Using variadic templates to emulate multiple inheritance

Question

I'm not entirely sure if this is possible or what syntax I would need to employ if it is, but I want to make a class that publicly derives from all of the classes provided in the variadic arguments so that I could derive from that single class and thereby derive from all the classes that were supplied to it.

What I am wanting to be able do is declare something like:

class MyClass : public InheritFrom<ClassA, ClassB,  ,,,> {
};

So that the InheritFrom template class effectively publicly derives from all of the classes listed. In my use case, each class provided will have a default constructor, so no special constructors will have to be called.

What I had tried to do was this:

template <class A, class ... Args> class InheritFrom : public A, public InheritFrom<Args...> {
...
};

template <class A> class InheritFrom : public A {
...
};

And effectively turn what would otherwise have been a multiple inheritance into a single inheritance heirarchy, but the compiler complains about the second InheritFrom declaration, which was supposed to be a specialization where only one class is given, as being a redeclaration with one parameter. There will ultimately be much more detail within the body of each of these classes, but for now I'm just trying to figure out how to get the syntax right.

super · Accepted Answer · 2022-11-27 17:25:23Z

3

In c++17 you just need to expand the pack, nothing more to it.

#include<iostream>
#include<type_traits>

template <class ...T> class InheritFrom : public T... {
};

struct Foo {};

struct Bar {};

struct Baz : InheritFrom<Foo, Bar> {};

int main(){
    std::cout << std::is_base_of_v<Foo, Baz> << std::is_base_of_v<Bar, Baz>;
}

answered Nov 27, 2022 at 17:25

super

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3 Comments

markt1964 Over a year ago

Apologies, I realize I'd didn't make this clear in my question (because I didn't know it was relevant), but InheritFrom needs to have the name of the first class in the list of classes passed to it. Without explicitly naming it as its own argument, is there any way to extract it from the variadic arguments?

super Over a year ago

@markt1964 Not sure exactly what Without explicitly naming it as its own argument means. You can do template <class First, class ...Rest> class InheritFrom : public First, public Rest... to get the name of the first one.

super Over a year ago

Or maybe you're looking for a recursive approach like the one you attempted so you can do something with each base class? If so, what is that extra thing?

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