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

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.

CodePudding user response：

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>;

Clang has not supported class template argument deduction for aggregates yet.