[Boost-users] metaprogramming help

Subject: [Boost-users] metaprogramming help
From: John Dlugosz (JDlugosz_at_[hidden])
Date: 2010-12-13 18:52:34

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<newclass&>(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<xxx,yyy> : std::tr1::true_type {};
to enable cisam_cast<xxx>(a_yyy) to have the same meaning as reinterpret_cast<xxx&>(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<Str*>((char**)A);
and I would want
          Str* SA= cisam_cast<Str*>(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

=====Begin Corporate Footer That I Can't Do Anything About=====

TradeStation Group, Inc. is a publicly-traded holding company (NASDAQ GS: TRAD) of three operating subsidiaries, TradeStation Securities, Inc. (Member NYSE, FINRA, SIPC and NFA), TradeStation Technologies, Inc., a trading software and subscription company, and TradeStation Europe Limited, a United Kingdom, FSA-authorized introducing brokerage firm. None of these companies provides trading or investment advice, recommendations or endorsements of any kind. The information transmitted is intended only for the person or entity to which it is addressed and may contain confidential and/or privileged material. Any review, retransmission, dissemination or other use of, or taking of any action in reliance upon, this information by persons or entities other than the intended recipient is prohibited. If you received this in error, please contact the sender and delete the material from any computer.