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digitalmars.D.learn - scope file.open

reply nobody <not possible.de> writes:
Hallo,

how can i saftly open a file with scope.
My  idea is:
import std.stream;
import std.stdio;

void main() {
        File file = new File;

        scope(failure) {
                writefln("Datei kann nicht geoeffnet werden ");
        }
        file.open("testdatei.txt",FileMode.In);

        while(!file.eof()) {
                printf("%.*s\n",file.readLine());

        }
        file.close();
}


But why must scope(failure) before the file.open statement.
I think it's look strange.
Mybe some on have a better idea to open a file with scope.

Sincerily
Jan 23 2007
parent reply Chris Nicholson-Sauls <ibisbasenji gmail.com> writes:
nobody wrote:

 Hallo,
 
 how can i saftly open a file with scope.
 My  idea is:
 import std.stream;
 import std.stdio;
 
 void main() {
         File file = new File;
 
         scope(failure) {
                 writefln("Datei kann nicht geoeffnet werden ");
         }
         file.open("testdatei.txt",FileMode.In);
 
         while(!file.eof()) {
                 printf("%.*s\n",file.readLine());
 
         }
         file.close();
 }
 
 
 But why must scope(failure) before the file.open statement.
 I think it's look strange.
 Mybe some on have a better idea to open a file with scope.
 
 Sincerily
 


Personally I don't think it really looks all that strange, but I'm odd like
that.  :)  In 
this case, if it worries you, you might try a traditional try-catch-finally
instead.


















-- Chris Nicholson-Sauls
Jan 23 2007
next sibling parent Frits van Bommel <fvbommel REMwOVExCAPSs.nl> writes:
Chris Nicholson-Sauls wrote:

 nobody wrote:

 Hallo,

 how can i saftly open a file with scope.
 My  idea is:




[snip]

 But why must scope(failure) before the file.open statement.
 I think it's look strange.
 Mybe some on have a better idea to open a file with scope.


 
 Personally I don't think it really looks all that strange, but I'm odd 
 like that.  :)  In this case, if it worries you, you might try a 
 traditional try-catch-finally instead.
 


















While you're at it, either
* put 'scope' in front of the 'File file' declaration, or
* move 'file.close()' into a finally{} block at the end of that 
try-catch, or
* put 'scope(exit) file.close();' right after the declaration of file

All of those will guarantee your file gets closed if an exception gets 
thrown. I'd personally prefer the first one, but if you're going to use 
try-catch anyway you might as well tack on a 'finally'.

Of course, it doesn't really matter in this case since it's directly in 
main(), and the exception will exit it...
Jan 23 2007
prev sibling parent reply nobody <not possible.de> writes:
Chris Nicholson-Sauls Wrote:


 nobody wrote:

 Hallo,
 
 how can i saftly open a file with scope.
 My  idea is:
 import std.stream;
 import std.stdio;
 
 void main() {
         File file = new File;
 
         scope(failure) {
                 writefln("Datei kann nicht geoeffnet werden ");
         }
         file.open("testdatei.txt",FileMode.In);
 
         while(!file.eof()) {
                 printf("%.*s\n",file.readLine());
 
         }
         file.close();
 }
 
 
 But why must scope(failure) before the file.open statement.
 I think it's look strange.
 Mybe some on have a better idea to open a file with scope.
 
 Sincerily
 


 
 Personally I don't think it really looks all that strange, but I'm odd like
that.  :)  In 
 this case, if it worries you, you might try a traditional try-catch-finally
instead.
 
















 
 -- Chris Nicholson-Sauls


The question is, how scope know it's a failure to open a file, before the
file.open statement 
is executed.
That is i didn't understand.
For this reason, i will put the scope(failure) after the file.open statement.

Thanks
Jan 23 2007
parent reply "Jarrett Billingsley" <kb3ctd2 yahoo.com> writes:
"nobody" <not possible.de> wrote in message 
news:ep7395$o92$1 digitaldaemon.com...


 The question is, how scope know it's a failure to open a file, before the 
 file.open statement
 is executed.
 That is i didn't understand.


What the scope(failure) statement does is "registers" the following piece of 
code to be executed whenever the current scope (brace block) is left.  So 
you can put it anywhere:

scope(failure) writefln("Datei kann nicht geoeffnet werden ");
File file = new File;
file.open("testdatei.txt",FileMode.In);
while(!file.eof()) writefln(file.readLine());
file.close();

Or:

File file = new File;
scope(failure) writefln("Datei kann nicht geoeffnet werden ");
file.open("testdatei.txt",FileMode.In);
while(!file.eof()) writefln(file.readLine());
file.close();

Or even:

File file = new File;
file.open("testdatei.txt",FileMode.In);
while(!file.eof()) writefln(file.readLine());
file.close();
scope(failure) writefln("Datei kann nicht geoeffnet werden ");

It doesn't matter where the scope(failure) appears.  It just means "if an 
exception passes out of this block, run this code."
Jan 24 2007
parent Frits van Bommel <fvbommel REMwOVExCAPSs.nl> writes:
Jarrett Billingsley wrote:

 "nobody" <not possible.de> wrote in message 
 news:ep7395$o92$1 digitaldaemon.com...
 

 The question is, how scope know it's a failure to open a file, before the 
 file.open statement
 is executed.
 That is i didn't understand.


 
 What the scope(failure) statement does is "registers" the following piece of 
 code to be executed whenever the current scope (brace block) is left.  So 
 you can put it anywhere:


1) It only gets executed when the scope is exited by exception unwinding.
2) It only gets executed when that exception happens *after* the scope 
statement.

http://www.digitalmars.com/d/statement.html#ScopeGuardStatement


 scope(failure) writefln("Datei kann nicht geoeffnet werden ");
 File file = new File;
 file.open("testdatei.txt",FileMode.In);
 while(!file.eof()) writefln(file.readLine());
 file.close();


That'll work.


 Or:
 
 File file = new File;
 scope(failure) writefln("Datei kann nicht geoeffnet werden ");
 file.open("testdatei.txt",FileMode.In);
 while(!file.eof()) writefln(file.readLine());
 file.close();


That'll only print if 'new File' is successful (i.e. there's enough 
memory and the constructor doesn't throw an exception) but an exception 
gets thrown in the rest of the code.


 Or even:
 
 File file = new File;
 file.open("testdatei.txt",FileMode.In);
 while(!file.eof()) writefln(file.readLine());
 file.close();
 scope(failure) writefln("Datei kann nicht geoeffnet werden ");


That will *not* work. It's just too late to affect any code.


 It doesn't matter where the scope(failure) appears.  It just means "if an 
 exception passes out of this block, run this code." 


It's "If an exception passes out of this block *after this statement*, 
run this code", actually.

See for yourself:
-----
urxae urxae:~/tmp$ cat test.d
import std.stdio;

void main() {
     scope(failure) writefln("will print");
     throw new Exception("will be thrown");
     scope(failure) writefln("won't print");
}
urxae urxae:~/tmp$ dmd -run test.d
will print
Error: will be thrown
-----
Jan 24 2007