Re: [boost] alignment problem in proposed any alternative

From: Rob Stewart (stewart_at_[hidden])
Date: 2005-08-11 09:38:59

From: Martin Bonner <martin.bonner_at_[hidden]>
> From: christopher diggins [mailto:cdiggins_at_[hidden]]
> 
> > 3.9(2) says:
> > For any complete POD object type T, whether or not the object holds a
> > valid value of type T, the underlying
> > bytes (1.7) making up the object can be copied into an array of char
> > or unsigned char.36) If the content of the array of char or unsigned
> > char is copied back into the object, the object shall subsequently
> > hold its original value.
> > 
> > This clearly implies char arrays are stringently aligned.
> 
> Err, no it doesn't.  It says that you can store the bits of a POD object
> into a char array, and then load the bits back into the same (correctly
> aligned) POD object.  
> 
> It does *NOT* say that you can use the address of the char array as if it
> were the address of the POD object.

I also agree with this analysis of the text.

> #include <assert.h>
  #include <string.h>
> int main()
> {
> 	long pod = 3141L;
> 	char pad;
> 	char c[sizeof(pod)];
> 	memcpy( c, &pod, sizeof(pod) );
> 	pod = 42L;
> 	memcpy( &pod, c, sizeof(pod) );
> 	assert (pod == 3141L);	// 3.9(2) requires this assert to pass.
> 	// nothing requires THIS assert to pass, and on some systems it fail
> 	// because c is not suitable aligned.
> 	assert ( *reinterpret_cast<long*>(&c[0]) == 3141L );
>       return 0;
> }

That fails on a Sun box.

-- 
Rob Stewart                           stewart_at_[hidden]
Software Engineer                     http://www.sig.com
Susquehanna International Group, LLP  using std::disclaimer;