Multiple inheritance with variadic templates: how to call function for each base class?

Question

I have a diamond inheritance scheme, where the last child should be able to inherit from many different parents.

     A
    /|\
   / | \
  B  C  ...
  |  |  |
    * *
    D E

Now imagine I have a class D : public B, class E : public B, public C, etc. From D I want to call the same function of all its parents, which I am guaranteed exists due to the inheritance. My thought was that I could wrap this in some variadic template.

Currently I have this:

template <typename T>
class A
{
public:
    A(T t) : mT(t) {}
    virtual ~A() {}
    virtual void doThings() = 0;
protected:
    T mT;
};

template <typename T, typename A = A<T>>
class B : public A
{
public:
    B(T t) : A(t) {}
    virtual ~B() {}
    virtual void doThings() { std::cout << "B" << std::endl; }
};

template <typename T, typename A = A<T>>
class C : public A
{
public:
    C(T t) : A(t) {}
    virtual ~C() {}
    virtual void doThings() { std::cout << "C" << std::endl; }
};

Now I thought I could do something like this, which obviously does not work:

template <typename T, typename ...Args>
class ChildGenerator : public Args...
{
public:
    ChildGenerator(T t) : Args(t)... {}

    // The unpacking of the variadic template does not work here.
    // Do I need to make it recursive somehow? How can I do that without having to instantiate new classes B and C?
    void doThings() override { Args...::doThings();}
};

My hope is that I can use it like so:

int main()
{
    using B = B<double>;
    using C = C<double>;
    B c1(0.0);
    C c2(1.0);
    ChildGenerator<double, B, C> c3(2.0);
    c1.doThings();
    c2.doThings();
    c3.doThings();
 }

Expected output (order does not matter):

B
C
B // <-- order of these two does not matter
C // <--

Is what I'm trying to achieve possible?

`int dummy[] = { (Args::doThings(), 0)... };` should do the trick — W.F., Apr 10 '17 at 12:18

Jarod42 · Accepted Answer · 2017-08-22T12:41:01.150

15

One way to iterate over the variadic bases:

template <typename T, typename ...Args>
class ChildGenerator : public Args...
{
public:
    ChildGenerator(T t) : Args(t)... {}

    void doThings() override {
        int dummy[] = {0, (Args::doThings(), void(), 0)...};
        static_cast<void>(dummy); // avoid warning for unused variable
    }
};

or in C++17, with folding expression:

    void doThings() override {
        (static_cast<void>(Args::doThings()), ...);
    }

edited Aug 22 '17 at 12:41

answered Apr 10 '17 at 12:18

Jarod42

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This does work like a charm! I'm a little bit lost on how it works however -- could you provide an explanation/resource? – pingul Apr 10 '17 at 12:22
3

You may look for *"iterate over a tuple"*. basically, `...` can be used in some context, and we use one which guaranty order of evaluation. `void()` is to handle possible overload of comma operator. first `0` is to handle empty `Args...`. – Jarod42 Apr 10 '17 at 12:51

n. m. could be an AI · Answer 2 · 2017-04-10T17:17:29.220

3

Use a fold-expression (C++17):

void doThings() override { ((Args::doThings()) , ...);}

Live demo

edited Apr 10 '17 at 17:17

answered Apr 10 '17 at 12:31

n. m. could be an AI

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2

As OP tag C++11, you should state that it is C++17. – Jarod42 Apr 10 '17 at 12:39
@Jarod42 it's C++14 actually. – n. m. could be an AI Apr 10 '17 at 14:22
1

c++17 according to [fold_expression](http://en.cppreference.com/w/cpp/language/fold) (and clang gives me *"warning: pack fold expression is a C++1z extension [-Wc++1z-extensions]"*). – Jarod42 Apr 10 '17 at 15:41
@Jarod42 hmm looks like it's one of those things in n4296 that are not in the official 14 standard. – n. m. could be an AI Apr 10 '17 at 17:17

Multiple inheritance with variadic templates: how to call function for each base class?

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