Standard C++ file size???

[Peter Olcott]

Is there any standard C++ way to determine the size of a
file before it is read?

[Victor Bazarov]

Peter Olcott wrote:

Is there any standard C++ way to determine the size of a
file before it is read?

No. The "standard C++ way" is to open the file for reading, seek to the
end of the file and get the position. If you need the size of the file
on disk (and you have the name of the file) without "touching" is in any
way, use the existing platform (OS) mechanisms to get the "file stats"
(statistics). RTFM on programming your OS.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[utab]

On Sun, 05 Oct 2008 10:46:43 -0500, Peter Olcott wrote:

Is there any standard C++ way to determine the size of a file before it
is read?

I guess the easiest would be to use the binary operation mode and try to
get the size in that mode.

Some binary file operations are treated in
http://www.cplusplus.com/doc/tutorial/files.html

with some examples, they can get you started, I suppose ;)

[Gennaro Prota]

Victor Bazarov wrote:

Peter Olcott wrote:

>Is there any standard C++ way to determine the size of a file before
it is read?

No. The "standard C++ way" is to open the file for reading, seek to the
end of the file and get the position. If you need the size of the file
on disk (and you have the name of the file) without "touching" is in any
way, use the existing platform (OS) mechanisms to get the "file stats"
(statistics). RTFM on programming your OS.

Note that "file size" is a less trivial notion than it might naively
appear: is it the number of bytes allocated on disk? The number of
characters you can read from the file in text mode? The number of bytes
you can read in binary mode? And what about symbolic links?

As Victor said, your platform is likely to expose a suitable function
yielding the number which corresponds to one particular definition of
"size" for the elements it can be applied to. POSIX systems must have
stat --note that this, modulo platform-specific extensions, doesn't know
what "size" is for some file types--; Win32 has GetFileAttribut esEx and
GetFileAttribut es, etc. Variants for "large file support" (e.g. stat64)
are also common.

Of course, Boost.Filesyste m may provide what you need just out of the
box.

--
Gennaro Prota | name.surname yahoo.com
Breeze C++ (preview): <https://sourceforge.net/projects/breeze/>
Do you need expertise in C++? I'm available.

[Peter Olcott]

"Victor Bazarov" <v.********@com Acast.netwrote in message
news:gc******** **@news.datemas .de...

Peter Olcott wrote:

>Is there any standard C++ way to determine the size of a
file before it is read?

No. The "standard C++ way" is to open the file for
reading, seek to the end of the file and get the position.

My best guess is that this is exactly what I need. I want to
read in an ASCII text file into a single contiguous block of
memory.

It would seem that I could do this using the method you
propose, and use a std::vector<uns igned charfor the memory
block, resized to position + 1. I would also guess that this
same method may also work for any possible type of data. Of
course I am assuming that the data is being read in binary
mode, in each case.

If you need the size of the file on disk (and you have the
name of the file) without "touching" is in any way, use
the existing platform (OS) mechanisms to get the "file
stats" (statistics). RTFM on programming your OS.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Maxim Yegorushkin]

On Oct 5, 10:54*pm, "Peter Olcott" <NoS...@SeeScre en.comwrote:

"Victor Bazarov" <v.Abaza...@com Acast.netwrote in message

news:gc******** **@news.datemas .de...

Peter Olcott wrote:

Is there any standard C++ way to determine the size of a
file before it is read?

No. *The "standard C++ way" is to open the file for
reading, seek to the end of the file and get the position.

My best guess is that this is exactly what I need. I want to
read in an ASCII text file into a single contiguous block of
memory.

It would seem that I could do this using the method you
propose, and use a std::vector<uns igned charfor the memory
block, resized to position + 1. I would also guess that this
same method may also work for any possible type of data. Of
course I am assuming that the data is being read in binary
mode, in each case.

