C++ for Scientists - Technische Universität Dresden

142 CHAPTER 5. META-PROGRAMMING
high-quality interface to BLAS, LAPACK and other libraries with similarly old-fashioned 6 interfaces.
Conversely, as long as we only use our own functions and classes we can avoid every
const cast. 7
We could implement a second view class for constant matrices and overload the trans function
to return this view:
template
class const transposed view
{
public:
typedef typename mtl::Collection::value type value type;
typedef typename mtl::Collection::size type size type;
};
const transposed view(const Matrix& A) : ref(A) {}
const value type& operator()(size type r, size type c) const { return ref(c, r); }
//private:
const Matrix& ref;
template
const transposed view inline trans(const Matrix& A)
{
return const transposed view(A);
}
This works fine and the user could use the trans function for both constant and mutable matrices.
However, a complete new class definition is a fair amount of work where just one piece of the
class definition needs to be altered. For this purpose we introduce two meta-functions.
Check for Constancy
Our problem with the view in Listing 5.1 is that it cannot handle constant types as template
argument. To modify the behavior for constant arguments we first need to find out whether
an argument is constant. The meta-function that provides this informantion is very simple to
implement by partial template specialization:
template
struct is const
{
static const bool value= false;
};
template
struct is const
{
static const bool value= true;
};
6 To phrase it diplomatically.
7 We disagree with Sutter and Alexandrescu on the other exception for using const cast [SA05, page 179],
this can be handled easily with an extra function. 8