Re: [eigen] FFT update

[Rohit Garg <rpg.314@xxxxxxxxx>]

On Sun, Jan 24, 2010 at 12:33 PM, Benoit Jacob <jacob.benoit.1@xxxxxxxxx> wrote:
> 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.

Alrighty, :)
>>
>> 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?

I'd suggest that since you are *forcing* a compiler error here, it
would be better to add a long descriptive error to go with it. Some
thing like this

http://eigen.tuxfamily.org/index.php?title=FAQ#I_need_help_with_compiler_errors.21

>
> 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
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
>
>



-- 
Rohit Garg

http://rpg-314.blogspot.com/

Senior Undergraduate
Department of Physics
Indian Institute of Technology
Bombay