## digitalmars.D.learn - C macros vs D can't do the right thing

```A stripped down problem to avoid fluff. The C macro:

#define FLOB(t) (sizeof(t))

Can be used in another macro:

#define THINGY(a, b) (_THING(a, FLOB(b)))

We can use this as in:

THINGY(10, __u32)

Now the D Way says use functions not macros. except that=20

size_t FLOB(T)(auto ref T t) {=C2=A0return t.sizeof; }

which is the result of DStep doesn't allow passing a type name as t.

Is this a problem in D or a problem in DStep?

--=20
Russel.
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=3D=3D
Dr Russel Winder      t: +44 20 7585 2200   voip: sip:russel.winder ekiga.n=
et
41 Buckmaster Road    m: +44 7770 465 077   xmpp: russel winder.org.uk
London SW11 1EN, UK   w: www.russel.org.uk  skype: russel_winder
```
Jun 03 2017
```Russel Winder wrote:

Now the D Way says...

..use templates! ;-)

it is not really possible to guess what macro author means (to do that,
dstep should be able to actually *understand* the code), so it tries to do
what is used more often.

that is, it's dstep failed guess.
```
Jun 03 2017
```On Saturday, 3 June 2017 at 13:17:46 UTC, Russel Winder wrote:
A stripped down problem to avoid fluff. The C macro:

#define FLOB(t) (sizeof(t))

Can be used in another macro:

#define THINGY(a, b) (_THING(a, FLOB(b)))

We can use this as in:

THINGY(10, __u32)

Now the D Way says use functions not macros. except that

size_t FLOB(T)(auto ref T t) { return t.sizeof; }

which is the result of DStep doesn't allow passing a type name
as t.

Is this a problem in D or a problem in DStep?

I think an alias template parameter will work here as aliases
take anything(types, literals, symbols).
```
Jun 03 2017
```On 2017-06-03 16:03, Nicholas Wilson wrote:

I think an alias template parameter will work here as aliases take
anything(types, literals, symbols).

No, it doesn't work for types:

void foo(alias a)() {}

void main()
{
foo!(int)();
}

Results in:

Error: template instance foo!int does not match template declaration
foo(alias a)()

Perhaps using the variadic template with a constraint on one element
trick will work. Ugly, but I think that will work.

--
/Jacob Carlborg
```
Jun 03 2017
```Jacob Carlborg wrote:

Perhaps using the variadic template with a constraint on one element
trick will work. Ugly, but I think that will work.

yeah. that's what Phobos does, for example.
```
Jun 03 2017
```On Saturday, 3 June 2017 at 14:19:00 UTC, Jacob Carlborg wrote:
Perhaps using the variadic template with a constraint on one
element trick will work. Ugly, but I think that will work.

We could also finally fix the frontend to get around this. At
DConf 2015, Walter officially agreed that this is a bug that
needs fixing. ;)

— David
```
Jun 03 2017
```On 2017-06-03 16:22, David Nadlinger wrote:

We could also finally fix the frontend to get around this. At DConf
2015, Walter officially agreed that this is a bug that needs fixing. ;)

That would be nice.

--
/Jacob Carlborg
```
Jun 03 2017    Jonathan M Davis via Digitalmars-d-learn writes:
```On Saturday, June 03, 2017 14:22:11 David Nadlinger via Digitalmars-d-learn
wrote:
On Saturday, 3 June 2017 at 14:19:00 UTC, Jacob Carlborg wrote:
Perhaps using the variadic template with a constraint on one
element trick will work. Ugly, but I think that will work.

We could also finally fix the frontend to get around this. At
DConf 2015, Walter officially agreed that this is a bug that
needs fixing. ;)

He did agree, but AFAIK, no one has ever actually done the work, and I
suspect that unless Walter gets frustrated over the problem himself, he
won't get around to fixing it himself, but I don't know. I also have no idea
how easy or hard the implementation would be. It really should be fixed at
some point though, since it's clearly causing problems and consistently
forcing us to have workarounds in the standard library.

- Jonathan M Davis
```
Jun 04 2017    David Nadlinger <code klickverbot.at> writes:
```On Saturday, 3 June 2017 at 13:17:46 UTC, Russel Winder wrote:
Is this a problem in D or a problem in DStep?

It's a limitation of DStep – for that use case, it would need to
transform one of the macro arguments into a template argument
rather than a runtime function parameter.

If you need to make the code work as-is, I suppose you could
create some aliases like `enum u32 = __u32.init;` and pass these
instead of the types – using runtime values just to convey their
type.

— David
```
Jun 03 2017