Mixins, variadic templates, and CRTP in C++
- by Eitan
Here's the scenario: I'd like to have a host class that can have a variable number of mixins (not too hard with variadic templates--see for example http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.103.144). However, I'd also like the mixins to be parameterized by the host class, so that they can refer to its public types (using the CRTP idiom).
The problem arises when trying to mix the too--the correct syntax is unclear to me.
For example, the following code fails to compile with g++ 4.4.1:
template <template<class> class... Mixins>
class Host : public Mixins<Host<Mixins>>... {
public:
template <class... Args>
Host(Args&&... args) : Mixins<Host>(std::forward<Args>(args))... {}
};
template <class Host> struct Mix1 {};
template <class Host> struct Mix2 {};
typedef Host<Mix1, Mix2> TopHost;
TopHost *th = new TopHost(Mix1<TopHost>(), Mix2<TopHost>());
With the error:
tst.cpp: In constructor ‘Host<Mixins>::Host(Args&& ...) [with Args = Mix1<Host<Mix1, Mix2> >, Mix2<Host<Mix1, Mix2> >, Mixins = Mix1, Mix2]’:
tst.cpp:33: instantiated from here
tst.cpp:18: error: type ‘Mix1<Host<Mix1, Mix2> >’ is not a direct base of ‘Host<Mix1, Mix2>’
tst.cpp:18: error: type ‘Mix2<Host<Mix1, Mix2> >’ is not a direct base of ‘Host<Mix1, Mix2>’
Does anyone have successful experience mixing variadic templates with CRTP?
