Re: [Boost-users] [preprocessor]:issues with expansion.

Subject: Re: [Boost-users] [preprocessor]:issues with expansion.
From: Nathan Ridge (zeratul976_at_[hidden])
Date: 2012-12-20 01:47:18

> Date: Wed, 19 Dec 2012 20:13:23 -0800 
> From: jeffrey.hellrung_at_[hidden] 
> To: boost-users_at_[hidden] 
> Subject: Re: [Boost-users] [preprocessor]:issues with expansion. 
>  
> On Wed, Dec 19, 2012 at 3:58 PM, Littlefield, Tyler  
> <tyler_at_[hidden]<mailto:tyler_at_[hidden]>> wrote: 
> hello all: 
> I have the following code: 
> #define REGISTER_MODULE(module)(\ 
> #ifdef BOOST_PP_EQUAL(BOOST_PP_LIST_SIZE(MODULES), 0)\ 
> //there are currently no modules in the list. 
> #define MODULES((module), BOOST_PP_NIL) 
> #else 
> BOOST_PP_LIST_APPEND(MODULES, module)\ 
> #endif 
> ) 
>  
> I am somewhat of a newb at preprocessor magic, so any advice would be  
> useful. For whatever reason this translates out to 'module)' when I try  
> to compile it with -E -P. 
> The idea is that it will add to the modules list (assuming the modules  
> list exists) the module to be registered, which should just be a type. 
> Thanks, 
>  
> Someone can correct me here, but I believe you can't use preprocessor  
> directives (#define, #if, etc.) within a macro definition, and, well,  
> who knows what'll happen if you try :/ 
>  
> - Jeff 

Right, and to address your desired use case a little more specifically,
I don't think you can store state in the preprocessor in between macro
invocations (if I'm wrong, please correct me, I would find such an
ability very useful as well).

Regards,
Nate