> >::value
   > that;
  return that;
}

Now, notice how it is cumbersome. An usual idiom in this case is to ue Boost MPL Integral Types instead of raw value whenever possible

#include <boost/mpl/max.hpp>

template<class B, class F>
class Foo
{};

template< class B1, class F1
        , class B2, class F2
        >
Foo< typename boost::mpl::max_<B1, B2>::type
   , typename boost::mpl::max_<F1, F2>::type
   >
operator+(Foo<B1,F1> const& a, Foo<B2,F2> const& b)
{
   Foo< typename boost::mpl::max_<B1, B2>::type
      , typename boost::mpl::max_<F1, F2>::type
      > that;
  return that;
}

Foo< boost::mpl::int_<4>, boost::mpl::int_<8> > x;

EDIT:
Also, this makes the return type of + a bit complex to write down in pre_C++11 auto.
Another classical stuff is to make this into a function object following the result_of protocol so you have a meta-function handy to compute the return type in an opaque way.