Question

I’ve been using the Curiously recurring template pattern The general code looks like this:

template <typename T> void genericFunction(T &);
template <typename T> struct Functionality {
    void genericMethod() {
        genericFunction(*((T *)this)) ;
    }
};

struct Klass : public Functionality<Klass> {};

void main() {
    Klass obj ;
    obj.genericMethod();
}

template <> void genericFunction<Klass>(Klass &obj) {
    //do stuff with Klass &obj here
}

I ran into an error today which cost me about 90 minutes of hair-pulling fustration, this error was caused by using an incorrect template parameter for my base class inheritance declaration, somewhat like so:

struct Klass : public Functionality<SomeOtherKlass> {}; //SomeOtherKlass wrong!!!

I’d like to enhance my code so that this mismatch between the derived class and the base class template parameter is detected (runtime, compile time, anytime 🙂 ), is this even possible?, thanks.

Answer 1

You could assert the relation in e.g. genericMethod() using Boost or C++11 features:

BOOST_STATIC_ASSERT(( boost::is_base_of<Functionality<T>, T>::value ));

… although that is assuming that the other class doesn’t derive from Functionality<T> as well.

An alternative could be to assert the relation at runtime in test-builds:

template <typename T> struct Functionality {
#ifdef TEST_BUILD
    virtual ~Functionality() {}
#endif
    void genericMethod() {
#ifdef TEST_BUILD
        assert(dynamic_cast<T*>(this));
#endif
        genericFunction(*((T *)this)) ;
    }
};

Note that the test won’t work inside constructors and destructors