Call templated constructor from non-templated constructor

Question

Let's say I have a class similar to:

template <typename T>
class MyClass
{
public:
    template <typename U>
    MyClass(const MyClass<U>&); // Not a copy constructor, as template.

    // How to delegate to above constructor?
    MyClass(const MyClass& a) : MyClass<T>(a) {}

    // ...
};

How to delegate the copy constructors to a (conversion) template constructor?

The code above results in a delegation cycle.

Normally, template argument deduction should be sufficient. The example is unclear because MyClass appears to inconsistently be a class or a class template. — François Andrieux
– François Andrieux, Commented Sep 27, 2022 at 22:20
Seems pretty clear to me. MyClass is a template and has a template argument T. It has a converting constructor template <typename U> MyClass(const MyClass<U>&); and OP wants to delegate to that from copy constructor. No idea if this is possible tho. — Yksisarvinen
– Yksisarvinen, Commented Sep 27, 2022 at 22:24
@FrançoisAndrieux MyClass is a class template with T being a template argument. — BlueCannonBall
– BlueCannonBall, Commented Sep 27, 2022 at 22:34
@BlueCannonBall Then update your question to make it clear. Provide a minimal example that compile except for the problematic constructor. I think that the easier workaround would be to have a "kind" of copy constructor with an extra argument. — Phil1970
– Phil1970, Commented Sep 28, 2022 at 0:08

Jarod42 · Accepted Answer · 2022-09-29 11:31:21Z

1

If the guess from @Yksisarvinen is correct then something like that should do:

template <class T> class MyClass
{
public:
    MyClass(const MyClass& a) : MyClass(a, true) {}

    template <class U>
    MyClass(const MyClass<U>& a) : MyClass(a, false) {}

private:
    template <class U>
    MyClass(const MyClass<U>& a, bool sameClass) 
    { 
        /* real code here */ 
    }
};

Alternatively, you can remove second copy constructor and put a default value for sameClass if you prefer (and make that constructor public).

edited Sep 29, 2022 at 11:31

Jarod42

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answered Sep 28, 2022 at 0:16

Phil1970

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4 Comments

Jarod42 Over a year ago

I would use a dummy tag instead of bool. std::is_same_v<T, U> has the information anyway, if required.

BlueCannonBall Over a year ago

@Jarod42 What do you mean by dummy tag?

Phil1970 Over a year ago

@Jarod42 Not a bad idea to use a dummy tag... I just made as simple as possible.

Jarod42 Over a year ago

@BlueCannonBall: I mean using some struct dummy_tag{}; and then Demo. bool sameClass has a meaning (which might be used wrongly).

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