Re: [Bug-gsl] using gsl callback routine from c++ / pointer to member function

[Jonathan Taylor]

> I'm trying to use GSL's multimin routine from C++, but the same issue
> arises with integration and any functionality that uses a callback function
> to calculate a user function f(x). The calls to GSL and to the callback
> function are in a class. I am using g++ 4.1.2 and GSL 1.13.
> 
> l define a function myF() to compute f(x), and pass GSL a pointer to myF().
> The problem is that the compiler won't convert from "pointer to myF ()" to
> the way that GSL declares the pointer. This is because C++ treats a
> "pointer to member function" differently from how it treats a pointer to a
> function that is not part of a class. I understand the C++ issue; I hope
> that somebody on this list knows how to get around it.

I'm afraid this is a completely standard issue, and I am pretty sure there is 
no particularly elegant solution to it. Your part 1 and part 2 will not and can 
not work, that's just basic violations of how the language requires you to 
behave, I'm afraid. The function types really are different. [As you may well 
know, a C++ function can be thought of as akin to a C function which takes an 
extra, first, parameter: the pointer to "this"]. No amount of typecasting can 
hide that.

Part 3 should work. The standard format is to do something like this (written 
in email, not compiled):

class MyClass
{
        double MyCallback(const gsl_vector *x);
        static double MyStaticCallback(const gsl_vector *x, void *params) { 
return ((MyClass*)params)->MyCallback(x); }
};

i.e. you declare a second static helper function for any class-based callback 
you require.


> Another suggestion was to make myF() static, ugly though that may be. I
> tried that, and got an error message that the minimization routine could
> not be found. Oddly, this was a runtime error. The linker did not complain,
> so presumably it found the nmsimplex2 routine, but somehow the routine
> could not be found at runtime.
> 
>    In the code: const gsl_multimin_fminimizer_type *T =
> gsl_multimin_fminimizer_nmsimplex2
> 
>    Runtime error: Unrecognized symbol gsl_multimin_fminimizer_nmsimplex2

I'm 99% sure this has nothing to do with your question about C++ callbacks. I 
can only guess that you are weakly linking to the GSL library and the weak 
linkage is failing at runtime, or something like that.

Hope this helps
Jonny