D - Slicing local variables?
- Matthew Wilson (9/9) Oct 18 2003 char[] fn()
- Walter (5/15) Oct 18 2003 Yes.
- Matthew Wilson (4/21) Oct 18 2003 Eeek! It makes sense, of course.
- Sean L. Palmer (8/21) Oct 18 2003 Wouldn't the compiler try to catch errors like this at compile time? Si...
- Matthew Wilson (8/30) Oct 18 2003 Simple
- Walter (10/32) Oct 18 2003 Simple
- Sean L. Palmer (6/19) Oct 18 2003 Ok, so long as it's officially an error, the good compilers can reject s...
char[] fn()
{
char ar[21];
int cch = sprintf(ar, "%d", 101);
return ar[0 .. cch];
}
Does this return a slice into something that (no longer) exists on the
stack, or does it allocated a new heap variable whose contents are
equivalent to the sliced section of ar?
Oct 18 2003
"Matthew Wilson" <matthew stlsoft.org> wrote in message news:bmqqet$dgb$1 digitaldaemon.com...char[] fn() { char ar[21]; int cch = sprintf(ar, "%d", 101); return ar[0 .. cch]; } Does this return a slice into something that (no longer) exists on the stack,Yes.or does it allocated a new heap variable whose contents are equivalent to the sliced section of ar?No. To do that: return ar[0..cch].dup;
Oct 18 2003
"Matthew Wilson" <matthew stlsoft.org> Eeek! It makes sense, of course. It's something that we'll need to make sure is well documented. "Walter" <walter digitalmars.com> wrote in message news:bmqv3c$lok$4 digitaldaemon.com..."Matthew Wilson" <matthew stlsoft.org> wrote in message news:bmqqet$dgb$1 digitaldaemon.com...char[] fn() { char ar[21]; int cch = sprintf(ar, "%d", 101); return ar[0 .. cch]; } Does this return a slice into something that (no longer) exists on the stack,Yes.or does it allocated a new heap variable whose contents are equivalent to the sliced section of ar?No. To do that: return ar[0..cch].dup;
Oct 18 2003
"Walter" <walter digitalmars.com> wrote in message news:bmqv3c$lok$4 digitaldaemon.com..."Matthew Wilson" <matthew stlsoft.org> wrote in message news:bmqqet$dgb$1 digitaldaemon.com...Wouldn't the compiler try to catch errors like this at compile time? Simple data flow. Local variables cannot outlive the function, and cannot be used in a return statement, even indirectly. Their values can be returned, however. I guess you have to allow passing local variables to other functions as parameters.. Seanchar[] fn() { char ar[21]; int cch = sprintf(ar, "%d", 101); return ar[0 .. cch]; } Does this return a slice into something that (no longer) exists on the stack,Yes.
Oct 18 2003
"Sean L. Palmer" <palmer.sean verizon.net> wrote in message news:bmr28m$plk$1 digitaldaemon.com..."Walter" <walter digitalmars.com> wrote in message news:bmqv3c$lok$4 digitaldaemon.com...Simple"Matthew Wilson" <matthew stlsoft.org> wrote in message news:bmqqet$dgb$1 digitaldaemon.com...Wouldn't the compiler try to catch errors like this at compile time?char[] fn() { char ar[21]; int cch = sprintf(ar, "%d", 101); return ar[0 .. cch]; } Does this return a slice into something that (no longer) exists on the stack,Yes.data flow. Local variables cannot outlive the function, and cannot beusedin a return statement, even indirectly. Their values can be returned, however. I guess you have to allow passing local variables to other functions as parameters..I agree it should be a compiler warning. I also agree that it should have the behaviour that it does, rather than trying to do something smart like an implicit .dup. (I'm not suggesting you were suggesting that, of course, just blithering on. ;)
Oct 18 2003
"Sean L. Palmer" <palmer.sean verizon.net> wrote in message news:bmr28m$plk$1 digitaldaemon.com..."Walter" <walter digitalmars.com> wrote in message news:bmqv3c$lok$4 digitaldaemon.com...Simple"Matthew Wilson" <matthew stlsoft.org> wrote in message news:bmqqet$dgb$1 digitaldaemon.com...Wouldn't the compiler try to catch errors like this at compile time?char[] fn() { char ar[21]; int cch = sprintf(ar, "%d", 101); return ar[0 .. cch]; } Does this return a slice into something that (no longer) exists on the stack,Yes.data flow. Local variables cannot outlive the function, and cannot beusedin a return statement, even indirectly. Their values can be returned, however. I guess you have to allow passing local variables to other functions as parameters..It is illegal to do that in D, and whether the compiler issues an error or not is a quality of implementation issue. The trouble is, for simple cases the compiler can detect it. For complex cases, it cannot, therefore the spec can't require that it diagnose the error. Also, it is an error, not a warning <g>. The exact same problem exists in C and C++.
Oct 18 2003
Ok, so long as it's officially an error, the good compilers can reject such code. Sean "Walter" <walter digitalmars.com> wrote in message news:bms3hr$23v2$3 digitaldaemon.com...specWouldn't the compiler try to catch errors like this at compile time?Simpledata flow. Local variables cannot outlive the function, and cannot beusedin a return statement, even indirectly. Their values can be returned, however. I guess you have to allow passing local variables to other functions as parameters..It is illegal to do that in D, and whether the compiler issues an error or not is a quality of implementation issue. The trouble is, for simple cases the compiler can detect it. For complex cases, it cannot, therefore thecan't require that it diagnose the error. Also, it is an error, not a warning <g>. The exact same problem exists in C and C++.
Oct 18 2003