Re: [eigen] FFT update

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

2010/1/24 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>:
> 2010/1/23 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>:
>> By the way, this "ReturnByValue::coeff() crashes by infinite
>> recursion" thing is really ugly, I wonder how to fix it!!
>>
>> C++0x at last brings the ability to remove a base class function from
>> a derived class... I wonder how to implement that idea in C++98. Is
>> that enable_if?
>
> Forget enable_if, forget C++0x, forget boost, i'm your new idol.
>
> If we want to make sure that using ReturnByValue::coeff() generates an
> error at compile time without generating an error until it's actually
> used, we can define a class:
>
> class Unusable
> {
>  // implement copy ctor/operator as private to disable them
>  Unusable(const Unusable&) {}
>  void operator=(const Unusable&) {}
> };
>
> We then let ReturnByValue::coeff() return that:
>
> class ReturnByValue
> {
> ...
>  Unusable coeff(int, int) { return Unusable(); }
> ...
> };
>
> The result is that when the user does e.g.
>  x = retval.coeff(i);
> or
>  x = retval.coeff(i) + 123;
> he gets compilation errors.
>
> Just doing
>  retval.coeff(i);
> doesn't give an error, but that should be optimizable as just a NOP anyway.
>
> With this, i go to bed full^Wproud of myself.

Ah, this needed a little adjustment. Doing
  Unusable coeff(int, int) { return Unusable(); }
was already copying a Unusable object, the return value of that
method. While the trick still worked with GCC, it was probably
fragile, and the error wasn't very explicit.

Here is a much better variant:
    const Unusable& coeff(int) const { return *reinterpret_cast<const
Unusable*>(this); }

Now this method is really not using any Unusable constructor/operator.

The error message when I do:
     float x = rotate(v).coeff(0);
is now just:
    returnbyvalue.cpp: In function ‘int main()’:
    returnbyvalue.cpp:122: error: cannot convert ‘const
Eigen::Unusable’ to ‘float’ in initialization

ok to commit?

Benoit

>
> Benoit
>
>>
>> Benoit
>>
>> 2010/1/23 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>:
>>> Hi,
>>>
>>> Here's an example: see attached file.
>>> Enjoy!!
>>> Benoit
>>>
>>> 2010/1/22 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>:
>>>> ok let me write an example...
>>>>
>>>> 2010/1/22 Jitse Niesen <jitse@xxxxxxxxxxxxxxxxx>:
>>>>> On Fri, 22 Jan 2010, Benoit Jacob wrote:
>>>>>
>>>>>> Our "policy" is now:
>>>>>> - try to return by value, optimizing with the ReturnByValue class,
>>>>>> wherever possible;
>>>>>> - otherwise you can just use references.
>>>>>
>>>>> Can somebody please explain how to use the ReturnByValue class?
>>>>> Or should I just extrapolate from the source code?
>>>>> I could not find any documentation.
>>>>>
>>>>> Jitse
>>>>>
>>>>>
>>>>>
>>>>
>>>
>>
>