Deducing type of template value parameter

Question

I was wondering if the type of a value template parameter could be deduced or obmitted when writing something similar to this:

enum class MyEnum {A, B}

enum class MyOtherEnum {X, Y}

template <typename T, T value>
struct GenericStruct {};

When using MyGenericStruct both T and value have to be passed, but T would be deducible from context typename T = decltype(value) except that value is not yet defined. template <auto value> isn't working either.

Is there any way to simply write MyGenericStruct<MyEnum::A> instead of MyGenericStruct<MyEnum, MyEnum::A> without usage of macros?

"`template ` isn't working either." why not? – 463035818_is_not_an_ai Sep 13 '22 at 14:47 — 463035818_is_not_an_ai, Sep 13 '22 at 14:47
apperently because I had c++11 active. works with c++17 :) – Myon Sep 13 '22 at 19:40 — Myon, Sep 13 '22 at 19:40

NathanOliver · Accepted Answer · 2022-09-13T14:49:37.397

7

If you can use C++17, you can use auto as the type of the non-type template parameter. That gives you

template <auto value>
struct GenericStruct {};

Now, value will have its type deduced by its initializer just like if you had declared a variable with type auto and gives you the desired syntax of GenericStruct<MyEnum::A> foo; as seen in this live example.

edited Sep 13 '22 at 14:49

answered Sep 13 '22 at 14:35

NathanOliver

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Deducing type of template value parameter

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