Re: How to pass a binary_function functor as an argument?
t_littell@yahoo.com 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>
{
public:
// I've tried with & without pure virtual method here, no luck.
virtual double operator()(const double& x, const double& y) const =
0;
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);
}
private:
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,
btw).
double foo(const vector<double>& vec, double init, BinFunctor& bf)
{
return std::accumulate(vec.begin(), vec.end(), init, bf); //
COMPILE ERROR HERE
// also tried: bf() bf::operator() nothing works
}
Three reasons for this:
1) You tried to provide a functor, which is not CopyConstructible,
this
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
--
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]