Re: [Boost-users] boost fusion and/or boost parameter: open ended question about constructing object from a string

Subject: Re: [Boost-users] boost fusion and/or boost parameter: open ended question about constructing object from a string
From: e r (erwann.rogard_at_[hidden])
Date: 2008-09-10 17:10:05

e r wrote:
> David Abrahams wrote:
>> on Wed Sep 10 2008, e r <erwann.rogard-AT-gmail.com> wrote:
>>
>>> hi,
>>>
>>> here's what i'd like to do
>>>
>>> A]
>>>
>>> hpp:
>>>
>>> struct my{
>>>  	template<typename ArgPack>
>>> 	my(const ArgPack& args):n_(args[tag::n]),k_(args[tag::k]){}
>>> }
>>>
>>>
>>> struct argpack{
>>>   arglist<???> operator()(const string& str){???}
>>> }
>>>
>>> cpp:
>>>
>>> string str("n=2; k=1");
>>>
>>> my(argpack(str));
>>>
>>> In words,I'm looking to construct an ArgPack from a string, what's the
>>> appropriate library for this? 
>> Well, unless the types of all your arguments are the same *and* you have
>> a limited set of parameter names, all known at compile time,
>> Boost.Parameter can not help at all... and even if you meet all those
> 
> Thanks.
> 
> - Actually, no the arguments are not all the same type.
> - A limited set of parameter names: limited as in small or known? Known,
> but not necessarily small.
> 
>> conditions, using Boost.Parameter would probably be more trouble than it
>> is worth.  The problem is that Boost.Parameter establishes an
>> association between keywords and argument values at compile time, and
>> your strings have to be parsed at runtime.
> 
> Quite a painful dilemma because I use Boost.Parameter (not unlike many
> folks I guess) to pass down arguments into nested objects (whose types
> may be parameterized in the host class) and I couldn't deal with
> positional arguments.
> 
>>> Does it seem like I should invest in Boost
>>> Spirit?
>> Almost any of Boost's text processing libraries might be useful to you.
>> In your case, it might be easiest to build something on Boost.Regex or
>> Boost.XPressive, or even tokenizer, since the job you're doing is so
>> simple.
>>
>>> The above is a simplified version of what i really need, which is,
>>> potentially,
>>> 1) having to construct a large number of objects (to be pushed at the
>>> back of a container)
>>> 2) each object taking a large number arguments (not just n and k as above)
>>> 3) each object an instance of one of a set of classes
>>> (my0,my1,my2,...,myN) each with its specific argument tags (keywords).
>>>
>>> So I'd like to put all these arguments in a file, each line
>>> corresponding to an object, rather than have a messy cpp file.
>> Well, if you're willing to compile that file as C++, you could go back
>> to using Boost.Parameter... ;-)
> 
> since the arguments to be passed into object constructors are known at
> compile time, i guess i could still do this?
> 
> a) have some sort of cpp template i.e. some, not all the code
> intentionally left out (say marked by "NA"), to be filled from a text
> file, via a text processor b) compile the modified cpp file.
> 
> i'm not all that experienced with parsers and such. is this where
> Boost.Xpressive etc fit in or should i just go ahead and use perl?
> 

PS: I've just noticed there was no need for mentioning boost::fusion in
the title. Totally out of context (I guess got confused with another
post I wrote).