C++ for Scientists - Technische Universität Dresden

6.5. BARTON-NACKMAN TRICK 193
6.4.2 Const Cast
const cast adds or removes the attributes const and/or volatile. The key word volatile informs the
compiler that a variable can be modified by other programs. It is therefore not hold or cached
in registers and accessed each time memory. This feature is not used in this script. Adding
an attribute is an implicit conversion in C ++. That is one can always assign an expression
to an variable of the same type with extra attributes without the need for a cast. Removing
an attribute requires a const cast and should be only done when unavoidable, e.g. to interface
old-style software that is lacking appropriate const attributes.
6.4.3 Reinterpretation Cast
This is the most aggressive form of casting and not used in this script. It takes an address or an
object’s memory location and interprets the bits there as it was of the target type. One can for
instance change a single bit in a floating point number by casting it to a bit chain. It is more
important for programming hardware drivers than complex flux solvers. Needless to say that
reinterpret cast is one of the most efficient ways to undermine the portability of an application.
6.5 Barton-Nackman Trick
This section describes the ‘Curiously Recurring Template Pattern’ (CRTP). It was introduced
by John Barton and Lee Nackman [?] and is therefore also referred to as the ‘Barton-
Nackman Trick’.
6.5.1 A Simple Example
⇒ crtp simple example.cpp
We will explain this with a simple example. Assume we have a class point with an equality
operator:
class point
{
public:
point(int x, int y) : x(x), y(y) {}
bool operator==(const point& that) const { return x == that.x && y == that.y; }
private:
int x, y;
};
We can program the unequality by using common sense or by applying de Morgan’s law:
bool operator!=(const point& that) const { return x != that.x || y != that.y; }
Or we can simplify our live and just negate the result of the equality:
bool operator!=(const point& that) const { return !(∗this == that); }