henry@zoo.toronto.edu (Henry Spencer) (08/02/90)
In article <MCGRATH.90Aug1225852@paris.Berkeley.EDU> mcgrath@paris.Berkeley.EDU (Roland McGrath) writes: >... However, we are talking about the standard for C++. As this standard >remains to be defined, and is independent from the existing standard for C, >I don't see the relavence of your statement. It is currently a defined property of C++ (see E&S) that any program which is both legal C and legal C++ means the same in both (with a couple of tiny and hard-to-avoid exceptions). Many people think that this limited upward compatibility is a good idea and should not be sacrificed. The name of the language is "C++", not "D". On a related point, people should bear in mind that the best language standards are the ones that codify *existing practice*, as opposed to throwing in every wild and wonderful feature that somebody thought was a good idea. X3J11 did, by and large, a very good job on this for C. (If you want to see what happens when standards committees start trying to invent the language, look at ANSI BASIC or FORTRAN 9x.) If the C++ standards effort does its job properly, ranting on the net about the vital need for your favorite feature will be entirely futile. What you need is an implementation of it and some favorable experience using that implementation, followed by a formal proposal to the committee (no, posting it to the net does not qualify). -- The 486 is to a modern CPU as a Jules | Henry Spencer at U of Toronto Zoology Verne reprint is to a modern SF novel. | henry@zoo.toronto.edu utzoo!henry
mcgrath@paris.Berkeley.EDU (Roland McGrath) (08/03/90)
I'm not suggesting codifying practice that does not yet exist. I'm merely suggesting not prohibiting it. -- Roland McGrath Free Software Foundation, Inc. roland@ai.mit.edu, uunet!ai.mit.edu!roland
mcgrath@paris.Berkeley.EDU (Roland McGrath) (08/03/90)
In article <1990Aug2.165111.25529@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes:
It is currently a defined property of C++ (see E&S) that any program which
is both legal C and legal C++ means the same in both (with a couple of tiny
and hard-to-avoid exceptions). Many people think that this limited upward
compatibility is a good idea and should not be sacrificed. The name of
the language is "C++", not "D".
I don't see how this would not remain true if member reordering were allowed.
The C compiler will generate accesses to the structure in the given order
throughout the program, so all the accesses will work. The C++ compiler will
generate accesses to the structure in whatever order it deems appropriate,
using this single order throughout the program, so all the accesses will work.
You can't expect to interlink different pieces of a program that is both legal
C and legal C++, some of which are compiled in C, some in C++, anyway. If you
could, `extern "C"' would not have been invented.
--
Roland McGrath
Free Software Foundation, Inc.
roland@ai.mit.edu, uunet!ai.mit.edu!rolandhenry@zoo.toronto.edu (Henry Spencer) (08/04/90)
In article <MCGRATH.90Aug3011745@paris.Berkeley.EDU> mcgrath@paris.Berkeley.EDU (Roland McGrath) writes: > It is currently a defined property of C++ (see E&S) that any program which > is both legal C and legal C++ means the same in both... > >I don't see how this would not remain true if member reordering were allowed. The problem is that in C, the ordering is a documented property of the language *and programs are therefore allowed to depend on it*. For example, it is explicitly guaranteed that a pointer to a later member compares high against a pointer to an earlier one, and there are probably programs out there somewhere that need this. Such a program will break, despite being legal C, if compiled with a reordering C++ compiler. -- The 486 is to a modern CPU as a Jules | Henry Spencer at U of Toronto Zoology Verne reprint is to a modern SF novel. | henry@zoo.toronto.edu utzoo!henry