The code I currently have is as follows:

import std.stdio;
import std.traits;
import std.typecons;

struct EmbeddedTest
{
    int bits;
}

struct Test
{
    //Other stuff
	
    EmbeddedTest embeddedTest;

    enum isSet(alias bit) = `cast(bool)(embeddedTest.bits & `
                                ~ bit.stringof ~ `)`;

    @property setBits()
    {
        enum Bits = EnumMembers!Bit;
        Nullable!(Bit, Bit.max)[Bits.length] ret;
        foreach (i, bit; Bits)
        {
            if (mixin(isSet!bit))
            {
                ret[i] = bit;
            }
        }
		
		return ret;
    }
}

enum Bit
{
    bit1 = 0x0001,
    bit2 = 0x0002,
    bit3 = 0x0004,
    bit4 = 0x0008,
}
									
void main()
{
	Test t;
	t.embeddedTest.bits = 0x1;
	foreach (bit; t.setBits)
	{
		if (!bit.isNull)
		{
			writeln(bit);
		}
	}
}

My problem is that I would like to not have to mixin the result of Test.isSet at each usage site, but I also want to avoid making it a function if possible. I tried to move the mixin into isSet like this:


    enum isSet(alias bit) = mixin(`cast(bool)(embeddedTest.bits & `
                                ~ bit.stringof ~ `)`);

This would now break isSet for usage outside of the struct Test. However, I don't see why it wouldn't work inside of Test, since the mixin expands, for example, to:

    cast(bool)(embeddedTest.bits & bit1)

So `if (isSet!bit1)` becomes `if (cast(bool)(embeddedTest.bits & bit1)`. That doesn't work, however. I get an error message saying "need 'this' for 'bits' of type 'int'". Is there a way to make this work, or am I stuck with the uglier mixin at the usage site?

On Wed, 24 Dec 2014 17:05:45 +0000
Meta via Digitalmars-d-learn <digitalmars-d-learn@puremagic.com> wrote:

> So `if (isSet!bit1)` becomes `if (cast(bool)(embeddedTest.bits & bit1)`. That doesn't work, however. I get an error message saying "need 'this' for 'bits' of type 'int'". Is there a way to make this work, or am I stuck with the uglier mixin at the usage site?

i don't think that you can cheat like this. ;-)

templates aren't macros, and `mixin` inside the template works immediately. i.e. it's trying to *execute* the mixined code in compile time. so your template with `mixin` in it tries to check `embedded.bits` in *compile* time, and then complains about missing 'this', as it should.

i don't think that you can do c-like macros in D.

On Wednesday, 24 December 2014 at 17:41:09 UTC, ketmar via Digitalmars-d-learn wrote:
> On Wed, 24 Dec 2014 17:05:45 +0000
> Meta via Digitalmars-d-learn <digitalmars-d-learn@puremagic.com> wrote:
>
>> So `if (isSet!bit1)` becomes `if (cast(bool)(embeddedTest.bits & bit1)`. That doesn't work, however. I get an error message saying "need 'this' for 'bits' of type 'int'". Is there a way to make this work, or am I stuck with the uglier mixin at the usage site?
>
> i don't think that you can cheat like this. ;-)
>
> templates aren't macros, and `mixin` inside the template works
> immediately. i.e. it's trying to *execute* the mixined code in
> compile time. so your template with `mixin` in it tries to check
> `embedded.bits` in *compile* time, and then complains about missing
> 'this', as it should.
>
> i don't think that you can do c-like macros in D.

Hmm, I confused myself over when the expression is interpreted. I got it into my head that the template expansion would somehow delay interpretation of the expression so it could be picked up at runtime, but of course `enum` forces it to be CTFE'd.

On Wed, 24 Dec 2014 19:52:31 +0000
Meta via Digitalmars-d-learn <digitalmars-d-learn@puremagic.com> wrote:

> Hmm, I confused myself over when the expression is interpreted. I got it into my head that the template expansion would somehow delay interpretation of the expression so it could be picked up at runtime, but of course `enum` forces it to be CTFE'd.

sometimes i want to have some kind of C-like macros in D too, which just inserts the generated string in the place of instantiation with implicit `mixin`. maybe someone will write a DIP/ER for this...

I am curious, however, why changing `enum` to `auto` (or bool) doesn't work. You said that the mixin tries to interpret the expression `cast(bool)(embeddedTest.bits & <enumName>)` at compile time, but I don't understand why that would be so when the storage is auto and not enum. I guess mixin doesn't work quite like I thought it did; it's not quite the equivalent of pasting the mixed-in code.

On Wednesday, 24 December 2014 at 20:08:07 UTC, Meta wrote:
> I am curious, however, why changing `enum` to `auto` (or bool) doesn't work. You said that the mixin tries to interpret the expression `cast(bool)(embeddedTest.bits & <enumName>)` at compile time, but I don't understand why that would be so when the storage is auto and not enum. I guess mixin doesn't work quite like I thought it did; it's not quite the equivalent of pasting the mixed-in code.

Ok, I understand now. I missed the fact that even when it's an auto or a bool variable, it is CTFE-initialized because it's outside the constructor. I'm so close, yet so far from making this work.