Re: [boost] Silent change in boost 1.36 alignment_of<> template withgcc 4.3

Subject: Re: [boost] Silent change in boost 1.36 alignment_of<> template withgcc 4.3
From: Jon Biggar (jon_at_[hidden])
Date: 2008-11-09 11:23:33

John Maddock wrote:
>>> In boost 1.36, boost::alignment_of<> was changed to use the builtin
>>> __alignof__() if gcc version was 4.3 or greater rather than using
>>> calculating alignement using:
>>>
>>> template <typename T>
>>> struct alignment_of_hack
>>> {
>>>     char c;
>>>     T t;
>>>     alignment_of_hack();
>>> };
>>>
>>> Unfortunately, this broke some code I had written that did structure
>>> introspection, because gcc's __alignof__() returns 8 for double on
>>> the
>>> x86 architecture, but actually only aligns doubles on 4 byte
>>> boundaries inside structures.
> 
> Hmmm, do you have a test case?
> 
> I ask because the type_traits tests compare the result of 
> alignment_of<>::value with the actual alignment inside a struct... and 
> the tests are all passing.

This is what the alignment_of test case outputs on my system:

alignment_of_test.cpp:78: The expression: 
"::boost::alignment_of<double>::value"
  had an invalid value (found 8, expected 4)
alignment_of_test.cpp:81: The expression: "::boost::alignment_of< 
::boost::long_
long_type>::value" had an invalid value (found 8, expected 4)

That's with this version of gcc from Fedora 9:

gcc version 4.3.0 20080428 (Red Hat 4.3.0-8) (GCC)

-- 
Jon Biggar
jon_at_[hidden]
jon_at_[hidden]
jonbiggar_at_[hidden]