[Numpy-discussion] Dealing with types in extension modules

Lane Brooks lbrooks@mit....
Wed Sep 10 22:59:59 CDT 2008

Travis E. Oliphant wrote:
> Lane Brooks wrote:
>> Travis E. Oliphant wrote:
>>> Lane Brooks wrote:
>>>> When writing an numpy extension module, what is the preferred way to 
>>>> deal with the all the possible types an ndarray can have?
>>>> I have some data processing functions I need to implement and they need 
>>>> to be generic and work for all the possible numerical dtypes.  I do not 
>>>> want to have to re-implement the same C-code for all the possible types, 
>>>> so the way I approached it was to use a C++ template function to 
>>>> implement the processing.  Then I have a dispatching function that 
>>>> checks the type of the input ndarray and calls the correct template.  Is 
>>>> there a better way?
>>> You could store the functions in an array of function pointers and 
>>> look-up the correct one using the typenum:
>>> resize_funcs[PyArray_Type(buf1)](PyArray_DATA(bufi))
>>> with resize_funcs filled appropriately.
>>> -Travis
>> Would this require implementing a unique function for each of the 
>> possible types, though?  That is mostly what I want to avoid.  I do 
>> not want to have to implement 10 to 15 different functions that all do 
>> the same exact thing but to different types of data.  I guess with 
>> your proposal I can still use templates to have a single function 
>> definition.
> You could have a default function which does type coercion or uses 
> type-indifferent code.  It really depends on what you are doing.
> But generally if you want to support multiple types you have to repeat 
> the code for that type, I don't know of anyway around that.
> Also, you don't have to fill in all the functions (some could be NULL 
> and you could use coercion to another type or some other default 
> implementation as needed).
> -Travis

Is such an approach preferred to templates?  I think duplicating the 
code for different types sound tedious and error-prone.

What is type-indifferent code?  Is there a way, for example, to add or 
multiple two variables in a type-indifferent way?

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