Re: [boost] Geometry/Vector Lib

From: Martin Bonner (martin.bonner_at_[hidden])
Date: 2006-09-22 09:52:11

----Original Message----
From: Sohail Somani 

> 
> Marcus Lindblom wrote:
>> 
>> Olivier Grant wrote:
>>> I agree on your point, on the other hand, you can do this :
>>> 
>>>    class vector3d
>>>    {
>>>    public:
>>>       float x, y, z;
>>> 
>>>       float operator[]( int index ) const
>>>       { return ((float *)this)[index]; }
>>> 
>>>       float & operator[]( int index )
>>>       { return ((float *)this)[index]; }
>>> 
>>>       /* ... */
>>>    };
>>> 
>>> in which case "v.x" and "v[0]" are the same thing. this is what I
>>> currently have. Now some people will argument its a hack maybe.
>>> 
>> I've sometimes done just:
>> 
>>    class vector3d
>>    {
>>    public:
>>       float x, y, z;
>> 
>>       float operator[]( int index ) const
>>       { return (&x)[index]; }
>> 
>>       /* ... */
>>    };
> Won't that not work in the presence of padding? 
In theory, yes.  In practice, I can't see why any compiler would bother
adding such padding.  

A more serious problem is if you start mixing usages.  A compiler is
quite likely to assume that assignments through the result of the index
operator don't affect .y or .z, and optimize on that basis.

> Sorry for the TP, lookout webmail. 
Reformatted :-)

-- 
Martin Bonner
Martin.Bonner_at_[hidden]
Pi Technology, Milton Hall, Ely Road, Milton, Cambridge, CB4 6WZ,
ENGLAND Tel: [phone]