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.
