Re: [boost] [interest] underlying type library

Subject: Re: [boost] [interest] underlying type library
From: John Bytheway (jbytheway+boost_at_[hidden])
Date: 2011-08-22 12:19:51

On 22/08/11 16:20, Julian Gonggrijp wrote:
> Touché! I'll have to accept that the bitwise approach is no solution 
> for any type at all. Could you quote the lines from the standard in 
> which memcpy over non-PODs is deemed undefined, just in order for me 
> to know the exact wording?

Well, I can't find it saying that explicitly; indeed there are hardly
any mentions on memcpy in the standard at all, but here's a relevant bit
for C++0x, from N3290 [basic.types] (3.9) p3:

For any trivially copyable type T, if two pointers to T point to
distinct T objects obj1 and obj2, where neither obj1 nor obj2 is a
base-class subobject, if the underlying bytes (1.7) making up obj1 are
copied into obj2,41 obj2 shall subsequently hold the same value as obj1.
[ Example:
T* t1p;
T* t2p;
// provided that t2p points to an initialized object ...
std::memcpy(t1p, t2p, sizeof(T));
// at this point, every subobject of trivially copyable type in *t1p
contains
// the same value as the corresponding subobject in *t2p
— end example ]

John Bytheway