[C++-sig] Exposing a generic template system in Boost.Python

[Francesco Biscani]

Greetings,

I have been trying to think of a solution about this for a few days now,
but I did not get very far.

I am exposing specific instances of C++ template classes using
Boost.Python. Let's say for the sake of argument that I am exposing
std::vector. What I would like to achieve on the Python
side would be some kind of unified interface of this kind:

# Define a vector-of-ints type.
vec_int = vector["int"]
# Define a vector-of-doubles type.
vec_float = vector["double"]
...

etc., possibly with exceptions being raised if a template type which has
not been exposed is requested. Additionally, the system should
automagically work with nested generic types:

# Define a vector-of-vectors-of-ints type.
vec_vec_int = vector[vector["int"]]

I have played around with some ideas, but it seems like I can never get to
a point where the design does not look bloated and/or brittle. Thus I am
wondering, is there anyone that tackled a problem like this before in
Boost.Python and has pointers/suggestions on how to do this properly?
Support for things like default template arguments would definitely be a
plus.

FWIW, I have access to C++11 on the C++ side, so I can deal with variadic
templates and other amenities in a not-so-kludgy way.

Thanks for any input,

  Francesco.
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Re: [C++-sig] Exposing a generic template system in Boost.Python

[Stefan Seefeld]

Francesco,

I have done something like what you are suggesting. It essentially boils
down to defining a function template like

template 
void define_vector(char const *name)
{
   class_<...> vector(name);
  ...
}


and then calling that multiple times:

  define_vector("IVector");
  define_vector("LVector");
  ...

Providing a factory function that instantiates one of those based on a
value-type selector as you want can be easily done on the Python side.

I don't think this can be automated any further. In particular, it is
clear that any type you may want to instantiate in Python has to be
compiled explicitly into the extension module. I.e., you need to
explicitly instantiate the templates above, by explicitly calling the
functions for the types you want to see supported at runtime. There is
no JIT compilation for this.

Stefan

-- 

  ...ich hab' noch einen Koffer in Berlin...

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Re: [C++-sig] Exposing a generic template system in Boost.Python

[Nikolay Mladenov]

To add to Stefan's answer:

I do the the same and also use visitors to add the same sets of members to
classes, with possible customizations via trait classes.

HTH


On Wed, Mar 19, 2014 at 1:36 PM, Stefan Seefeld  wrote:

> Francesco,
>
> I have done something like what you are suggesting. It essentially boils
> down to defining a function template like
>
> template 
> void define_vector(char const *name)
> {
>class_<...> vector(name);
>   ...
> }
>
>
> and then calling that multiple times:
>
>   define_vector("IVector");
>   define_vector("LVector");
>   ...
>
> Providing a factory function that instantiates one of those based on a
> value-type selector as you want can be easily done on the Python side.
>
> I don't think this can be automated any further. In particular, it is
> clear that any type you may want to instantiate in Python has to be
> compiled explicitly into the extension module. I.e., you need to
> explicitly instantiate the templates above, by explicitly calling the
> functions for the types you want to see supported at runtime. There is
> no JIT compilation for this.
>
> Stefan
>
> --
>
>   ...ich hab' noch einen Koffer in Berlin...
>
> ___
> Cplusplus-sig mailing list
> Cplusplus-sig@python.org
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>
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