C++ - Why does aggregate initialization not work with template struct

Question

This code works, without having to specify a constructor:

struct Foo
{
    int a;
    int b;
};

//...

    int a1, b1;

    Foo foo = {a1, b1};

If I make Foo a template, it doesn't work.

template<typename T1, typename T2>
struct Foo
{
    T1 a;
    T2 b;
};

//...

    int a1, b1;

    Foo foo = {a1, b1};

It says deduction failed / 2 arguments were provided while 1 expected. If I add a constructor like Foo(T1, T2){} then it works. I thought, that sort of construction just works by default for structs. What am I getting wrong?

EDIT: I'm using Clang, which seems not to support it. Both MSVC and GCC compile it with c++20 compiler flag.

It works when you explicitly specify `Foo foo = {a1, b1};`. https://godbolt.org/z/Eqcc7Pefn — kiner_shah, Jan 19 '22 at 07:21
Does this answer your question? [initializer\_list and template type deduction](https://stackoverflow.com/questions/12431495/initializer-list-and-template-type-deduction) — Ari0nhh, Jan 19 '22 at 07:24
This should make compilation possible `Foo foo = {{a1, b1}};`. — 273K, Jan 19 '22 at 07:25
Your compiler probably doesn't implement the whole of C++20. An aggregate deduction guide should be synthesised. — StoryTeller - Unslander Monica, Jan 19 '22 at 07:26

score 10 · Accepted Answer · edited Aug 07 '23 at 15:31

10

Since C++20 aggregates have implicitly-generated deduction guide so class template argument deduction works for aggregates too.

int a1, b1;
Foo foo = {a1, b1}; // works since C++20; T1 and T2 are deduced as int

Before C++20, you need to add user-defined deduction guide, e.g.

template<typename T1, typename T2>
struct Foo
{
    T1 a;
    T2 b;
};

template<class T1, class T2> Foo(T1 a, T2 b) -> Foo<T1, T2>;

As of Aug 2023, Clang has not supported class template argument deduction for aggregates yet, but the most recent Clang trunk does support it and, apparently, this will be supported in Clang 17.

edited Aug 07 '23 at 15:31

lisyarus

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answered Jan 19 '22 at 07:27

songyuanyao

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I see. it works with GCC on godbolt, it doesn't work with Clang which is what I'm using. – Newline Jan 19 '22 at 07:31
@Newline Yes, [Clang](https://clang.llvm.org/cxx_status.html#cxx20) hasn't supported that. You cand add a user-defined deduction guide. – songyuanyao Jan 19 '22 at 07:35
I've tweaked the final remark slightly - the feature was [commited](https://reviews.llvm.org/D139837) in December 2022, and will probably be in Clang 17. – lisyarus Aug 07 '23 at 15:32

C++ - Why does aggregate initialization not work with template struct

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