C++ for Scientists - Technische Universität Dresden

144 CHAPTER 5. META-PROGRAMMING
The Solution
Now we have all we need to revise the view from Listing 5.1. The problem was that we returned
an entry of a constant matrix as mutable reference. To avoid this we can try to make the
mutable access operator disappear in the view when the referred matrix is constant. This is
possible but too complicated for the momemt. We will come back to this in Section 5.2.4.
An easier solution is to keep both the mutable and the constant access operator but choose the
return type of the former depending on the type of the template argument:
1 template
2 class transposed view
3 {
4 public:
5 typedef typename mtl::Collection::value type value type;
6 typedef typename mtl::Collection::size type size type;
7 private:
8 typedef typename if ::type vref type;
12 public:
13 transposed view(Matrix& A) : ref(A) {}
14
15 vref type operator()(size type r, size type c) { return ref(c, r); }
16 const value type& operator()(size type r, size type c) const { return ref(c, r); }
17
18 private:
19 Matrix& ref;
20 };
Listing 5.3: Const-safe view implementation
This implementation returns a constant reference in line 15 when the referred matrix is constant
and a mutable referrence for mutable referred matrix. Let us see if this is what we need. For
mutable matrix references, the return type of operator() depends on the constancy of the view
object:
• If the view object is mutable (line 15) then operator() returns a mutable reference (line 10);
and
• If the view object is constant (line 16) then operator() returns a constant reference.
This is the same behavior as in Listing 5.1.
If the matrix reference is constant, then a constant reference is always returned:
• If the view object is mutable (line 15) then operator() returns a mutable reference (line 9);
and
• If the view object is constant (line 16) then operator() returns a constant reference.
Altogether, we implemented a view object that provides read and write access whereever appropriate
and disables it where inappropriate.