Multiple inheritance with variadic templates: how to call a function for each base class?
I have a diamond inheritance scheme where the last child must inherit from many different parents.
A
/|\
/ | \
B C ...
| | |
* *
D E
Now imagine what I have class D : public B
, class E : public B, public C
etc. From D
I want to call the same function of all my parents, which I am guaranteed to exist due to inheritance. My thought was that I could wrap this in some kind of variational pattern.
I currently 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 clearly doesn't 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();}
};
I hope I can use it like this:
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 result (order doesn't matter):
B
C
B // <-- order of these two does not matter
C // <--
Am I trying to achieve something?
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One way to repeat variable 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 a folding expression:
void doThings() override {
(static_cast<void>(Args::doThings()), ...);
}
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