Re: [Boost-users] const-correctness and lambda expression

From: Jaakko Järvi (jajarvi_at_[hidden])
Date: 2005-04-27 13:13:48

Hi Matthias,

On Apr 17, 2005, at 4:17 AM, Matthias Kaeppler wrote:

> Hi,
>
> in my program, I have this function to compute the sum of sizes of a 
> bunch of files:
>
> Glib::ustring DirBrowser::get_total_file_size()
> {
>     using namespace boostfs; // boostfs = boost::filesystem
>     using namespace lambda;  // lambda = boost::lambda
>     boost::intmax_t sum = 0;
>     
> std::for_each(m_first_file,m_files.end(),sum+=bind(&file_size,*_1));
>     return make_size_human_readable( sum );
> }

> The line with for_each computes the sum of file sizes, by iterating 
> over a container of pointers to boostfs::pathS, starting at the first 
> file (there are also directories, before the files), till the end, 
> invoking the lambda functor on each.
>
> Since this method does not change the state of the object, I wanted to 
> declare it const, but suddenly the lambda expression starts 
> complaining:
> Somehow, those path objects inside the lambda expression become const 
> now, too, and it doesn't compile anymore.
>

You mean you are making get_total_file_size to be a const member 
function?
Are m_first_file, and m_files members of DirBrowser?

If so, this most likely makes *_1 (with a path object substituted for 
_1) return a const object.
How does file_size take its argument? By non-const reference perhaps? 
If so, then that would be a reason.

If this reasoning does not hold, can you provide some more context?

   Best, Jaakko Järvi

> How come?
>
> -- 
> Matthias Kaeppler
>
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-- Jaakko Järvi, Assistant Professor
-- Texas A&M University, Computer Science
-- jarvi_at_[hidden]