Re: Chained Catch Statements
What is the idiom for handling a case where multiple exceptions of different types may be thrown? I think you could catch a common baseclass like Exception and test if it's a specific Exception by casting.
Re: Chained Catch Statements
On Monday, 30 January 2012 at 14:37:19 UTC, Jared wrote:
In Java and C++, I can do something to the effect of:
That works in D too.
I believe it does it linearly though, so it will use the
first catch that matches.
try {}
catch (Exception e) {} // most throwable objects derive from
Exception
catch (SpecialException e) {} // never used, because Exception
matches it all
Try putting the more specific catches first, and the generic
base classes at the end of the list.
Re: Chained Catch Statements
I suppose that works, though it does seem a little hackish (and I'm pretty sure it's considered bad form in Java). What was the line of reasoning for omitting this (to me, at least) extremely useful construct?
Re: Chained Catch Statements
30.01.2012 16:37, Jared пишет:
However, this doesn't seem to be possible in D.
Why not?
import std.stdio;
class Exception1 : Throwable { this( string s ) { super( s ); } }
class Exception2 : Throwable { this( string s ) { super( s ); } }
void foo() {
throw new Exception1( foo );
}
void bar() {
throw new Exception2( bar );
}
int main() {
enum tryBar = false;
try {
static if( tryBar ) {
bar();
} else {
foo();
}
} catch( Exception1 e1 ) {
writeln( First catch block: , e1.msg );
} catch( Exception2 e2 ) {
writeln( Second catch block: , e2.msg );
}
return 0;
}
Re: Chained Catch Statements
On 30-01-2012 15:37, Jared wrote:
In Java and C++, I can do something to the effect of:
try
{
//Some code
}
catch (Exception1)
{
}
catch (Exception2)
{
}
//etc...
However, this doesn't seem to be possible in D. What is the idiom for handling
a case where multiple exceptions of different types may be thrown?
Huh? I do this several places in my code. Works For Me (TM).
- Alex
Re: Chained Catch Statements
On Monday, 30 January 2012 at 14:50:23 UTC, Adam D. Ruppe wrote:
On Monday, 30 January 2012 at 14:37:19 UTC, Jared wrote:
In Java and C++, I can do something to the effect of:
That works in D too.
I believe it does it linearly though, so it will use the
first catch that matches.
try {}
catch (Exception e) {} // most throwable objects derive from
Exception
catch (SpecialException e) {} // never used, because Exception
matches it all
Try putting the more specific catches first, and the generic
base classes at the end of the list.
To me this seems like a mistake. Since likely your catching the
current exception and not one of the previously stored ones; A
codepath like that should either:
A) Fail at compile time, hopefully telling you a suggested order
so there's no problems.
B) Reorder the catch blocks on it's own during compile time,
since only one can get caught at a time anyways.
At least that's how I see it.
Re: Chained Catch Statements
On 1/30/12, Era Scarecrow rtcv...@yahoo.com wrote: To me this seems like a mistake. You could throw SpecialException in the handler for Exception. So it's legal code.
Re: Chained Catch Statements
On Monday, 30 January 2012 at 17:17:46 UTC, Andrej Mitrovic wrote:
On 1/30/12, Era Scarecrow rtcv...@yahoo.com wrote:
To me this seems like a mistake.
You could throw SpecialException in the handler for Exception.
So it's
legal code.
Yes, thanks to inheritance it is legal code. However it's almost
the same as this, which is legal code too.
try {
} catch (Throwable t) {
} catch {Exception e) { //never executed
} catch (StreamException st) { //never executed
} //and the list can go on forever.
See the problem? Everything that COULD be caught by exception is
now caught by throwable since it's inherited. At the very least
the compiler should warn you that certain sections won't be
executed, But then I would have to manually look up the
relationships when the compiler already knows them. In GCC known
code blocks that are never executed aren't even compiled in. If
the compiler reorders the blocks for you, the above would become
this.
try {
} catch (StreamException st) { //specific case
} catch {Exception e) { //less specific but no other inheritances
of this kind
} catch (Throwable t) { //everything else.
