Category Archives: C++

Everything related to the use of the C++ language

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.     $result = PyArray_FromDims(1, &amp;length, NPY_DOUBLE);
  4.     memcpy(PyArray_DATA((PyArrayObject*)$result),&amp;((*(&amp;$1))[0]),sizeof(double)*length);
  5. }

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.

Creating a Python module with Scons and SWIG

Some times ago, I proposed an optional build for SWIG if the SWIG binary was not found on the system. Here I propose an enhancement, a new library builder that will be registered in the environment env as PythonModule. It takes the same arguments as a classical SharedLibrary, but it does some additional steps :

  • It forces SWIG to create a Python wrapper (flag -python)
  • It checks if SWIG is present at all
  • It suppresses every prefix that the system might need (as lib in Linux)
  • On Windows and for Python >= 2.5, it changes the extension as pyd

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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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Building with Scons and an optional SWIG

I now regularly use Scons as a cross-platform software construction tool. It is easy, written in Python, and I know Python, so no problem learning a new language as for CMake. In some cases when I use SWIG, the target platform does not have the SWIG executable. But when compiling a module, Scons must use this executable, whatever you try to do. In this case, one need to create a new SharedLibrary builder, so that this attribute will determine if SWIG is present or if the generated .c or .cpp files must be used instead.
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