C++ for Scientists - Technische Universität Dresden

4.8. FUNCTORS 113
struct functor base
{
virtual double operator() (double x) const= 0 ;
} ;
double finite difference( functor base const& f, double x, double h )
{
return ( f(x+h) − f(x) ) /h ;
}
The functor class has a pure 22 virtual function operator() and thus can not be used. We can
however alter the functor para sin plus cos such that it inherits from the abstract base class and
specializes operator().
class para sin plus cos
: public functor base
{
public:
para sin plus cos(double p) : parameter(p) {}
double operator() (double x) const // Is virtual function in base
{
return sin( parameter ∗ x ) + cos(x);
}
private:
double parameter;
};
Now we can use an object of this class as the first argument of finite difference.
The whole program looks as follows:
#include
#include
struct functor base {
virtual double operator() ( double x ) const = 0 ;
} ;
double finite difference( functor base const& f, double x, double h ) {
return ( f(x+h) − f(x) ) /h ;
}
class para sin plus cos
: public functor base
{
public:
para sin plus cos( double const& p )
: parameter ( p )
{}
double operator() ( double x ) const { // Virtual function
return sin( parameter ∗ x )+ cos(x) ;
22 TODO: undefined