[Issue 4423] New: enums of struct types

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=4423

           Summary: enums of struct types
           Product: D
           Version: D2
          Platform: Other
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: jmdavisProg at gmail.com


--- Comment #0 from Jonathan M Davis <jmdavisProg at gmail.com> 2010-07-04 03:23:53 PDT ---
I would like to be able to create enums of struct types, but it doesn't seem
possible at the moment. You can create manifest constants of a struct using
enum, but not an enum with an actual list of values. So, if I were to create a
struct that looks like this

struct S
{
    this(string phrase, int num)
    {
        this.phrase = phrase;
        this.num = num;
    }

    int opCmp(const ref S rhs)
    {
        if(phrase < rhs.phrase)
            return -1;
        else if(phrase > rhs.phrase)
            return 1;

        if(num < rhs.num)
            return -1;
        else if(num > rhs.num)
            return 1;

        return 0;
    }

    string phrase;
    int    num;
}


I could create enums like this

enum a = S("hello", 1);
enum b = S("goodbye", 45);


and the compiler is fine with it. However, if I try and create a bona fide enum

enum E : S {a = S("hello", 1),
            b = S("goodbye", 45),
            c = S("world", 22)};


the compiler complains like so

t.d(28): Error: variable __ctmp1 cannot be read at compile time
t.d(28): Error: cannot evaluate __ctmp1.this("hello",1) at compile time
t.d(28): Error: cannot evaluate __ctmp1.this("hello",1) at compile time
t.d(29): Error: cannot evaluate __ctmp2.this("goodbye",45) at compile time
t.d(29): Error: cannot evaluate __ctmp2.this("goodbye",45) at compile time
t.d(28): Error: cast(const(S))__ctmp1.this("hello",1) is not an lvalue
t.d(29): Error: cannot evaluate __ctmp2.this("goodbye",45) at compile time
t.d(29): Error: cannot evaluate
__ctmp2.this("goodbye",45).opCmp(cast(const(S))__ctmp1.this("hello",1)) at
compile time
t.d(29): Error: Integer constant expression expected instead of
__ctmp2.this("goodbye",45).opCmp(cast(const(S))__ctmp1.this("hello",1)) < 0
t.d(28): Error: cast(const(S))__ctmp1.this("hello",1) is not an lvalue
t.d(29): Error: cannot evaluate __ctmp2.this("goodbye",45) at compile time
t.d(29): Error: cannot evaluate
__ctmp2.this("goodbye",45).opCmp(cast(const(S))__ctmp1.this("hello",1)) at
compile time
t.d(29): Error: Integer constant expression expected instead of
__ctmp2.this("goodbye",45).opCmp(cast(const(S))__ctmp1.this("hello",1)) > 0
t.d(30): Error: cannot evaluate __ctmp3.this("world",22) at compile time
t.d(30): Error: cannot evaluate __ctmp3.this("world",22) at compile time
t.d(28): Error: cast(const(S))__ctmp1.this("hello",1) is not an lvalue
t.d(30): Error: cannot evaluate __ctmp3.this("world",22) at compile time
t.d(30): Error: cannot evaluate
__ctmp3.this("world",22).opCmp(cast(const(S))__ctmp1.this("hello",1)) at
compile time
t.d(30): Error: Integer constant expression expected instead of
__ctmp3.this("world",22).opCmp(cast(const(S))__ctmp1.this("hello",1)) < 0
t.d(28): Error: cast(const(S))__ctmp1.this("hello",1) is not an lvalue
t.d(30): Error: cannot evaluate __ctmp3.this("world",22) at compile time
t.d(30): Error: cannot evaluate
__ctmp3.this("world",22).opCmp(cast(const(S))__ctmp1.this("hello",1)) at
compile time
t.d(30): Error: Integer constant expression expected instead of
__ctmp3.this("world",22).opCmp(cast(const(S))__ctmp1.this("hello",1)) > 0


It doesn't look like the CTFE stuff can handle this for some reason. Granted,
what CTFE can do is a definite work in progress, but I think that the lack of
ability to create an enum of structs is seriously limiting and rather odd given
that you can create a manifest constant of a struct using an enum. So, I'd love
it if it were possible to create actual enums of a struct type.

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