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D - Slicing local variables?

reply "Matthew Wilson" <matthew stlsoft.org> writes:
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
parent reply "Walter" <walter digitalmars.com> writes:
"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
next sibling parent "Matthew Wilson" <matthew stlsoft.org> writes:
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
prev sibling parent reply "Sean L. Palmer" <palmer.sean verizon.net> writes:
"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.

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.. Sean
Oct 18 2003
next sibling parent "Matthew Wilson" <matthew stlsoft.org> writes:
"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$4 digitaldaemon.com...
 "Matthew Wilson" <matthew stlsoft.org> wrote in message
 news:bmqqet$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.

Wouldn't the compiler try to catch errors like this at compile time?

 data flow.  Local variables cannot outlive the function, and cannot be

 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..

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
prev sibling parent reply "Walter" <walter digitalmars.com> writes:
"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$4 digitaldaemon.com...
 "Matthew Wilson" <matthew stlsoft.org> wrote in message
 news:bmqqet$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.

Wouldn't the compiler try to catch errors like this at compile time?

 data flow.  Local variables cannot outlive the function, and cannot be

 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..

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
parent "Sean L. Palmer" <palmer.sean verizon.net> writes:
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...
 Wouldn't the compiler try to catch errors like this at compile time?

 data flow.  Local variables cannot outlive the function, and cannot be

 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..

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

 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