CODEWITHSUNDEEP
c++
AUK1939
AUK1939

Reputation: 659

A function pointer issue. How to efficiently interface with C API (ie. GSL) from C++ class?

This question seems to have been asked numerous times before, for example here1, here2 and here3.

What I am trying to do is, set the member function of the C-Struct gsl_function to a member function of my class.

class MyClass{

    double foo(double x)
    {
        return ...;
    }

    double bar(double x)
    {
        ...
        gsl_function F;
        // Problem I cant do this (compiler error)
        F.function = &(this->foo);
    }
};

The third link above provides a solution, I think it is based on the wrapper approach described here4.

So my question is can I do better. Is there an easier way? For example, possibly by using Boost's function and Bind objects.

I am weighing up the option of using a gsl wrapper, such as o2scl. But am a bit releuctant as I may pay the price later if the wrapper is not well maintained. Any suggestions?

Upvotes: 2

Views: 753

Answers (1)

Managu
Managu

Reputation: 9039

Since GSL allows you to pass in arbitrary parameters, you can abuse this to hold the pointer to the instance under question. Then use a static member function to forward to the member function:

class MyClass
{
    double foo(double x)
    {
       ...
    }
    static double foo_wrapper(double x, void *params)
    {
        return static_cast<MyClass*>(params)->foo(x);
    }

    double bar(double x)
    {
        ...
        gsl_function F;
        F.function=&MyClass::foo_wrapper;
        F.params=this;

        // invoke GSL function passing in F
        ...
    }
};

Can you do better? Is there an easier way? Not really. Any approach you take will be doing this under the covers somewhere.

But you can write a simple wrapper which hides some of this:

class gsl_function_pp : public gsl_function
{
public:
    gsl_function_pp(boost::function<double(double)> const& func) : _func(func)
    {
        function=&gsl_function_pp::invoke;
        params=this;
    }
private:
    boost::function<double(double)> _func;

    static double invoke(double x, void *params)
    {
        return static_cast<gsl_function_pp*>(params)->_func(x);
    }
};

This should give you (possibly at a moderate performance penalty due to multiple indirections involved) the type of functionality you'd want:

class MyClass
{
    double foo(double x)
    {
        ...
    }

    double bar(double x)
    {
        gsl_function_pp F(boost::bind(&MyClass::foo, this, _1));
        // invoke GSL function passing in F
        ...
    }
};

The caveat is that you'll have to ensure that any gsl_function_pp object stays in scope for the entire time that the GSL might invoke it. So, don't try to set up a root finder/etc in one function (using a local gsl_function_pp), return, and then perform root finding iterations in another -- you'll get a crash or worse.

Upvotes: 7

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