> In the general case, the hardware requires it; that's how computers work. In rust, it creates the trait object right as it's calling a function that requires

> In the general case, the hardware requires it; that's how computers work. In rust, it creates the trait object right as it's calling a function that requires one. But "creates" is too strong a word -- it's just sending the virtual function table pointer along with the structure pointer and any other arguments, probably in registers. Only if you are actually storing the trait object somewhere does it matter much. In your example, you have to construct the "derived" type each time you come from C to C++. Using an extra type is annoying. I think a better C++ example would be trying to do something closer to what rust does and use the C struct pointer most places, and construct the C++ object right before you need it. Would work, but doesn't come out quite as clean as it does in rust. In any case, the main point is that the normal mode of C++ is to add vtable pointers to the structures to achieve dynamic dispatch, and adding stuff to the beginning of a struct does not interoperate with C code very well. The normal mode of rust is to not touch the struct layout and achieve these features through other means. I'm sure there's a way to work against the grain in C++ and make it work, but if you ask me, rust gets it a little cleaner in this particular case. Or you could argue that working against the grain is the normal mode of C++ ;-).