Partial specialization C++

Question

Good day!

I am trying to build a template class with its several partial specializations as follows:

#include <iostream>

struct MainT {};

struct Foo {};

template <int dim>
struct Tensor {};

template <int dim>
struct Bar {
    using Sub = Tensor<dim>;
};

template<typename T, typename R>
struct A {
    A() { std::cout << "A<T,R>" << std::endl; }
};

template<typename T>
struct A<T, Foo> {
    A() { std::cout << "A<T,Foo>" << std::endl; }
};

template<typename T, int dim>
struct A<T, Bar<dim>> {
    A() { std::cout << "A<T,Bar<dim>>" << std::endl; }
};

template<typename T, int dim>
struct A<T, typename Bar<dim>::Sub> {
    A() { std::cout << "A<T,Bar<dim>::Sub>" << std::endl; }
    Tensor<dim> t;
};

int main() {
    A<MainT,double> a1; // OK
    A<MainT,Foo> a2; // OK
    A<MainT,Bar<2>> a3; // OK
    A<MainT,Bar<2>::Sub> a4; //  error: template parameters not deducible in partial specialization, note: 'dim'

    return 0;
}

The 4th specialization fails to compile. I can not get why it does so. Could you please give a hint on where to look at to understand why it happens and possible solutions?

Short answer: `Bar::Sub` is in so-called non-deduced context. — SergeyA, May 03 '21 at 16:50

463035818_is_not_an_ai · Accepted Answer · 2021-05-03T16:58:00.123

4

There is no 1:1 relation between Bar<dim>::Sub and dim as there is between Bar<dim> and dim.

Consider you are a compiler.

I tell you Bar<2>, then you know "ah ok, they want dim=2". That is: Given Bar<2> you can deduce dim=2.

I tell you Tensor<42>, then you cannot possibly know that I actually meant the Tensor<42> that is aliased as Bar<42>::Sub. It could be that Bar<24>::Sub, via a specialization, is the exact same type.

More formally, as SergeyA mentions in a comment, Bar<dim>::Sub is a so-called non-deduced context. For details I refer you to What is a nondeduced context?.

edited May 03 '21 at 16:58

answered May 03 '21 at 16:50

463035818_is_not_an_ai

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This answer would benefit from a more formal explanation. – SergeyA May 03 '21 at 16:52
@SergeyA indeed, I guess you dont mind if I steal from your comment ;) – 463035818_is_not_an_ai May 03 '21 at 16:52
sometimes I like to see how far one can get by handwaving. This answer takes it a bit far, but at least it links to something less sloppy ;) – 463035818_is_not_an_ai May 03 '21 at 16:55
@largest_prime_is_463035818 thank you for the explanation, quite clear in fact, and thanks for the link, it clarified the topic nicely :) – Vladimir Ivannikov May 03 '21 at 17:32
@largest_prime_is_463035818 by all means, still away :) – SergeyA May 03 '21 at 17:43

Partial specialization C++

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