C++ for Scientists - Technische Universität Dresden

120 CHAPTER 4. GENERIC PROGRAMMING
4.8.3 The function accumulate with a functor argument
TODO: Again, I don’t like the use of pointers here — Peter
Recall the function accumulate from section 4.2.1 that we used to introduce Generic Programming.
In this section, we will generalize this function. We introduce a binary functor (concept
BinaryFunctor) that implements an operation on two arguments as function or callable class
object. 27 Then we can accumulate values with respect to this binary operation:
template
T accumulate( T∗ a, T∗ a end, T init, BinaryFunctor op ) {
T sum( init ) ;
for ( ; a!=a end; ++a ) {
sum = op( sum, ∗a ) ;
}
return sum ;
}
The concept BinaryFunctor is defined as follows: 28
• Let op be of type BinaryFunctor.
– has the method op( first argument type, second argument type ) with result type being
convertible to T. T should be convertible to the first and second argument types.
From this generic example, it is quite clear that the conceptual conditions are becoming complicated
when we are mixing types. Usually, we make sure that the first argument type, second
argument type and result type are the same, but strictly speaking, this is not required, since
the compiler is allowed to perform conversions.
The main program could be as follows:
struct sum functor
{
double operator() ( double a, double b ) const {
return a + b ;
}
} ;
struct product functor
{
double operator() ( double a, double b ) const {
return a ∗ b ;
}
} ;
int main()
{
int n=10;
double a[n] ;
double s = accumulate( a, a+n, 0.0, sum functor() ) ;
s = accumulate( a, a+n, 1.0, product functor() ) ;
}
27 TODO: Introduce term.
28 TODO: revisit