C++ for Scientists - Technische Universität Dresden

148 CHAPTER 5. META-PROGRAMMING
template
struct enable if
: public enable if c
{};
The real enabling behavior is realized in enable if c whereas enable if is merely a convience function
to avoid type ‘::value’.
Now we have all we need to implement the L1 norm in the generic fashion we aimed for:
1 template
2 typename boost::enable if::type
3 inline one norm(const T& A)
4 {
5 using std::abs;
6 typedef typename Magnitude::type mag type;
7 mag type max(0);
8 for (unsigned c= 0; c < num cols(A); c++) {
9 mag type sum(0);
10 for (unsigned r= 0; r < num cols(A); r++)
11 sum+= abs(A[r][c]);
12 max= max < sum ? sum : max;
13 }
14 return max;
15 }
16
17 template
18 typename boost::enable if::type
19 inline one norm(const T& v)
20 {
21 using std::abs;
22 typedef typename Magnitude::type mag type;
23 mag type sum(0);
24 for (unsigned r= 0; r < size(v); r++)
25 sum+= abs(v[r]);
26 return sum;
27 }
The selection is now driven by enable if in line 2 and 18. Let us look at line 2 in detail for a
matrix argument:
1. is matrix is evaluated to (i.e. inherited from) true ;
2. enable if passes true ::value i.e. true to enable if c;
3. enable if c< >::type is set to typename Magnitude::type;
4. This is the return type of the function overload.
What happens in this line when the argument is not a matrix type:
1. is matrix is evaluated to (i.e. inherited from) false ;
2. enable if passes false ::value i.e. false to enable if c
3. enable if c< >::type is not set in this case;