C++ for Scientists - Technische Universität Dresden

6.5. BARTON-NACKMAN TRICK 195
Likewise every class U with an equality operator can be derived from unequality. A collection
of such CRTP templates for operator defaults is provided by Boost.Operators from Jeremy
Siek and David Abrahams.
Alternatively to the above implementation where the this pointer is dereferred and casted as
reference, one can cast the pointer first and derefer it afterwards:
template
struct unequality
{
bool operator!=(const T& that) const { return !(∗static cast(this) == that); }
};
There is no difference, this is just a question of taste.
6.5.2 A Reusable Access Operator
⇒ matrix crtp example.cpp
We still owe the reader the reusable implementation of the matrix bracket operator promised
in Section 3.7.4. Back then we did not know enough language features.
First of all we had no templates which are indispensable for a proxy. We will show you why.
Say we have a matrix class as in § 3.7.4 and we just want to call the binary operator() from the
unary operator[] via a proxy:
class matrix; // Forward declaration
class simple bracket proxy
{
public:
simple bracket proxy(matrix& A, int r) : A(A), r(r) {}
double& operator[](int c){ return A(r, c); }
private:
matrix& A;
int r;
};
class matrix
{
// ...
double& operator()(int r, int c) { ... }
};
simple bracket proxy operator[](int r)
{
return simple bracket proxy(∗this, r);
}
This does not compile because operator[] from simple bracket proxy calls operator() from matrix but
this is not defined yet. The forward declaration of matrix is not sufficient because we need the
complete definition of matrix not only the assertion that the type exist. Vice versa if we define
matrix first, we would miss the constructor of simple bracket proxy in the operator[] implementation.