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copied!<p>It is indeed interesting that people ask this over and over again. One reason may be that the proposed solutions are not easy to understand. I for one had problems understanding and implementing them. (the solutions did not work out of the box for me, as you might expect.)</p> <p>With the help of <a href="https://stackoverflow.com/users/423791/tlamadon">tlamadon</a> I just figured out a solution that may be helpful here as well. Let's see what you guys think. </p> <p>So just to recap, the problem is that you have a class that contains a member function on which you want to operate with something from the GSL library. Our example is useful if the GSL interface requires a </p> <pre><code>gsl_function F; </code></pre> <p><a href="http://www.gnu.org/software/gsl/manual/html_node/Providing-the-function-to-solve.html#Providing-the-function-to-solve" rel="nofollow noreferrer">see here for a definition.</a></p> <p>So here is the example class:</p> <pre><code>class MyClass { private: gsl_f_pars *p; // not necessary to have as member public: double obj(double x, void * pars); // objective fun double GetSolution( void ); void setPars( gsl_f_pars * xp ) { p = xp; }; double getC( void ) ; // helper fun }; </code></pre> <p>The objective of this exercise is to be able to </p> <ol> <li>initiate <code>MyClass test</code>, </li> <li>supply it with a paramter struct (or write a corresponding constructor), and</li> <li>call <code>test.GetSolution()</code> on it, which should return whatever the GSL function was used for (the minimum of <code>obj</code>, a root, the integral or whatever)</li> </ol> <p>The trick is now to put have an element in the parameter <code>struct gsl_f_pars</code> which <em>is a pointer to MyClass</em>. Here's the struct:</p> <pre><code>struct gsl_f_pars { double a; double b; double c; MyClass * pt_MyClass; }; </code></pre> <p>The final piece is to provide a wrapper that will be called inside <code>MyClass::GetSolution()</code> (the wrapper is a stand in for the member function <code>MyClass::obj</code>, which we cannot just point to with <code>&obj</code> inside the class). This wrapper will take the parameter struct, dereference <code>pt_MyClass</code> and evaluate <code>pt_MyClass</code>'s member <code>obj</code>: </p> <pre><code>// Wrapper that points to member function // Trick: MyClass is an element of the gsl_f_pars struct // so we can tease the value of the objective function out // of there. double gslClassWrapper(double x, void * pp) { gsl_f_pars *p = (gsl_f_pars *)pp; return p->pt_MyClass->obj(x,p); } </code></pre> <p>The full example is a bit too long to post here, so I put up a gist. It's a <a href="https://gist.github.com/floswald/7103615" rel="nofollow noreferrer">header file</a> and a <a href="https://gist.github.com/floswald/7103639" rel="nofollow noreferrer">cpp file</a>, it should be working wherever you have GSL. Compile and run with </p> <pre><code>g++ MyClass.cpp -lgsl -o test ./test </code></pre>

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