On Monday, 15 April 2019 at 08:39:24 UTC, Anton Fediushin wrote:
Hello! I am currently trying to add a custom `toString` method
to an enum so that:
1. Enum members would still have numeric values and can be
easily compared (things like `enum a { foo = "FOO", bar =
"BAR”}` won't do, I want `a.foo < a.bar)`)
2. More custom methods can be implemented in the future
Obvious solution is to wrap an enum in a structure and utilize
'alias this' for subtyping like this:
```
struct Enum {
private {
enum internal {
foo,
bar
}
internal m_enum;
}
this(internal i) { m_enum = i; }
alias m_enum this;
string toString() {
// custom implementation of toString
}
}
```
This seems to work just fine for assigning and comparisons but
passing Enum as a function argument does not work:
```
void fun(Enum e) {}
fun(Enum.foo);
---
Error: function fun(Enum e) is not callable using argument
types (internal)
Cannot pass argument foo of type internal to parameter Enum e.
```
Of course, I could just define a bunch of functions that accept
my enum as the first argument and call them using UFCS but it'd
require to explicitly specify functions instead of D taking
care of that (toString() for structures is called automagically
by functions like writeln) and those functions would hang
around here and there totally unorganized. I prefer to keep
functions inside of structures and classes.
If there are other ways of achieving the same *and* keeping
code clean and organized, please share.
Thank you in advance,
Anton.
yes,
import std.stdio, std.meta, std.traits, std.conv;
enum _MyEnum : int { a,b,c}
struct _Enum(T)
{
T value;
alias value this;
// generate static field members
static foreach(e, v; EnumMembers!T)
{
pragma(msg, "static MyEnum "~to!string(v)~" =
MyEnum(T."~to!string(v)~");");
mixin("static MyEnum "~to!string(v)~" =
cast(MyEnum)(T."~to!string(v)~");");
}
}
alias _Enum!_MyEnum MyEnum;
void foo(MyEnum e)
{
writeln(to!int(e));
}
void main()
{
foo(MyEnum.a);
foo(MyEnum.b);
foo(MyEnum.c);
}
https://run.dlang.io/is/WOcLrZ
Note that value is never used, it just makes the cast work and
treats the struct as an enum. Not sure if there is a way around
that.