In this case you don't need to know the size. It could be as simple
as:

#include <fstream>
#include <iterator>
#include <vector>

int main()
{
std::ifstream file("text.file ");
std::vector<cha rfile_in_memory (
(std::istream_i terator<char>(f ile))
, (std::istream_i terator<char>() )
);
// the file has been read into file_in_memory
}

However, if performance is paramount, or you need to know the exact
file errors, or the file is too big to fit into memory, you may like
to use your platform's native functions (like POSIX open(), fstat()
and mmap()).

--
Max

[James Kanze]

On Oct 5, 6:21 pm, Victor Bazarov <v.Abaza...@com Acast.netwrote:

Peter Olcott wrote:

Is there any standard C++ way to determine the size of a
file before it is read?

No. The "standard C++ way" is to open the file for reading,
seek to the end of the file and get the position.

That's a frequently used method, but it certainly isn't standard
C++. There's no guarantee that the position is convertable to
an integral type, and there's no guarantee that the integral
value means anything if it is.

In practice, this will probably work under Unix, and with binary
(but not text) files under Windows. Elsewhere, who knows?

If you need the size of the file on disk (and you have the
name of the file) without "touching" is in any way, use the
existing platform (OS) mechanisms to get the "file stats"
(statistics). RTFM on programming your OS.

Supposing, of course, that the system has some sort of request
for determining what you mean by file size. The most obvious
meaning is the number of bytes you will read before encountering
EOF. And as far as I know, Unix is the only system which has a
request which will return this. Another reasonable meaning is
the number of bytes the file occupies on the disk, but I don't
know of any system which has a request for this. (Unix
certainly doesn't.)

--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[Gennaro Prota]

James Kanze wrote:
[file size]

Another reasonable meaning is
the number of bytes the file occupies on the disk, but I don't
know of any system which has a request for this. (Unix
certainly doesn't.)

I have never tried it, but I think a few math (and path manipulation),
using GetDiskFreeSpac eEx and GetDiskFreeSpac eA should do it for (recent)
Windows. There might be gotchas I'm not seeing offhand, though.

Hopefully as off-topic as occasionally tolerable,

--
Gennaro Prota | name.surname yahoo.com
Breeze C++ (preview): <https://sourceforge.net/projects/breeze/>
Do you need expertise in C++? I'm available.

[Matthias Buelow]

James Kanze wrote:

Another reasonable meaning is
the number of bytes the file occupies on the disk, but I don't
know of any system which has a request for this. (Unix
certainly doesn't.)

stat(), lstat(), fstat() will determine the number of blocks used.

[PeteOlcott]

On Oct 6, 5:21*am, James Kanze <james.ka...@gm ail.comwrote:

On Oct 5, 6:21 pm, Victor Bazarov <v.Abaza...@com Acast.netwrote:

Peter Olcott wrote:

Is there any standard C++ way to determine the size of a
file before it is read?

No. *The "standard C++ way" is to open the file for reading,
seek to the end of the file and get the position.

That's a frequently used method, but it certainly isn't standard
C++. *There's no guarantee that the position is convertable to
an integral type, and there's no guarantee that the integral
value means anything if it is.

In practice, this will probably work under Unix, and with binary
(but not text) files under Windows. *Elsewhere, who knows?

Why would it not work for Text files under Windows?
(I am only looking for the size that can be block read into memory)

>

If you need the size of the file on disk (and you have the
name of the file) without "touching" is in any way, use the
existing platform (OS) mechanisms to get the "file stats"
(statistics). *RTFM on programming your OS.

Supposing, of course, that the system has some sort of request
for determining what you mean by file size. *The most obvious
meaning is the number of bytes you will read before encountering
EOF. *And as far as I know, Unix is the only system which has a
request which will return this. *Another reasonable meaning is
the number of bytes the file occupies on the disk, but I don't
know of any system which has a request for this. *(Unix
certainly doesn't.)

--
James Kanze (GABI Software) * * * * * * email:james.ka. [e-mail]
Conseils en informatique orientée objet/
* * * * * * * * * *Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]