mirror of https://github.com/CGAL/cgal
112 lines
2.7 KiB
TeX
112 lines
2.7 KiB
TeX
|
|
\cleardoublepage
|
|
\chapter{Function Objects}
|
|
|
|
Function objects are objects with an \ccc{operator()(..)} defined. This results
|
|
in faster code than passing function pointers, as the operator can
|
|
even be inlined. The following function object classes are defined in \stl.
|
|
|
|
\section{Arithmetic operations}
|
|
|
|
\ccInclude{functional}
|
|
|
|
\stl\ defines the following function object classes \ccStyle{plus<T>},
|
|
\ccStyle{minus<T>}, \ccStyle{times<T>}, \ccStyle{divides<T>}, and
|
|
\ccStyle{modulus<T>}, which have an operator() with two arguments.
|
|
Furthermore, there is the function object class \ccStyle{negate<T>} with
|
|
a single argument. The arguments as well as the return value are of
|
|
type \ccStyle{T}.
|
|
|
|
Pars pro toto we give the more formal definition for tha class \ccc{plus<T>}.
|
|
|
|
\begin{ccClassTemplate}{plus<T>}
|
|
|
|
\ccDefinition
|
|
An object of the class \ccClassName\ is a function object that allows
|
|
to add two objects of type \ccc{T}.
|
|
|
|
\ccCreationVariable{add}
|
|
|
|
\ccOperations
|
|
|
|
\ccMethod{T operator()(const T& t1, const T& t2) const;}
|
|
{returns \ccc{t1 + t2}.
|
|
\ccPrecond `+' must be defined for type \ccc{T}.}
|
|
\end{ccClassTemplate}
|
|
|
|
|
|
\ccExample
|
|
|
|
The following example shows how the function object \ccc{negate<T>}
|
|
is applied on each element of an array.
|
|
|
|
\begin{cprog}
|
|
{
|
|
const int n = 10;
|
|
int A[n];
|
|
A[0] = 23;
|
|
...
|
|
A[9] = 56;
|
|
for_each(A, A+n, negate<int>());
|
|
}
|
|
\end{cprog}
|
|
|
|
\newpage
|
|
|
|
\section{Comparisons}
|
|
|
|
\ccInclude{functional}
|
|
|
|
\stl\ defines the following function object classes \ccStyle{equal_to<T>},
|
|
\ccStyle{not_equal_to<T>}, \ccStyle{greater<T>}, \ccStyle{less<T>},
|
|
\ccStyle{greater_equal<T>}, \ccStyle{less_equal<T>}. They all have an
|
|
operator() with two arguments of type \ccStyle{T} and the return value
|
|
is of type \ccStyle{bool}.
|
|
|
|
\begin{ccClassTemplate}{greater<T>}
|
|
|
|
\ccDefinition
|
|
An object of the class \ccClassName\ is a function object that allows
|
|
to compare two objects of type \ccc{T}.
|
|
|
|
\ccCreationVariable{g}
|
|
|
|
\ccOperations
|
|
|
|
\ccMethod{T operator()(const T& t1, const T& t2) const;}
|
|
{returns \ccc{t1 > t2}.
|
|
\ccPrecond `$>$' must be defined for type \ccc{T}.}
|
|
\end{ccClassTemplate}
|
|
|
|
|
|
\ccExample
|
|
|
|
A {\em set} is a container that stores objects in a linear
|
|
order. Instead of having global {\em compare} functions for a type we
|
|
pass a function object as template argument. Set \ccStyle{S} stores
|
|
integers in a decreasing order. The first template argument is the
|
|
type of the data in the set, the second template argument is the type
|
|
of the comparison function object class.
|
|
|
|
|
|
\begin{cprog}
|
|
{
|
|
set< int, greater<int> > S;
|
|
}
|
|
\end{cprog}
|
|
|
|
|
|
The following code fragment shows how to sort an array using the
|
|
\stl\ function \ccc{sort}.
|
|
|
|
\begin{cprog}
|
|
{
|
|
const int n = 10;
|
|
int A[n];
|
|
A[0] = 23;
|
|
...
|
|
A[9] = 56;
|
|
sort(A, A+n, greater<int>());
|
|
}
|
|
\end{cprog}
|