Re: [boost] [GTL] - geometric template library - determining interest

From: Michael Marcin (mmarcin_at_[hidden])
Date: 2007-10-05 12:52:39

Miles Bader wrote:
> "Phil Endecott" <spam_from_boost_dev_at_[hidden]> writes:
>>> http://www.gamedev.net/community/forums/topic.asp?topic_id=261920
>> Yes, a very good page showing several solutions:
>>
>> 1. A vector of member pointers (may have some overhead).
>> 2. Anonymous structs and unions (may be non-standard).
>> 3. x, y and z implemented as references (makes object bigger).
>> 4. Relying on (&x)[1] pointing to y.
> 
> Regarding (2), note that using a union can cause severe performance
> degradation with some compilers (recent versions of gcc, at least).  I
> guess that the compiler makes pessimistic assumptions about aliasing
> when it sees a union.
> 
> As an example:  I have some code that offers both ".x/.y/.z" and
> array-style access to the members of 3d vectors.
> 
> The code originally only used ".x/.y/.z" fields.  When I first added the
> "array-style" feature, I used a construct like:
> 
>    union {
>      struct { float x, y, z; };
>      float els[3];
>    };
>    float &operator[] (unsigned i) { return els[i]; }
>    const float &operator[] (unsigned i) const { return els[i]; }
> 
> The resulting object code was _much_ worse than the original.
> 

Not to mention that you can't put non-POD types into a union which makes 
this option a non-starter for generic code.

- Michael Marcin