CODEWITHSUNDEEP
c++templates
Andy Brown
Andy Brown

Reputation: 12999

How to declare a self referencing template type

I've a situation that is like this contrived example:

template<class TFeature> struct Controller {};

template<class TController,typename T> struct Feature {
  typedef Feature<TController,T> FeatureType;
};

typedef Controller<Feature::FeatureType,int> DefaultController;

The Controller is templated to accept features and my problem is that some of the features need the type of the controller as a template parameter. This makes the typedef on the last line of the sample not compile.

Is this possible or do I need to rethink the design?

Upvotes: 5

Views: 1321

Answers (4)

BigBoss
BigBoss

Reputation: 6914

In order to accomplish this you should do some meta programming magic(and believe me it is not an easy task). But if you really nead it and using boost is an option for you take a look at boost::mpl and you can have something like this:

template< class ControlerT >
struct FeatureEx {
    typedef ControlerT controler_type;
};
template< class FeatureT >
struct ControlerEx {
    typedef ControlerEx<FeatureT> this_type;
    typedef typename boost::mpl::apply<
        FeatureT, boost::mpl::identity<this_type>
    >::type feature_type;

    feature_type const& get_feature() const {return f_;}

private:
    feature_type f_;
};

typedef ControlerEx<FeatureEx<boost::mpl::placeholders::_1> > DefaultControler;

Upvotes: 2

SomeWittyUsername
SomeWittyUsername

Reputation: 18368

Seems that you're trying to pass 2 params to you Controller template, while it can accept only one.

Upvotes: 0

Rontogiannis Aristofanis
Rontogiannis Aristofanis

Reputation: 9063

You are passing to the Controller class two template parameters, but you have declared it to take only one. Do you need something like the following?

typedef Controller<Feature<Controller<int>,int>::FeatureType> DefaultController;

Upvotes: 1

bdonlan
bdonlan

Reputation: 231273

One option would be to use a dummy subclass instead of a typedef:

struct DefaultController : public Controller<Feature<DefaultController,int>::FeatureType> {};

Upvotes: 0

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