C++ for Scientists - Technische Universität Dresden

108 CHAPTER 4. GENERIC PROGRAMMING
really the object itself but only use it for the calculation, we can as well create an anonymous
object and perform the creation/construction and calcution in one expression:
template
typename abs functor::result type
inline abs(const T& x)
{
return abs functor()(x);
}
In this expression we have two pairs of parentheses: the first one contains the arguments of the
constructor, which are empty, and the arguments of the application operator, which is/are the
argument(s) of the function. If would write:
template
typename abs functor::result type
inline abs(const T& x)
{
return abs functor(x); // error
}
then x would be interpreted as argument of the constructor and an object of the functor class
would be returned. 18
Now we have to implement our functor classes:
template
struct abs functor
{
typedef T result type;
};
T operator()(const T& x)
{
return x < T(0) ? −x : x;
}
template
struct abs functor
{
typedef T result type;
};
T operator()(const std::complex& x)
{
return sqrt(real(x)∗real(x) + imag(x)∗imag(x));
}
We wrote a general implementation that works for all fixed-point and floating-point types.
18 Many years and versions ago, g++ tolerated this expression (sometimes) despite it is not standard-compliant.