On Monday, November 13, 2017 12:40:39 bauss via Digitalmars-d-learn wrote: > On Monday, 13 November 2017 at 01:12:59 UTC, Adam D. Ruppe wrote: > > On Monday, 13 November 2017 at 01:03:17 UTC, helxi wrote: > >> In this program, casting using to does not work as intended > >> (returning 23/11) on the struct. However, calling opCast > >> directly seems to do the job. Why is that? > > > > to!string calls a function called `string toString() {}` on the > > struct, not the cast operator. > > Which is generally what you want to use anyway and not a cast > overload. > > My rule of thumb (Which can of course differ per preference.) is > that cast overload should only be done between the following: > > struct <-> scalar types > class <-> scalar types > struct <-> class > > Any string conversions should always be done with `toString()`. > > Anything else should not be implemented with casts or conversion > methods. An exception of course is creating slices, which is > acceptable using `opSlice`, but generally I avoid using something > like `opCast` to an array, UNLESS it's an array wrapper, which > you most likely won't have in D anyway, because you'd be better > off creating a range.
There's nothing wrong with using opCast to define other conversions - it's generally what std.conv.to is going to use (the other options being alias this and a constructor if the type being converted to has a constructor that would work). It's just that in the case of string, there's a standard function for converting to it other than opCast. The main problem with overloading opCast in general is that doing so unfortunately disables all of the built-in casts, which can be a big problem - though hopefully that gets fixed at some point. https://issues.dlang.org/show_bug.cgi?id=5747 https://issues.dlang.org/show_bug.cgi?id=9249 There's also the argument that using an explicitly named function is clearer - particularly if it's not obvious what converting from one type to another would mean - but those won't work with std.conv.to. So, it depends on what you're trying to do. - Jonathan M Davis