variadic mixin templates and other stuff

[Timon Gehr]

On 04/20/2012 03:23 PM, Martin Drasar wrote:
> Hi,
>
> I am migrating a C++ project to D and I have hit a roadblock that I hope
> you might help me with.
>
> My code is heavily inspired by the COM architecture, so I have naturally
> take a look at std/c/windows/com.d, just to find out that it does not
> contain all I need.
>
> In the C++ code I have several interfaces and a class. The class
> inherits from these interfaces, exposes some of them and it is possible
> to get one interface from another using the QueryInterface function by
> supplying correct GUID:
>
> class C : IA, IB, IC
> {
>    EXPOSE_INTERFACE(IA, IC);
>    ...
> }
>
> where EXPOSE_INTERFACE is macro that expands to something like this:
>
> void QueryInterface (IComponent** comp, GUID* guid)
> {
>    ...
>    if (guid == IAGuid) *comp = ...some casting
>    if (guid == IBGuid) *comp = ...some casting
>    ...
> }
>
> So I was thinking that in D I could use mixin templates with variadic
> arguments, so I tried something like that:
>
> string interfaceGuid(string ifaceName)
> {
>    return ifaceName ~ "Guid";
> }
>
> mixin template EXPOSE_INTERFACE(string ifaceName)
> {
>    if (mixin(interfaceGuid(ifaceName)) == guid) { ... some casting }
> }
>
> mixin template EXPOSE_INTERFACES(T...)(T args)
> {
>    void QueryInterface(ref IComponent comp, string guid)
>    {
>      foreach (arg; args)
>      {
>        mixin EXPOSE_INTERFACE!arg;
>      }
>    }
> }
>
> But I got this:
>
> component.d(8): Declaration expected, not 'if'
> component.d(15): no identifier for declarator args
> component.d(15): semicolon expected, not ')'
> component.d(15): Declaration expected, not ')'
> component.d(19): unrecognized declaration
>
> Then I got confused and realized that my D is as sharp as is my German -
> studied it for some time, but still has a problem to order a bread in a
> store...
>
> My questions are following:
> - can mixin templates be used this way?

They can only mixin declarations.

> - why are they complaining?

Because if is a statement and not a declaration.

> - is there a better way to do this other than reproducing C macros?
>

Inline EXPOSE_INTERFACE into EXPOSE_INTERFACES and it'll work.