Re: [boost] Remarkably efficient file copy

From: Fedder Skovgaard (fsk_at_[hidden])
Date: 2002-03-19 07:56:44

Hi,

I think it's a viable route to take. Nicolai Josuttis  writes: "This is
probably the fastest way to copy files by using C++ I/O streams."
See"The C++ standard Library" (ISBN 0201379260) Chapter 13.14.3.

You should, obviously be able to get better performance by using platform
specific calls and technologies (such as Asynchroneous IO and Memory mapped
files on Win32).

But the point lies (in my perspective) in the subphrase "by using C++ I/O
streams". Most operating systems also sports memory management routines
that out-perform new / delete for specific purposes, but that doesn't mean
that new / delete is a bad thing.

Just my two cents.

Fedder

                                                                                                                                              
                                                                                                                                              
                        "Alisdair Meredith"            To: <boost_at_[hidden]>                                                            
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                        enaultf1.com>                  Subject:  Re: [boost] Remarkably efficient file copy                                   
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                        19-03-2002 12:04                                                                                                      
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However, it surely performs well as a lowest-common-denominator approach
for a cross-platform library.

Hopefully we can support more optimised solutions for specific platforms,
but something similar should be widely supportable as a fall-back
implementation.  Here, it's very simplicity is in its favour!

AlisdairM

>>> gila_at_[hidden] 19 March 2002 06:24:36 >>>
In my tests, I've found a combination of stat, mmap (there's a similar
win32 api), and memcpy to be the fastest implementation available. I
don't have any benchmarks available but it also stands to reason that
handing the file copy routine over to the kernal will yield the best
possible results for a general purpose file copy.

Given that your astoundingly simple implementation is so, well,
simple, it hardly seems to warrent inclusion as a file copy routine
and should just be one of the many tricks in your bag.

On Tue, Mar 19, 2002 at 04:37:33PM +1100, Dylan Nicholson wrote:
> If we definitely decide to put in a "copy_file" routine into
boost::filesystem,
> then believe it or not, the following single line of code is remarkably
> efficient (at least under HP aCC 3.33, which is what I tested it with):
>
> bool copy(const char* src, const char* dest)
> {
>            return std::ofstream(dest) << std::ifstream(src).rdbuf();
> }
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