Passing member function pointer to class-less function

Question

In the code below, I cannot figure out a way of passing a member function to a generic root-finder.

#include <stdio.h>

double OneDimBisector(double (*fun)(float), float a, float b, float tol){
 double val;                                                                                                                                                                           
 val = (*fun)(0.5*(b-a));    // actually: do proper bisection                                                                                                                          
 return val;                                                                                                                                                                           
}                                                                                                                                                                                      

class EOS {                                                                                                                                                                            
 public:                                                                                                                                                                               
  double S_array[10][10];    // actually: filled by constructor                                                                                                                        
  double S(double T, double P);                                                                                                                                                        

  double T_PS(double P, double S);                                                                                                                                                     
  double functForT_PS(double T);                                                                                                                                                       
  double (EOS::*pfunctForT_PS)(double);                                                                                                                                                
  double Sseek, Pseek;                                                                                                                                                                 
};                                                                                                                                                                                     


double EOS::S(double T, double P){                                                                                                                                                     
  double val = T+P;          // actually: interpolate in S_array                                                                                                                       
  return val;                                                                                                                                                                          
}                                                                                                                                                                                      

double EOS::functForT_PS(double T){                                                                                                                                                    
 return S(T,Pseek)-Sseek;                                                                                                                                                              
}                                                                                                                                                                                      

// Find T from P and S (T is invertible), assuming the intervals are ok
double EOS::T_PS(double P, double S0){
  double Tmin = 2., Tmax = 7., T1, tol=1e-8;
  pfunctForT_PS = &EOS::functForT_PS;
  Sseek = S0;
  Pseek = P;

  printf("\n %f\n", (*this.*pfunctForT_PS)(4.));         // no problem
  T1 = OneDimBisector(pfunctForT_PS, Tmin, Tmax, tol);   // wrong type for pfunctForT_PS

  return T1;
}

int main() {
  double P=3., S=8;
  EOS myEOS;

  printf("\n %f %f %f\n",P,S,myEOS.T_PS(P,S));
}

I do not want to make the root-finder a member because it is not specific to this class, and the solution of making everything static seems very inelegant. Would someone have an idea? This must be a common situation yet I did not find a relevant post that was also understandable to me.

Thanks!

Edit: Actually, I also meant to ask: Is there a proper, thread-safe way of setting the Pseek variable other than what I did? Just to make it clear: I am doing one-dimensional root finding on a two-dimensional function but fixing one of the two arguments.

Answer 1

One way would be to change the signature of the root finder (add #include <functional>):

double OneDimBisector(std::function<double(float)> f, float a, float b, float tol);

Then invoke it with bind:

T1 = OneDimBisector(std::bind(pfunctForT_PS, this, std::placeholders::_1),
                    Tmin, Tmax, tol);

This carries a certain overhead. If you don't mind having lots of duplicate code, you can make the function a template:

template <typename Func>
double OneDimBisector(Func f, float a, float b, float tol);

You invoke it the same way, but every time you have a new function type, a new instance of the template is created in your compilate.

The "traditional" solution would be to have a free (or static) function that accepts an additional instance argument.

---

Update: The "traditional solution":

double OneDimBisector(double(*f)(float, void *), void * data, ...);

double EOSBisect(float f, void * data)
{
    EOS * e = static_cast<EOS *>(data); // very "traditional"
    return e->functorForT_PS(f);
}

Usage: T1 = OneDimBisector(EOSBisect, this, Tmin, Tmax, tol);

Answer 2

You cannot pass a member function pointer as a function pointer, because the latter lacks the context pointer (the this) to properly invoke the member function pointer.

The general way to solve this (as in the standard C++ library) is to use a template:

template <typename F>
double OneDimBisector(F fun, float a, float b, float tol){
   double val;
   val = fun(0.5*(b-a));
   return val;                                                        
}

and pass a function object to it

struct Evaluator
{
   EOS* this_;

   Evaluator(EOS* this_) : this_(this_) {}  // constructor

   double operator()(double value) const    // call the function
   {
       return this_->functForT_PS(value);
   }
};

T1 = OneDimBisector(Evaluator(this), Tmin, Tmax, tol);

You could also use std::bind1st(std::mem_fun(&EOS::functForT_PS), this), but what it does is just the same as the structure above. _{^{(BTW, both std::bind1st and std::mem_fun have been deprecated.)}}

If you don't like templates, you could accept a polymorphic function instead (e.g. using Boost.Function or std::function in C++11), but it will be slower:

double OneDimBisector(const boost::function<double(double)>& fun,
                      float a, float b, float tol)
{
    return fun(0.5 * (b-a));
}

and finally, if you can use C++11, you could use a lambda function on calling OneDimBisector:

T1 = OneDimBisector([=](double value){ return functForT_PS(value); },
                    Tmin, Tmax, tol);

Answer 3

The problem you face is that a function pointer is something different to a member funcgtion pointer.

A common (Java World) Approach to circumvent the problem is using the Strategy pattern (fun of the Bisector would be some Implementation of a Strategy).

A common C++-Approach would be using functors/binding, e.g. with boost:

typedef boost::function<double (double)> MyFun;
double OneDimBisector(const MyFun & fun, float a, float b, float tol){
    double val;                                                                                                                                                                           
    val = fun(0.5*(b-a));    // actually: do proper bisection                                                                                                                          
    return val;                                                                                                                                                                           
}

// Calling
T1 = OneDimBisector (boost::bind (&EOS::functForT_PS, *this), Tmin, Tmax, tol));