Re: [Boost-users] [preprocessor] have macro do different things based on structure of its argument

Subject: Re: [Boost-users] [preprocessor] have macro do different things based on structure of its argument
From: Edward Diener (eldiener_at_[hidden])
Date: 2011-03-02 20:02:57

On 3/2/2011 4:38 PM, Nathan Ridge wrote:
>
>> From: eldiener_at_[hidden]
>>> MY_MACRO(x) // a single token
>>> MY_MACRO((x, y)) // a parenthesized list of two tokens
>>> MY_MACRO((x, y, z)) // a parenthesized list of three tokens
>>> MY_MACRO(x y) // two tokens
>>> MY_MACRO(x y z) // three tokens
>>> MY_MACRO(x = y) // three tokens with the second one being something specific
>>
>> I will assume your examples are numbered from 1 to 6.
>>
>> In numbers 2 and 3 the parameter is a pp-lib tuple. Normally you would
>> have to know in advance how many elements are in a tuple but if your
>> compiler supports variadic macros you can use my variadic_macro_data
>> library in the sandbox to know how many elements are in the pp-lib tuple
>> and then you can subsequently extract any one of them.
>>
>> Numbers 1,4,5,6 are single elements. There is no way of which I know to
>> determine how many space separated sequences of text there are in each
>> element or extract any one of them. But there is a way to check whether
>> the sequence starts with a given sequence, and you may be able to work
>> from there via Lorenzo Caminiti's clever example, which he worked out
>> for his Local library:
>>
>> "NOTE: This is more of a curiosity because for my application I can
>> manipulate the storage classifiers using preprocessor metaprogramming.
>> In fact, `auto` and `register` are known alphanumeric tokens (i.e.,
>> keywords) that always appear at the beginning of the parameter type
>> expression so the following macros can be used to manipulate them:
>>
>> [snip]
>>
>> REMOVE_AUTO(auto int x) // int x
>> REMOVE_AUTO(int x) // int x
>>
>> ADD_AUTO(auto int x) // auto int x
>> ADD_AUTO(int x) // auto int x
>
> Interesting. I'm trying to do something slightly similar. I'm trying to accept both
> "typename identifier" and "identifier", and add "auto" (the C++0x kind)
> if there is no typename.
>
> So "ADD_AUTO(int x)" would remain "int x", but ADD_AUTO(x)
> would become "auto x".
>
> Do you think that's possible?

I would think you would have to know each type on which you want that to 
work, but you should be able to do that using Lorenzo's code as a basis. 
Essentially if you know the beginning text you should be able to work 
with the rest in the same way that Lorenzo shows.