metaprogramming help

13 Dec 2010

      A couple of times I've run into the situation where a "new" C++ class is written to exactly match "legacy" C structures.  At the edges where old code meets new code, you can cast one to the other in-place.  For example, given a huge mess of data that includes
          oldstruct s1;
you could write the line
          newclass& s2= reinterpret_cast(s1);
because s2 is designed to exactly overlay s1.  It might even derive from it, and just add member functions and friendlier accessors.
Another example I've seen is a class that contains nothing but a char* pointer member, that contains overloaded operators to replace the ancient str* functions.  In particular, it has an operator==, so casting an array of char* to an array of that class allowed std::find to be used!

I don't like the wild and unchecked use of reinterpret_cast, but I can see the point for these and think the idiom can be tamed.  Basically, I want to write a cisam_cast that behaves like a reinterpret_cast but only for those conversions I've explicitly nominated as being cisam's.  CISAM is "compatible in structure and meaning", and I avoid calling it "upgrade" because it works in either direction or sideways between different code bases' definitions of the same wire data.

I'm thinking that I should be able to declare something like
          template<> cisam : std::tr1::true_type {};
to enable cisam_cast<xxx>(a_yyy) to have the same meaning as reinterpret_cast(a_yyy), etc. (foreshadowing hard question...)

Since it would be common to make a new class to wrap a primitive type or old struct, a shortcut would be to include a typedef yyy cisam; as a member of xxx.

Now I get around to my actual question:  the "etc." part above.  Given that xxx and yyy are cisam-mates, I want to enable not only xxx to yyy and vice versa, but handle const and volatile qualified forms with correct cv-modifier carry-through, and handle pointers to them to any depth, which is complicated by the fact that there could be cv-qualifiers at any level.

In the earlier example, the original code was something like:
          char* A[7];  char* val;
          //...
          Str* SA= reinterpret_cast((char**)A);
and I would want
          Str* SA= cisam_cast(A);
to be accepted.  Note that the cisam-ness is between (char*) and (Str), so there are a different number of stars on the before and the after.

So, how might I write the cisam_cast machinery to elaborate the basic cisam definitions into the full set of allowed casts?

Thanks,
--John

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John Dlugosz

Steven Watanabe

John Dlugosz

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Sebastian Redl

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