}
Re: Chained Catch Statements
On Monday, 30 January 2012 at 18:23:56 UTC, Era Scarecrow wrote:
try {
} catch (Throwable t) {
} catch {Exception e) { //never executed
} catch (StreamException st) { //never executed
} //and the list can go on forever.
See the problem?
No?
Error: catch at test.d(3) hides catch at test.d(4)
The compiler does not reorder the exceptions because it enforces
order in the written code. As you say the compiler knows the
relationship, the one reading it may not.
Re: Chained Catch Statements
On Monday, 30 January 2012 at 19:25:03 UTC, Jesse Phillips wrote:
On Monday, 30 January 2012 at 18:23:56 UTC, Era Scarecrow wrote:
try {
} catch (Throwable t) {
} catch {Exception e) { //never executed
} catch (StreamException st) { //never executed
} //and the list can go on forever.
See the problem?
No?
Error: catch at test.d(3) hides catch at test.d(4)
The compiler does not reorder the exceptions because it
enforces order in the written code. As you say the compiler
knows the relationship, the one reading it may not.
So long as it errors from the hiding (as it appears it does) then
it's fine, and the previous example shown was wrong.
try {}
catch (Exception e) {} // most throwable objects derive from
Exception
catch (SpecialException e) {} // never used, because
Exception matches it all
Re: Chained Catch Statements
On 1/30/12, Era Scarecrow rtcv...@yahoo.com wrote: If the compiler reorders the blocks for you A warning, maybe. But I'm against compilers which try to be too smart and rewrite code to change its semantics. If there's something wrong with my code I want to know about it (warning or error) and fix it, I don't want the compiler rewriting bad code behind-the-scenes. That only encourages writing sloppy code, and I end up having to know implementation details of each compiler to really know what some function body actually does (it could be someone else's code and not just mine).
Re: Chained Catch Statements
On Tuesday, January 31, 2012 01:05:20 Andrej Mitrovic wrote: On 1/30/12, Era Scarecrow rtcv...@yahoo.com wrote: If the compiler reorders the blocks for you A warning, maybe. But I'm against compilers which try to be too smart and rewrite code to change its semantics. If there's something wrong with my code I want to know about it (warning or error) and fix it, I don't want the compiler rewriting bad code behind-the-scenes. That only encourages writing sloppy code, and I end up having to know implementation details of each compiler to really know what some function body actually does (it could be someone else's code and not just mine). I see no reason why the compiler should be reordering anything here. That changes the semantics of the code. It's not like it's a great burden on the programmer to just reorder the blocks so that they're in the correct order. Having the compiler give a warning or error would be plenty, and a lot better than letting the programmer continue to screw up their code in ignorance. If they don't understand how the catch blocks work, they're likely to screw up more than just the order. I do think that having the compiler complain when one catch block hides another makes a lot of sense, but I think that that's as far as it should go. It's the programmer's job to fix their code, not the compiler's. - Jonathan M Davis
Re: Chained Catch Statements
Jonathan M Davis: I do think that having the compiler complain when one catch block hides another makes a lot of sense, but I think that that's as far as it should go. It's the programmer's job to fix their code, not the compiler's. I agree. Do we create a new diagnostic enhancement request then? Bye, bearophile
Re: Chained Catch Statements
I see no reason why the compiler should be reordering anything here. That changes the semantics of the code. It's not like it's a great burden on the programmer to just reorder the blocks so that they're in the correct order. Having the compiler give a warning or error would be plenty, and a lot better than letting the programmer continue to screw up their code in ignorance. If they don't understand how the catch blocks work, they're likely to screw up more than just the order. I do think that having the compiler complain when one catch block hides another makes a lot of sense, but I think that that's as far as it should go. It's the programmer's job to fix their code, not the compiler's. You are right.. I just have wishful thinking of the compiler being smart enough to auto-fix some things, but when you start making assumptions then errors and problems tend to arise more. That and if the compiler DOES reorder those blocks, then you re-ordering them for a specific reason may not work and could have unintended results. Course I don't see how with Try/catch since only one block (plus finally) can be used at a time.
