Re: [Glsdk-devel] The use of anonymous structures

[Branan R. <br...@gm...>]

I don't think the problem is in structs, it's in a union of
struct/array that makes things complicated.

Having mat2f defined as:

typedef struct GLMmat2f_t
{
float m00, m01, m10, m11;
} GLMmat2f;

is perfectly fine. It's the union that's complicated and confusing.

Branan

On Sat, Aug 23, 2008 at 2:32 PM, Henri Häkkinen <hen...@gm...> wrote:
> No no... We had this conversation already in the forums few days ago.
>
> The struct approach is superior over typedef GLfloat GLMmat2f[4]; for the
> following reason:
>
> - it allows matrix data to be accessed as either using index or members. You
> can see the memory layout of the matrix from the ordering of the mXX
> members.
>
> - the following piece of code:
>
> typedef float FOO[2];
>
> FOO *test (FOO *out, const FOO *f)
> {
>   (*out)[0] = (*f)[0];
>   (*out)[1] = (*f)[1];
>   return out;
> }
>
> int main ()
> {
>   FOO a, b;
>   test (&a, &b);
>   return 0;
> }
>
> Produces the following warning on GCC 4.2.3:
> warning: passing argument 2 of 'test' from incompatible pointer type
>
> Compare this:
>
> typedef struct {
>   union {
>     float f[2];
>     struct {
>       float x, y;
>     };
>   };
> } FOO;
>
> FOO *test (FOO *out, const FOO *f)
> {
>   out->x = f->x;
>   out->y = f->y;
>   return out;
> }
>
> This code is a lot more clear and usable than the previous one. Also:
>
> vec[0] = -vec[0];
>
> You cannot be sure if this vec is an array of vectors or are you accessing
> the first element of a 2D vector.
>
> Implementing math datatypes as structure is a lot more cleaner approach than
> the array approach. Perhaps we should think if the union there is redundant
> but typedef GLfloat GLMvec2f[2]; is not a good idea. And I don't think we
> should hinder our design by the assumed ignorance of the afromentioned
> newbies. Besides, I have already implemented the library by the struct
> approach design. See the OpenGL forums for the discussion we underwent there
> fore more points.
>
>
> On Sun, Aug 24, 2008 at 12:04 AM, Jason McKesson <ko...@gm...> wrote:
>>
>> Henri Häkkinen wrote:
>>
>> Somebody mentioned in the OpenGL forums that we should avoid the use of
>> anonymous structures. Is this worth the concern?
>>
>> For example, compare these:
>>
>> typedef struct GLMmat2f_s {
>>   union {
>>     GLfloat m[4];
>>     struct GLMmat2fdata_s {
>>       GLfloat m00, m01, m10, m11;
>>     };
>>   };
>> } GLMmat2f;
>>
>> vs.
>>
>> typedef struct {
>>   union {
>>     GLfloat m[4];
>>     struct {
>>       GLfloat m00, m01, m10, m11;
>>     };
>>   };
>> } GLMmat2f;
>>
>> We need to enclose the mXX variables into a struct in order for the union
>> to take them as a whole.
>>
>> --
>> Henri 'henux' Häkkinen
>>
>> I would suggest that those struct definitions violate the design principle
>> of simplicity. You need to pick one: either a matrix is an array of floats
>> or it is a struct with member fields. It should not be both. If you're
>> having to use a union, you've already done something wrong. Unions are
>> confusing and scary to neophyte programmers.
>>
>> I'd prefer that a matrix were defined as:
>>
>> typedef GLfloat[4] GLMmat2f;
>>
>> It's simpler and gets the job done without being confusing to a programmer
>> as to what is going on. Yes, it will need to have conventions associated
>> with it, but that's a small price to pay for the obviousness of what the
>> matrix is doing.
>>
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>
>
>
> --
> Henri 'henux' Häkkinen
>
>
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