Nico Brailovsky's thought repository

For some reason I couldn't find many examples of how to use a parameter pack as a mixin, to enable different features with no runtime overhead. Here is a full example of you might implement this (be aware there are some nasal daemons in the code below!). The technique is really based on this one line:

 int dummy[sizeof...(Config)] = { (Config::apply(p), 0)... };

This idiom will unpack a parameter pack and call T::apply, for each T in the parameter pack. You can use this idiom to build very clean mixin-type interfaces with static dispatch, or to build job security.

Full example:

struct EnableFeatureA {
  template <typename T> static void apply(T *a) {
    cout << a->a() << endl;
  }
};
struct EnableFeatureB {
  template <typename T> static void apply(T *a) {
    cout << T::b() << endl;
  }
};
template <typename Impl, typename... Config>
struct Foo {
  Foo(){
    // Call apply() for each type in Config
    Impl *p = nullptr;
    int dummy[sizeof...(Config)] = { (Config::apply(p), 0)... };
  }
};
struct Bar;
using FwdFoo = Foo<Bar, EnableFeatureA, EnableFeatureB>;
struct Bar : FwdFoo {
   int a() { return 4; }
   static int b() { return 2; }
};