C++ for Scientists - Technische Universität Dresden

4.6. TEMPLATE SPECIALIZATION 101
an appropriate technique to achieve polymorphism.
4.6 Template Specialization
Although one of the advantages of a generic implementation is that the same code can be used
for all objects that satisfy the corresponding concept, this is not always the best approach.
Sometimes the same behavior can be implemented more efficiently for a specific type. In
principle, one can even implement a different behaviour for a specific type but this is not
advisable in general because the program becomes much more complicated to understand and
using the specialized classes can require a whole chain of further specialization (bearing the
danger of errors when imcompletely realized). C ++ provides an enormous flexibility and the
programmer is in charge to use this flexibility responsibly and for being consistent to himself.
4.6.1 Specializing a Class for One Type
In the following, we want to specialize our vector example from page 96 for bool. Our goal is
to save memory by packing 8 bools into one byte. Let us start with the class definition:
template
class vector
{
// ..
};
Although our specialized class is not type-parametric, we still need the template key word and
the empty triangle brackets. After the class the complete type list must be given. This syntax
looks a bit cumbersome in this context but makes more sense for multiple template arguments
where only some are specialized. For instance, if we had some container with 3 arguments and
specialize the second one:
template
class some container
{
// ..
};
Back to our boolean vector class. In the class we define a default constructor for empty vectors,
a constructor for vectors of size n and a destructor. In the size of the array, we have to pay
some attention if the vector size is not disible by 8 because the integer division simply cuts off
the remainder.
template
class vector
{
public:
explicit vector(int size)
: my size(size), data(new unsigned char[(my size + 7) / 8] )
{}
vector() : my size(0), data(0) {}