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From: dlagno@mail.nnov.ru (Denis Lagno)
Newsgroups: comp.std.c++
Subject: proposal: "virtual restrict" inheritance
Date: Sun, 2 Feb 2003 20:23:52 +0000 (UTC)
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[Problem description]
Imagine that you have typical rhomb shaped inheritance framework

     B
   /   \
   C   D
   \   /
     X

If compiler knows that nothing is grafted badly to this structure in
other translation units, it can implement one of inheritances
C:virtual B or D:virtual B not as it usually does -- thru pointer and
indirect access but directly thru statically known offset.

But since compiler sees only one translation unit at a time it cannot
do such optimization.

[Proposed solution]
add "virtual restrict" inheritance.
Now standard says

# For an object of class AA, all virtual occurrences
# of base class B in the class lattice of AA correspond
# to a single B subobject within the object of type AA,
# and every other occurrence of a (non-virtual) base
# class B in the class lattice of AA corresponds
# one-to-one with a distinct B subobject within
# the object of type AA. 

"virtual restrict" inheritance logically shall be the same as usual
"virtual" but with one restriction -- if for any derived class some
base class occurs more than 1 time as "virtual restrict" then the
program is ill-formed.

So changes to standard required are very small and local and without
possible interference with other language features -- because
semantically "virtual restrict" shall everywhere act as usual
"virtual".  But beforementioned restriction enables compiler to
optimize virtual inheritance.

I assumed that keyword "restrict" already included in C99 so it is
good to reuse it in another sense in another place.

[example]
former inheritance framework may be now written as:

class B { ... };

class C: public virtual B { ... };

class D: public virtual restrict B { ... };

class X: public C, public D { ... };

I'll be glad to hear comments.
--
Denis

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