How to avoid big memory allocation with std::make_shared

Question

Let\'s say that I have some arbitrary class, A:

class A {
 //... stuff
};

I want to call into an external API that takes in a shared pointe

Answer 1

Assuming class A supports move semantics, do this:

std::shared_ptr newA = make_shared (std::move (_a));

Do not use _a anymore, use only newA. You can now pass newA to the function.

If class A does not support move semantics, there is no safe/sane way to do this. Any hack will only happen to work, and may break in the future. If you control enough of the class code, you may be able to add support for move semantics.

But from what I can tell, a new instance of A is still created.

Why do you care? What you're trying to avoid is copying all the data in the instance, and this does that.

The point of move semantics is to move the data from one instance to another without having to do an allocate/copy/free. Of course, this makes the original instance "empty", so don't use that anymore.