C++ for Scientists - Technische Universität Dresden

5.2. PROVIDING TYPE INFORMATION 149
4. The function overload has no return type;
5. Is therefore ignored.
For short, the overload is only enabled if the argument is a matrix — as the names of the
meta-functions say. Likewise the second overload is only available for vectors. A short test
demonstrates this:
mtl::dense2D A(3, 3);
A= 2, 3, 4,
5, 6, 7,
8, 9, 10;
mtl::dense vector v(3);
v= 3, 4, 5;
std::cout ≪ ”one norm(A) is ” ≪ tst::one norm(A) ≪ ”\n”;
std::cout ≪ ”one norm(v) is ” ≪ tst::one norm(v) ≪ ”\n”;
For types that are neither matrix or vector it will look as there is no function one norm at all.
Types that are considered both matrix and vector would cause an ambiguity.
Draw-backs: The mechanism of enable if is very powerful but not particularly pleasant to
debug. Error messages caused by enable if are usually rather long but not very meaningful. If
a function match is missing for a given argument type, it is hard to determine why because no
helpful information is provided to the programmer, he/she is only told that no match is found,
period. The enabling mechanism can not select the most specific condition. For instance,
we cannot specialize implementation for say is sparse matrix. This can be achieved by avoid
ambiguities in the conditions:
template
typename boost::enable if c::type
inline one norm(const T& A);
template
typename boost::enable if::type
inline one norm(const T& A);
Evidently, this will become quite confusing if too many hierarchical conditions are considered.
The SFINAE paradigm only applies to template arguments of the function itself. Therefore,
member functions cannot be enabled depending on the class’ template argument. For instance,
the mutable access operator in line 9 of Listing 5.1 cannot be hidden with enable if for views on
constant matrices because the operator itself is not a template function. There are possibilities
to introduce a template argument artificially for a member function to enable enable if but this
really does not contribute to the clarity of the program.
Concepts can handle hierarchies in conditions, non-template member functions and provide also
more helpful error messages. Unfortunately, they will not be available in C ++0x and it is not
clear yet when they will usable for mainstream programming.