on Fri Nov 02 2007, "Boris Gubenko" <Boris.Gubenko-AT-hp.com> wrote:
David Abrahams wrote:
I can't imagine how your change would fix an endianness problem.
It does not fix "an endianness problem". The bug is in the code conditionalized with '#ifdef BOOST_BIG_ENDIAN'. This is the only connection between endianness and the fix.
Also, your change relies on the nonportable (implementation-defined) behavior of reinterpret_cast.
It does. As well as other Boost code using reinterpret_cast :-)
Scanning Boost sources (trunk) for reinterpret_cast yields 700 hits, including 8 instances in the Boost python library (two of them are commented out):
Those are most likely all wrong; they should be looked at one by one and fixed.
bash-2.03$ find ./boost/libs/python -type f -exec grep reinterpret_cast {} \; instance<>* self = reinterpret_cast<instance<>*>(inst); _doc += str(reinterpret_cast<const char*>(detail::py_signature_tag)); // _doc += "\n"+str(reinterpret_cast<const char*>(detail::cpp_signature_tag)); _doc += str(reinterpret_cast<const char*>(detail::cpp_signature_tag)); instance<>* self = reinterpret_cast<instance<>*>(inst); _doc += str(reinterpret_cast<const char*>(detail::py_signature_tag)); // _doc += "\n"+str(reinterpret_cast<const char*>(detail::cpp_signature_tag)); _doc += str(reinterpret_cast<const char*>(detail::cpp_signature_tag)); bash-2.03$
I am apparently responsible for one of those, though only a careful combing through history will tell for sure. I need to bother other people about the other instances.
You are, probably, referring to 5.2.10 - Reinterpret cast [expr.reinterpret.cast], para 3:
-3- The mapping performed by reinterpret_cast is implementation-defined. [Note: it might, or might not, produce a representation different from the original value. ]
In my fix, reinterpret_cast performs a mapping from non-null pointer to long to pointer to char. If reinterpret_cast cannot be used in this case without invoking implementation-defined behavior, I'm not sure how it can be used in a multiplatform code like Boost at all.
Very perceptive. There are almost no ways to use reinterpret_cast portably. If you want to get a char* that traverses the bytes of a long, you do it as below:
I think the correct change to silence the type conversion error is
char * first = static_cast<char *>(static_cast<void *>(const_cast<long*>(& l)));
I'm not sure my change is incorrect.
It is if you're trying to write portable code. You've already found the place in the standard where it says reinterpret_cast performs an implementation-defined mapping. If you want to assert your change is portable, please show me where in the standard it defines the behavior of your specific usage.
In terms of style, I think that your change is better.
Hah, I think it's stylistically distasteful, but at least it's correct. I wish that it were properly spelled reinterpret_cast<char*>(&l) ;-) -- Dave Abrahams Boost Consulting http://www.boost-consulting.com