Clarification on the differences between `Ptr` and `Ref`

[yuyichao]

@kdheepak

Is there any difference between the following code? If so, can someone elaborate on what the difference is?

Yes. The second version is garbage. (i.e. the value you get in C is not usable.)

@kdheepak

Why does the documentation say “ generally be used for types of input arguments”?

It’s talking about the argument type in the ccall, i.e. where you have Ptr{Cvoid} and it mosly apply to when you know the type you’ll use so that you can write ccall(.... (Ref{Int}, ...), 1).

@kdheepak

In what situations would it be appropriate to call pointer_to_objref on an instance of a mutable Julia struct?

Almost never. In the context of ccall the function is there to implement unsafe_convert. Using it would be equivalent to using unsafe_convert. If you don’t have a case where you need unsafe_convert, don’t use pointer_from_objref.