Re: [Boost-users] [Lambda] Help with if_then_else

From: Jaakko Jarvi (jarvi_at_[hidden])
Date: 2006-03-08 16:47:48

On Mar 8, 2006, at 3:21 PM, Scott Meyers wrote:

> Consider this (working) code using Boost.FileSystem:
>
>    fs::directory_iterator di(dir);
>    fs::directory_iterator end;
>    while (di != end) {
>      if (is_directory(*di)) cout << "D\n";
>      else cout << "~D\n";
>      ++di;
>    }
>
> This looks like it should be lambda-able.  Here's my attempt:
>
>    for_each(fs::directory_iterator(dir), fs::directory_iterator(),
>             if_then_else(bind(&fs::is_directory, _1),
>                          cout << "D\n",
>                          cout << "~D\n"));
>
> VC7.1 doesn't like this -- the diagnostics are below.  Any idea how  
> I can make
> this work?
>

I can spot one thing at least:
Arguments to if_then_else must be lambda functions:

>             if_then_else(bind(&fs::is_directory, _1),
>                          var(cout) << "D\n",
>                          var(cout) << "~D\n"));

This delays the evaluation of the branches (which would otherwise  
just print D or ~D once,
when the lambda function is created, rather than during every  
iteration of for_each

Best, Jaakko