Re: How to pass a binary_function functor as an argument?

"=?iso-8859-1?q?Daniel_Kr=FCgler?=" <>
Tue, 27 Mar 2007 19:17:15 CST
<> schrieb:

I want to pass a binary_function<double, double, double> functor
object as an argument into another function. So, I tried the
following which does not work with std::accumulate():

It does not work, because you provided a functor, which is not
constructible. Algorithms are allowed to copy those functors, which
is not possible for a pure, abstract class. The simple solution is to
provide a non-polymorphic wrapper functor, v.i.

class BinFunctor : public std::binary_function<double, double, double>
   // I've tried with & without pure virtual method here, no luck.
   virtual double operator()(const double& x, const double& y) const =

why not by-value?

    virtual double operator()(double x, double y) const = 0;


Add this wrapper functor:

struct BinFunctorWrapper : std::binary_function<double, double,
double> {
  BinFunctorWrapper(const BinFunctor& f) : f(&f) {}
  double operator()(double x, double y) const {
    return (*f)(x, y);
  const BinFunctor* f;

Note that by using this functor you actually don't need your abstract
functor derive from std::binary_function<> (which is not virtual,

double foo(const vector<double>& vec, double init, BinFunctor& bf)
   return std::accumulate(vec.begin(), vec.end(), init, bf); //
// also tried: bf() bf::operator() nothing works

Three reasons for this:

1) You tried to provide a functor, which is not CopyConstructible,
is fixed be using the BinFunctorWrapper instead.
2) You transferred the functor as non-const reference, which would
not allow to call the const operator() overload.
3) I assume a prefixing std:: before vector.

This leads to:

double foo(const std::vector<double>& vec, double init, const
BinFunctorWrapper& bf)
   return std::accumulate(vec.begin(), vec.end(), init, bf);

int main()
   SumFunctor sf;
   double res = foo(vec, 0, sf);

Add the missing vector and modify the call of foo sligthly:

int main()
   SumFunctor sf;
   std::vector<double> vec = ...; // Initialize
   double res = foo(vec, 0, BinFunctorWrapper(sf));

Greetings from Bremen,

Daniel Kr|gler

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