Newbie: copy, assignment of class instances

[bearophile]

Larry Luther:

I'm nonplussed. Could you expand on why D class instances don't need to copy their contents and instances of D structs do?  While migrating C++ code to D I've had to convert "struct"s to "class"es because of the need for inheritance.  Why would the need to copy an instance's contents to another instance disappear?<

You are an experienced programmer, so if you think well about this topic you can probably understand the situation and the purposes behind D design as well or better than me. You probably know already what I can tell you about this topic.

D is different from C++, it's GC-based, and this changes many things and the way they have to be designed and used. D is designed to be a little "simpler" than C++, this means in some cases it prefers to do something in a safer way, instead in the most efficient way. D doesn't follow the zero overhead policy of C++, because it can lead to hairy code and because Java and C# have plenty shown it's often a waste of programmers time with no real performance gain.

In D structs don't support inheritance because they are designed to avoid the slicing bug, to be simpler, to be Plain Old Data. So D is designed to have different classes and structs because their usage style and purposes are different. When more complexity is needed, classes are there to be used. Even if currently DMD is not very good at optimizing away the class-derived overhead, better future D compilers can solve this. For some situations there's even the "scope" (that Walter is not so sure to keep, while other people like me have suggested the opposite, that is to extend its purpose to allocate an object inside the memory of another object) that can help performance. In Java the HotSpot is able to perform Escape Analysis on objects to avoid many heap allocations, and LDC is able to detect some simple cases and do the same.

By design D has no standard way to copy classes. I have not written very large object oriented D programs, but from what I have seen, I generally don't need to copy objects. I have often the need to move them around, put them in a collection, pull them out and put them in another collection, or create them in single instance, and so on. But I don't remember the last time I've had to copy a class instance in D. I just copy and add and remove class references. When I want I can keep many references to the same object and so on. In C++ programs this has to be done with care, because there is no GC, or you need some kind of smart pointer, while in D (in theory) you can relax and just let the GC do its thing.

My experience tells me that when you try to translate a program written in language X to language Y you always find some impedance (unless X and Y are almost the same language). But most times in language Y there are ways to solve the problem in a different way. You, as programmer, have to learn the way Y programs are usually written, its idioms and to avoid to force X idioms in Y and then complain that X idioms are not well represented in Y.

D design is far from perfect, and in future some things will probably need to be fixed (I have written enough text to fill two books about D features that can enjoy some change), but behind its design there is also lot of thought. So, you can show us some D code where you think there is a need to copy objects. If such uncommon need arises you can usually write your own copy functions.

Bye,
bearophile