Re: [Boost-users] Seeding boost::mt19937 with bytes from CryptoGenRandom

Subject: Re: [Boost-users] Seeding boost::mt19937 with bytes from CryptoGenRandom
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2010-02-11 17:01:56

AMDG

Ovanes Markarian wrote:
> Yes I read the paragraph as well. But as far as I understand my use case is
> legal (assumed uint64_t is POD), which I think it is. Or am I mistaken?
>   

It's legal for char or unsigned char to alias any type, so it would
probably actually be okay in this specific case.

> This is what 9.2  Clause 16 states (you have a snippet from 9.5 which just
> refers to that one):
> "If a POD-union contains two or more POD-structs that share a common initial
> sequence, and if the PODunion
> object currently contains one of these POD-structs, it is permitted to
> inspect the common initial part
> of any of them. Two POD-structs share a common initial sequence if
> corresponding members have layoutcompatible
> types (and, for bit-fields, the same widths) for a sequence of one or more
> initial members."
>
>
> I understand that that it is allowed to use union for byte conversions of
> POD types.
>   

"Common initial sequence" means something like

struct { int x; double y; };
struct { int x; float y; };

int x is common to the two structs.
You can't use a union to convert between arbitrary PODs.

In Christ,
Steven Watanabe