How do you declare an extern "C" function pointer

Question

So I have this code:

#include "boost_bind.h"
#include <math.h>
#include <vector>
#include <algorithm>

double foo(double num, double (*func)(double)) {
  return 65.4;
}

int main(int argc, char** argv) {
  std::vector<double> vec;
  vec.push_back(5.0);
  vec.push_back(6.0);
  std::transform(vec.begin(), vec.end(), vec.begin(), boost::bind(foo, _1, log));
}

And receive this error:

        return unwrapper<F>::unwrap(f, 0)(a[base_type::a1_], a[base_type::a2_]);
.............................................................^
%CXX-E-INCOMPATIBLEPRM, argument of type "double (* __ptr64 )(double) C" is
          incompatible with parameter of type "double (* __ptr64 )(double)"
          detected during:
            instantiation of ...5 pages of boost

So this error is because 'log' is extern "C"'d in math.h

I was wondering how to declare my function pointer argument in foo() so it handles extern "C"'d functions.

Owl

not that it answers your question, but this particular situation doesn't require boost::bind, std::bind2nd would do the job just fine. – Versus

Answer 1

You can try including cmath instead, and using static_cast<double(*)(double)>(std::log) (cast necessary to resolve to the double overload).

Otherwise, you will limit your function to extern C functions. This would work like

extern "C" typedef double (*ExtCFuncPtr)(double);

double foo(double num, ExtCFuncPtr func) {
  return 65.4;
}

Another way is to make foo a functor

struct foo {
  typedef double result_type;
  template<typename FuncPtr>
  double operator()(double num, FuncPtr f) const {
    return 65.4;
  }
};

Then you can pass foo() to boost::bind, and because it's templated, it will accept any linkage. It will also work with function objects, not only with function pointers.

Seals

Amazing, I tried with a functor but I didn't think of fixing it for all types with a template. Thanks – Owl

Answer 2

Try using a typedef:

extern "C" {
  typedef double (*CDoubleFunc)(double);
}

double foo(double num, CDoubleFunc func) {
  return 65.4;
}

Prostitution