c++11 - using c++ aggregate initialization in std::make_shared

Question

Per my understanding, the following code constructs an object of type Foo and then moves that object into the memory allocated by std::make_shared

struct Foo
{
    std::string s;
    int i;
    char c;
};

int main(int argc, char* argv[])
{
    auto foo = std::make_shared<Foo>(Foo{"hello", 5, 'c' });
}

Is it possible to aggregate initialize Foo directly into the memory allocated by std::make_shared?

Answer 1

You could create an adapter with a variadic constructor template to forward the arguments, something like:

template<class T>
struct aggregate_adapter : public T {
    template<class... Args>
    aggregate_adapter(Args&&... args) : T{ std::forward<Args>(args)... } {}
};

And then you can do:

auto foo = std::make_shared<aggregate_adapter<Foo>>("hello", 5, 'c');

Since aggregate_adapter<Foo> and Foo are related, foo is convertible to std::shared_ptr<Foo> as well.

Caveats

---

Unfortunately, the use of forwarding also makes it impossible to brace-init any of the members like std::make_shared<aggregate_adapter<Foo>>({'h','e','l','l','o'}, 5, 'c'); without specifying the type explicitly, but the same restriction applies to make_shared already.