CODEWITHSUNDEEP
c++templates
Clifford
Clifford

Reputation: 93476

Creating an alias for an instantiated template

I originally had a number of signal processing filter classes that were identical apart from a few constants defining the filter characteristic, so I decided to change these to a template class for maintainability and extensibility. There are performance and memory management reasons for preferring a template over constructor arguments in this case; it is for an embedded system.

Consequently I have a template class of the form:

template <int SIZE, int SCALE_MULTIPLIER, int SCALE_SHIFT>
class cBoxcarFilter
{
public:
    // Allow access to SIZE at runtime.
    static const int FILTER_SIZE = SIZE ;
    ...
}

Which I explicitly instantiate thus, for example:

template class cBoxcarFilter<8, 1, 3>

The problem is when I need to access the FILTER_SIZE member it requires:

cBoxcarFilter<8, 1, 3>::FILTER_SIZE

which rather makes access to FILTER_SIZE redundant since it must be repeated in the arguments. My solution to this problem is this:

// Create an alias for filter
#define cSpecialistBoxcarFilter cBoxcarFilter<8, 1, 3>
template class cSpecialistBoxcarFilter ;

then I can access FILTER_SIZE thus:

cSpecialistBoxcarFilter::FILTER_SIZE

this also has the advantage of meaningful unique names for each filter instance as in the original non-templated versions, but it seems somewhat smelly to me using a macro that looks like a class since it has different scope semantics.

Is there a better way of creating alias class names for a template instance?

Upvotes: 1

Views: 586

Answers (2)

Alex Brown
Alex Brown

Reputation: 42872

You can ask concrete instances (objects) of your class for their filter size:

template <int X> class A
{
  public:
  int y;
  static const int size = X;
};

int main(int, char**)
{
  A<3> a;
  printf("size is %i\n", a.size);
}

result

size is 3

Upvotes: 0

Oliver Charlesworth
Oliver Charlesworth

Reputation: 272507

Yes, typedef!

typedef cBoxcarFilter<8, 1, 3> cSpecialistBoxcarFilter;

Upvotes: 10

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