Re: [boost] any interesting in a foreach looping construct?

From: Thomas Witt (witt_at_[hidden])
Date: 2003-10-09 15:22:10

Eric,

You asked why there was so little public reply to your posting. Well as 
far as I am concerned, I have trouble coming to a conclusion.

Eric Niebler wrote:
> 
> You can use it as follows to loop over an STL collection, for example:
> 
> std::list<int> int_list;
> ...
> BOOST_FOREACH( int &i, int_list )

What I don't like about this syntax is that the assignment to i in the 
loop is far from obvious.

> {
>     // mutates the int in the list:
>     i += 20;
>     // break works as you would expect
>     if ( i == 100 )
>         break;
> }

In generally I pretty much hate that kind of macros. Constructs that 
look very similar have given me a lot of trouble. Though in this case 
being against it is more a feeling than that I can come up with a good 
technical argument (at this time).

> 
> It has several advantages over std::for_each. In particular:
>   - You don't have to define a predicate at namespace scope
>   - You can break, continue, goto or return from the middle
>     of the loop body.

This is not neccessarily an advantage. It can also be seen as 
encouraging bad style.

>   - In the loop body, you have complete access to the local
>     variables from the surrounding code.
>   - You don't need to worry about off-by-one errors, iterators,
>     half-open sequences, binders, adapters, lambdas....

As far as lambda is concerned, I do believe that it is worrisome only 
because of some restrictions of the language. If I had a builtin lambda 
or a library that can make use of a decltype I would always prefer 
std::for_each over the macro.

> 
> So, any interest?
> 

I would be interested about real world experience. In that sense I am 
interested in a boost implementation. That being said I wouldn't use it 
in my code (yet).

Regards

Thomas