Storing function pointers

Question

The following uses a simple function pointer, but what if I want to store that function pointer? In that case, what would the variable declaration look like?

#include <iostream>
#include <vector>

using namespace std;

double operation(double (*functocall)(double), double wsum);
double get_unipolar(double);
double get_bipolar(double);

int main()
{
    double k = operation(get_bipolar, 2); // how to store get_bipolar?
    cout << k;
    return 0;
}
double operation(double (*functocall)(double), double wsum)
{
    double g = (*functocall)(wsum);
    return g;
}
double get_unipolar(double wsum)
{
    double threshold = 3;
    if (wsum > threshold)
        return threshold;
    else
        return threshold;
}
double get_bipolar(double wsum)
{
    double threshold = 4;
    if (wsum > threshold)
        return threshold;
    else
        return threshold;
}

Answer 1

You already (almost) have it in your code:

double (*functocall)(double) = &get_bipolar;

This defines a function pointer named functocall which points to get_bipolar.

Answer 2

typedef double (*func_t)(double); 
func_t to_be_used = get_bipolar

Answer 3

double (*foo)(double);

where foo is the variable name.

Answer 4

typedef double (*PtrFunc)(double); 

PtrFunc ptrBipolar = get_bipolar;


OR


typedef double (Func)(double); 

Func *ptrBipolar = get_bipolar;

which ever you are comfortable to use.

Answer 5

You should consider using a typedef:

 typedef double (*MyFunc)(double);
 MyFunc ptr_func = &get_bipolar;
 (*ptr_func)(0.0);


double operation(MyFunc functocall, double wsum)
{
    double g;
    g = (*functocall)(wsum);
    return g;
}

May I recommend also the identity template trick:

template<class T>
class Id
{
    typedef T type;
};

Id<double(double)>::type * ptr_func = &get_bipolar;
MyFunc func = &get_bipolar;
(*ptr_func)(0.0);


double operation(Id<double(double)>::type * functocall, double wsum)
{
    double g;
    g = (*functocall)(wsum);
    return g;
}

Answer 6

You code is almost done already, you just seem to call it improperly, it should be simply

double operation(double (*functocall)(double), double wsum)
{
    double g;
    g = functocall(wsum);
    return g;
}

If you want to have a variable, it's declared in the same way

double (*functocall2)(double) = get_bipolar;

or when already declared

functocall2 = get_bipolar;

gives you a variable called functocall2 which is referencing get_bipolar, calling it by simply doing

functocall2(mydouble);

or passing it to operation by

operation(functocall2, wsum);

Answer 7

Have a look at boost function, it's a header only library that tidies things up a little (IMHO): http://www.boost.org/doc/libs/1_42_0/doc/html/function.html

typedef boost::function<double (double)> func_t;
func_t to_be_used = &get_bipolar;

(NB: different syntax required for VC6)