Re: [Boost-users] Convert big endian to little endian (floating point values)

Subject: Re: [Boost-users] Convert big endian to little endian (floating point values)
From: Agustín K-ballo Bergé (kaballo86_at_[hidden])
Date: 2015-12-11 15:11:50

On 12/11/2015 4:54 PM, Ion Gaztañaga wrote:
>> Reading from an inactive member of a union is plain and simply undefined
>> behavior. To read and write the bits of the `float` in a well defined
>> way use `std::memcpy`.
>
> Curiously, it is undefined in C++, whereas well-defined in C:
>
> "6.5.2.3 Structure and union members
>
> If the member used to read the contents of a union object is not the
> same as the member last used to store a value in the object, the
> appropriate part of the object representation of the value is
> reinterpreted as an object representation in the new type as described
> in 6.2.6 (a process sometimes called ‘‘type punning’’). This might be a
> trap representation."
>
> Since C++ includes C as a normative reference it would be a good idea to
> allow it also in C++. memcpy is fine if the compiler optimizes it as an
> intrinsic, but inefficient in some implementations to copy only a couple
> of bytes.

C has *vastly* weaker aliasing rules than C++ (to the point that several 
C implementations choose to follow the C++ rules instead).
For more details see the thread at 
http://lists.boost.org/Archives/boost/2015/11/226663.php

If your implementation of choice gives you poor codegen for `memcpy` 
then I'll suggest you file a bug against it. We would all benefit from that.

Regards,
-- 
Agustín K-ballo Bergé.-
http://talesofcpp.fusionfenix.com