Re: [boost] [filesystem] Some questions about string use

From: Bo Persson (bop_at_[hidden])
Date: 2006-03-05 02:34:53

"Ion Gaztañaga" <igaztanaga_at_[hidden]> skrev i meddelandet 
news:4409CCA4.6050107_at_gmail.com...
> Hi to all,
>
>  While revising a bit the boost::filesystem error/exception use for
> looking a model for interprocess library and after reading the N1934
> "Filesystem Library Proposal for TR2 (Revision 2)" I'm a bit 
> concerned
> about the heavy use of std::string/string_type in the library.
>
> The following functions return some kind of path as value type:
>
> // observers
> const string_type string() const;
> const string_type file_string() const;
> const string_type directory_string() const;
>
> const external_string_type external_file_string() const;
> const external_string_type external_directory_string() const;
>
> string_type  root_name() const;
> string_type  root_directory() const;
> basic_path   root_path() const;
> basic_path   relative_path() const;
> string_type  leaf() const;
> basic_path   branch_path() const;
>
> I know that syntactically this is nicer and we can have RVO:
>
> string_type str = path.string();
>
> But when iterating through directories I find that that returning a
> temporary object that must allocate/copy/deallocate hurts my 
> performance
> paranoia. Even with move semantics we have a an overhead:

On the other hand, I would guess that the actual OS and hard drive / 
network level operations take a lot more time than copying a few 
strings.

>
> std::vector<std::path> paths;
> //fill with paths
>
> std::path::iterator beg = paths.begin(), end = paths.end();
>
> for(; beg != end; ++it){
>    std::path::string_type str = it->root_name();//temporary created
>    str += "append some data";
>    std::cout << str;
> }

or

std::cout << it->root_name() << "append some data";

Who says you want to store the result? A temporary might be just fine.

>
> Couldn't be better (although uglier) to take a reference to a string
> that will be filled?
>
> void fill_root_name(string_type &root_name) const;

Really ugly!  :-)

It also assumes that the caller has somewhere to store the result. And 
the intention to keep it.

> ...
>
> ////////////////////////////////////////////////////////////////////
>
> std::vector<std::path> paths;
> //fill with paths
>
> std::path::string_type root_name;
>
> root_name.reserve(PATH_LENGTH);

So now you are wasting space instead.  :-)

>
> This way we only allocate memory if we don't have enough place in 
> our
> string. We can also reserve it beforehand to speed up code.

How do we know that speed is gained (and important), while space is 
cheaper?

Bo Persson