C++ for Scientists - Technische Universität Dresden

196 CHAPTER 6. INHERITANCE
Another disadvantage of the implementation above is that we would need another proxy for the
constant access.
This is an interesting aspect of templates. It does not only enable writing type-parametric software
but can also help to break mutual dependencies thanks to its post-poned code generation.
By templetizing the proxy the dependency is gone:
template
class bracket proxy
{
public:
bracket proxy(Matrix& A, int r) : A(A), r(r) {}
Result& operator[](int c){ return A(r, c); }
private:
Matrix& A;
int r;
};
class matrix
{
// ...
bracket proxy operator[](int r)
{
return bracket proxy(∗this, r);
}
};
With this implementation we can now write A[i][j] and it is realized by the binary operator()
however this is implemented. Such a bracket operator is useful in every matrix class and the
implementation will be always the same.
For this reason we like to have this implementation only once in our code base and reuse
whereever appropriate. The only way to achieve this is with the CRTP paradigm:
template
class bracket proxy
{
public:
bracket proxy(Matrix& A, int r) : A(A), r(r) {}
Result& operator[](int c){ return A(r, c); }
private:
Matrix& A;
int r;
};
template
class crtp matrix
{
public:
bracket proxy operator[](int r)
{
return bracket proxy(static cast(∗this), r);
}