Templated Barton and Nackman Trick Problems

Question

I'm trying to implement class using the Barton and Nackman trick to avoid dynamic dispatch. (I'm writing MCMC code where performance matters.) I'm not a C++ expert but the basic trick is working for me elsewhere. However I now have a case where the second derived class needs to be templated. This seems to cause problems. The outline of my code is:

// Generic step class
template<class DerivedStepType>
class Step {
public:
  DerivedStepType& as_derived() {
    return static_cast<DerivedStepType&>(*this);
  }

  void DoStep() {
    return as_derived.DoStep();
  }
};

// Gibbs step
template<class DerivedParameterType> // THIS IS THE PROBLEM
class GibbsStep : public Step<GibbsStep> {
public:
  GibbsStep(DerivedParameterType new_parameter) {
  }
  void DoStep() {
  }
};

The problem is template<class DerivedParameterType> and the following <GibbsStep> (from the Barton and Nackman trick). Using g++ v 4.01 (OSX), I get the following error:

./src/mcmc.h:247: error: type/value mismatch at argument 1
 in template parameter list for 'template<class DerivedStepType> class Step'
./src/mcmc.h:247: error:   expected a type, got 'GibbsStep'

Everything compiles fine if a drop template<class DerivedParameterType> and replace DerivedParameterType with, say, double.

Any ideas?

Answer 1

GibbsStep is a template class and requires instantiation while you are using in public Step<GibbsStep> . You should change it to public Step<GibbsStep<DerivedParameterType> >

Answer 2

Naveen is right, but what you show here is not a Barton-Nackman trick. This is CRTP (Curiously Recurring Template Pattern). You can read about it here:

http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Curiously_Recurring_Template_Pattern

Burton-Nackman trick deals with restricted template expansion and was superseded by template specialization semantics. You can read more about it here:

http://en.wikipedia.org/wiki/Barton-Nackman_trick

With Kind Regards, Ovanes