SWIG interfaces

karlnapf edited this page Oct 26, 2014 · 16 revisions
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How SWIG works

Shogun exposes its modular interfaces via SWIG (activated via cmake). SWIG reads a bunch of interface files, located in shogun/src/interfaces/modular/*.i, which contain a list of classes to expose to the modular interfaces (and options and technical things).

Roughly speaking, SWIG scans all header files of the to be exposed classes, and for every public class/method in those, generates some code that is all put into a C++ file. This file is located in src/interfaces/python_modular/modshogunPYTHON_wrap.cxx (Python version) and is gets quite big. This happens because all methods of the classes in the interface files are cause SWIG to generate some code in there. Even more crazy, all templated classes are instantiated with all possible template parameters: SGVector<float64_t> aka RealVector, SGVector<float32_t> aka ShortRealVector, and so on.

The file is eventually compiled with the C++ compiler and produces the modshogun.py (Python) that contains the interface for the modular target language. Note that the compilation process requires quite some memory (2GiB currently, Oct 2014).

SWIG also contains a bunch of typemaps: Those are responsible for mapping memory from say a Shogun SGVector to a numpy.ndarray, etc. All signatures can be used with the target language types. Great!

Two public interfaces: SWIG and C++

In order to avoid the SWIG output to explode, we want to minimise the number of methods being exposed in SWIG. We thought a lot about how to do this. D-pointers seemed an option at first as they keep the interface clean. However, they don't allow to have two public interfaces, which we realised is exactly what we need -- there are two types of public interfaces in Shogun:

  • Public C++ methods
  • SWIG methods (subset of the above)

Note that there are methods that belong to the first but not the second type. For example helper methods that need to be called from another C++ class, but are usually not called from within the modular interface. A Python example is


Here, CSVFile(fname) is an instance of CSVFile, whose C++ interface implements a bunch of methods to access individual vectors, e.g.,

virtual void get_vector(float64_t*& vector, int32_t& len);

in CSVFile. However, those methods should not be exposed in Python as SWIG's output gets too large (method exists for every type) and users actually do not need those methods -- they just want to pass the instance of CSVFile to another class (RealFeatures here), which then calls the get_vector methods to load data.

How to do this

  • Write Shogun C++ interface
    • Only includes from <shogun/*> are allowed in header files, no external ones
    • Public methods can only contain Shogun data-types
      • Allowed: SGVector, float64_t, CSGObject
      • Not allowed: Eigen3, STL, etc
    • Make all helper methods protected/private (SWIG ignores them then)
      • This also means that one can use non-Shogun types in them (only forward declared)

Finally: For helper methods that need cannot be private (as called by other classes), one can do

#ifndef SWIG //SWIG should ignore this
     SGVector<T> public_method_to_ignore();
#endif // #ifndef SWIG //SWIG should ignore this

This will cause SWIG to ignore the method in between. It will not be available in the modular interfaces (and though not appear in the C++ file that SWIG outputs). Note that the SWIG flag is true if and only if SWIG scans the header files. That is why the above code leaves only hides the method when SWIG scans the file. C++ itself will see it.

Alternatively, one can do

#ifdef SWIG
%ignore public_method_to_ignore;
#endif // #ifdef SWIG

The first option is clearly better as no pattern/type/signature matching has to be done.

Shogun also has a blacklist where we used to add methods to ignore, modshogun_ignores.i. This file is very hard to maintain, and is therefore not recommended to use. Excluding methods from SWIG in the C++ interface file puts both of Shogun's public interfaces (as listed above) in one place and therefore allows to easily get an overview.


When writing a new Shogun class

  • Think about the interface, make it clean (maybe use D-pointers)
  • Don't include any non-Shogun objects in there
  • Try to make all helper methods private/protected to hide them from SWIG
  • For helper methods that cannot be private (as need to be called by other classes), mark them to be ignored by SWIG.

World domination.