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

user_10248920 0
Viewed 148

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.

ctad c++20 initialization templates c++
1 Answers

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.

user_33097900
Related