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

From: Peter Dimov (pdimov_at_[hidden])
Date: 2006-03-08 17:12:02

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?

The cout << "D\n" subexpression is evaluated immediately since it doesn't 
have any lambda components. Use var(cout) << "D\n" instead, or the ?: 
variant:

    cout << if_then_else_return( bind(&fs::is_directory, _1), "D\n", 
"~D\n" )

You could also make an alias for var(std::cout):

    var_type<std::ostream>::type cout = var( std::cout );

and use that in the lambda expressions.