Using template base class template constructor in derived class

Question

i know that you can use constructors from a base class in a derived class like

class A {
public:
    A() {};
}

class B : public A {
public:
    using A::A;
}

Furthermore you can use a constructor from a template base class like

template<typename T>
class A {
public:
    A() {};
}

template<typename T>
class B : public A<T> {
public:
    using A<T>::A;
}

Suppose that class A now has a template function as constructor:

template<typename T1>
class A {
public:
    template<typename T2>
    A() {};
}

template<typename T1>
class B : public A<T1> {
public:
    using A<T1>::A;                           // nope
    using A<T1>::A<>;                         // neither             
    template<typename T2> using A<T1>::A<T2>; // sounds good, doesn't work

}

How could you use the base class constructor in the derived class?

How would you call that constructor?

Evg
– Evg

2021-07-13 19:36:36 +00:00
Commented Jul 13, 2021 at 19:36 — Evg
– Evg, Commented Jul 13, 2021 at 19:36

HolyBlackCat · Accepted Answer · 2021-07-13 19:36:53Z

2

Two things:

template<typename T2> A() {} is unusuable as a constructor, since there is no way to deduce T2.
You can only inherit all constructors at once. You can't choose specific ones.

Other than that, using A<T1>::A; is correct.

answered Jul 13, 2021 at 19:36

HolyBlackCat

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2 Comments

syntaxerror147 Over a year ago

You're totally right, i've edited the signature of the constructor in the last code snippet so that T2 can be deduced. Well, that's easier than i thought it was. Certainly there was another problem with my more complex code, tested your answer with this minimal sample and it works well! Thanks :)

HolyBlackCat Over a year ago

@syntaxerror147 You're welcome! Usually it's discouraged to make significant changes to the question after getting an answer, to avoid confusing future readers. So I've rolled back the question edit, I hope you don't mind.

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