Tag Archives: C++

Parallel computing in large-scale applications

In March 2008 issue, IEEE Computers published a case study on large-scale parallel scientific code development. I’d like to comment this article, a very good one in my mind.

Five research centers were analyzed, or more precisely their development tool and process. Each center did a research in a peculiar domain, but they seem share some Computational Fluid Dynamics basis.

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Transforming a C++ vector into a Numpy array

This question was asked on the Scipy mailing-list last year (well, one week ago). Nathan Bell proposed a skeleton that I used to create an out typemap for SWIG.

  1. %typemap(out) std::vector<double> {
    
  2.     int length = $1.size();
    
  3.     $result = PyArray_FromDims(1, &amp;length, PyArray_DOUBLE);
    
  4.     memcpy(PyArray_DATA($result),&amp;((*(&amp;$1))[0]),sizeof(double)*length);
    
  5. }
    
  6. </double>

This typemap uses obviously Numpy, so don’t forget to initialize the module and to import it. Then there is a strange instruction in memcpy. &((*(&$1))[0]) takes the address of the array of the vector, but as it is wrapped by SWIG, one has to get to the std::vector by dereferencing the SWIG wrapper. Then one can get the first element in the vector and take the address.

Using Scons to create Python modules with Visual Studio 2005

Starting from Visual Studio 2005, every executable or dynamic library must declare the libraries it uses with a manifest file. This manifest can be embedded in the executable or library, and this is the best way to deal with it.

When using Scons, this embedding does not occur automatically. One has to overload the SharedLibrary builder so that a post-action is made after building the library :

def MSVCSharedLibrary(env, library, sources, **args):
  cat=env.OriginalSharedLibrary(library, sources, **args)
  env.AddPostAction(cat, 'mt.exe -nologo -manifest ${TARGET}.manifest -outputresource:$TARGET;2')
  return cat
 
 env['BUILDERS']['OriginalSharedLibrary'] = env['BUILDERS']['SharedLibrary']
 env['BUILDERS']['SharedLibrary'] = MSVCSharedLibrary

With this method, the embedding is made for every library, which is handy. The same can be done for the Program builder with the line :

  env.AddPostAction(cat, 'mt.exe -nologo -manifest ${TARGET}.manifest -outputresource:$TARGET;1')

Wrapping a C++ container in Python

When moving to Python, the real big problem that arises is the transformation of a Python array into the C++ container the team used for years.

Let’s set some hypothesis :

  • there is a separation between the class containing the data and the class that uses the data (iterators, …)
  • the containing class can be changed (policy or strategy pattern)

The first hypothesis is derived from the responsibility principle, the two classes have two distinct responsibilities, the first allocates the data space and allows simple access to it, the second allows usual operations (assignation, comparison tests or iterations for instance).

The second one will be the heart of the wrapper. It allows to change the way data is stored and accessed in a simple way.
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