On Wednesday, 19 June 2019 at 12:55:09 UTC, Jonathan M Davis wrote:

Even in C++, using const ref is not as good a practice as it once was, because they added move constructors, finally making object moveable. The result is that in many cases, it's actually more efficient to just copy values in C++ rather than use const &, but which is better does depend on the code.

As for D, unless you're dealing with large objects, odds are that worrying about passing by value is pointless. D classes are reference types, and D structs have move semantics built-in. So, you don't get as many copies as you would in C++98, and the situation is probably better than newer versions of C++, since IIRC, C++ classes aren't moveable by default, whereas D structs are. In general, you're probably better off just passing by value unless you find that a particular piece of code is inefficient when benchmarking. Either way, you don't want to be slapping const on everything the way you would in C++, because D's const is far more restrictive. So, while it still can be quite useful, odds are that if you start using it heavily, you're going to run into problems fast - especially since casting away const and mutating an object is undefined behavior in D. D's const has no back doors. If something is const, then you can't mutate it unless you also have a mutable reference to the same data. And because const is transitive, you pretty much can't get mutable stuff from const stuff like you frequently can in C++ (e.g. in C++, it's possible to have a const container of mutable objects, wherein D, once part of something is const, everything within that part is const).

As for the DIP, I'd suggest watching Andrei's recent dconf talk on the subject:


- Jonathan M Davis

I am not talking about cases that would be candidate for moving, or where const would be any problem. If you want an example for the sake of argument:

struct Matrix3D
Matrix3D opBinary(string op)(const ref Matrix3D rhs) const;
        auto a = Matrix3D.random;
        assert(a == a * Matrix3D.identity);
        assert(a == a + Matrix3D.zeros);

I did watch Andrei's talk, actually this is where I started and learned about the DIP(s), then I was confused that 1016 had been rejected, and smelling that it may be "reopening" I was not sure where I can find the "index" of DIPs under discussion or whatever... :)

IIRC, C++ classes aren't moveable by default, whereas D structs are.

What do you mean that structs are movable? I know about RVO (in both D and C++, supposedly guaranteed by all compilers in practice, but not by language spec -- why not D?), but what about passing up the stack as here?

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