Re: [Boost-users] Python 'in' keyword-like in C++

From: Simon Buchan (simon_at_[hidden])
Date: 2005-09-23 03:08:57

David Hall wrote:
> Tzu-Chien Chiu wrote:
> 
>> Any boost library can make me write the expression like
>>
>>  int x = 3;
>>  if ( x in list(3, 5, 6, 7) ) {
>>   ...
>>  }
>>
>>  
>>
> I don't know about the boost libraries, but I quickly sketched out 
> something you might be interested in, based on something I saw on one of 
> the news groups. All you have to do is to write a binary predicate 
> functor with bool operator()(int,<whatever your list type is>). It's 
> untested, but a quick syntax check with g++ 3.4.2 works fine.
> 
> Usage will be
> if (x <in> list(3,5,6,7) ) { ... }
> 
> I hope that's more or less what you're after; I don't know if you want 
> that list type/function as well.
> 
> The code itself is attached. Be sure to fix the template parameter in 
> the unnamed namespace..
> 
> David Hall
> 
> 
> ------------------------------------------------------------------------
> 
> _______________________________________________
> Boost-users mailing list
> Boost-users_at_[hidden]
> http://lists.boost.org/mailman/listinfo.cgi/boost-users
Huh.. I never thought to overload them that way, I like it!
Some small issues with the implementation:
* in operator_t<F>: your ctor init is... dodgy at best. Rename the 
parameter _f or something (ie: operator_t(F _f = F()) : f(_f) {})
* Cleaner layout! indent evenly, return types and init lists on their 
own lines, not seperating & from types, spaces after comma's in 
parameter lists, etc... Makes comprehension easier, for you _and_ your 
readers.
* related note: more meaningful template parameter names: BinFn instead 
of F, ValueT instead of T, etc...
* It should be designed as an expression template library, 'in' is 
something that operator< can recognise as an infix op, and generate some 
  binder functional that when it sees an operator> can call the op on 
the values to its left (stored) and right. I'd have to take a look at 
what some of the expression template functions do to come up with an 
implementation. This way <in> works for everything, and, if you are 
careful, you could generalise to pretty much operator that you want, and 
the same operator< template function can create the right thing. I'll 
play around with it over the weekend and see what I can come up with.
Boost.Infix?