Re: [boost] temp file

From: Dylan Trotter (trotterdylan_at_[hidden])
Date: 2005-09-08 17:02:02

After I read the original Post I became interested in the problem as 
well. The realization that I came to is that there are more objects 
involved than just a istreams and ostreams. If you were to create an 
ostream that deleted the file on destruction, you'd find it to be 
useless since you wrote some data, but have no way whatsoever of 
retrieving it again. Coversely, opening a temporary istream would serve 
no purpose, since it would obviously contain no data.

One possible solution, I felt was to have a sort of handle to the file. 
For example:

class temp_file
{
public:
     temp_file(const path &root) { /* create file in root */ }
     ~temp_file() { /* delete the file */ }
     const path &get_path() const { return path_; }
};

Then any i/ostreams that use the file hold a shared_ptr to it. The 
lifetime of the temp_file is arbitrary and will exist only so long as 
some stream is holding its pointer. So add:

typedef shared_ptr<temp_file> temp_file_ptr;

class temp_ofstream
{
public:
     temp_ofstream(temp_file_ptr tempFile) : tempFile_(tempFile) { }
     temp_ofstream(const path &root) : tempFile_(new temp_file(root) { }

private
     temp_file_ptr tempFile_;
};

Actually, the implementation I came up with used a stream_buffer derived 
class, but the effect is the same. To share the temp_file between 
ostreams and istreams, do this:

temp_file_ptr file(new temp_file(""));
{ /* some scope */
     temp_ofstream out(file);
     /* do stuff with out */
}
{ /* some other scope */
     temp_ifstream in(file);
     /* do stuff with in */
}

However, there are problems with this layout. In particular, there's 
race conditions with the actual filesystem object. Also, there needs to 
be some kind of access control to the file supplied by the temp_file 
object so you don't attempt to open a reader while you're writer or 
vice-versa. In any case, just some preliminary thoughts.

Dylan

Beman Dawes wrote:
> "Tomek" <at_at_[hidden]> wrote in message 
> news:Pine.LNX.4.58.0509071815370.26925_at_rex.anfa.pl...
> 
>>Hi,
>>
>>
>>Can you add to boost::filesystem namespace class for temporary files
>>which will remove file on object destruction ?
>>
>>I prepared same sample code : http://psycho.pl/pub/src/tmpfile.cc
>>
>>If you say that this is ok, I will write it using boost library and
>>also working on win32.
> 
> 
> I'm definitely interested, although the specification is of more interest 
> than the actual code. I'm particularly interested in resolving any 
> differences between POSIX and Windows, and avoiding security issues.
> 
> It is a bit surprising that this is the first request I can recall for a 
> temporary file facility. Seems like a natural addition.
> 
> --Beman 
> 
> 
> 
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