Partial emplate Argument Deducion C++

Question

I have the following code:

template <class B, class A>
class C {

    A m_a;

public:

    explicit C(A a) : m_a(a) {}

};

int main() {
    C<int>(16);
    return 0;
}

that can not be compiled. My purpose is to automatically deduce class A using the constructor parameter but use manually mentioned class B. Is it possible?

There is no partial CTAD in C++. You either specify none of the arguments or all of them. — NathanOliver, Aug 22 '22 at 13:47
@RoQuOTriX this is an example, B is used but can not be deduced from the constructor — Кирилл Волков, Aug 22 '22 at 13:49
Refer to [how to ask](https://stackoverflow.com/help/how-to-ask) and you'll find plenty of related SO posts. One example is : [C++17 Partial Deduction Guide](https://stackoverflow.com/questions/56351144/c17-partial-deduction-guide) — Jason, Aug 22 '22 at 13:54

score 3 · Accepted Answer · answered Aug 22 '22 at 13:50

3

Sure, but only with a helper function.

As currently written, C<int> has to be the complete name of a type, which it obviously isn't. The constructor argument isn't considered until after the type of the object is determined.

So write

template <class B, class A>
C<B,A> make_c(A a) { return C<B,A>{a}; }

int main() {
    auto c = make_c<int>(16);

and now you have a function template whose second type parameter can be deduced as you want, from the argument, before you have to write out the type it returns.

answered Aug 22 '22 at 13:50

Useless

64,155
6
88
132

So the difference is that partial arguments deduction is ok for usual function and not for the constructor? – Кирилл Волков Aug 22 '22 at 13:52
1

You can't invoke the constructor until you know the type you're constructing, so the type you're constructing can't depend on the the constructor. It's a circular dependency. Having a separate function just breaks this loop. – Useless Aug 22 '22 at 13:54

Partial emplate Argument Deducion C++

1 Answers1