On Mon, 22 Feb 2010 08:32:20 -0500, strtr <st...@spam.com> wrote:
Daniel Keep Wrote:
strtr wrote:
> On winXP (D1) I can compile/run this code without a problem.
> Not even a warning.
>
> void main() {
> char[] s= "immutable literal?";
> s[$-1] = '!';
> writefln(s);
> }
> Codepad runs into a segmentation fault.
> http://codepad.org/NQfsRoR5
>
> Why doesn't it result in a compiler error or warning?
> If it did I would have noticed this quirk earlier.
There's no compiler error because D1 doesn't have a const/immutable
system.
There's no crash because Windows doesn't write-protect the data segment
which contains the literal.
But according to the specs, it does constitute an error and I suspect
string literals are placed in a specific memory location.
Wouldn't it be possible to error on such code?
Yes, it is possible -- use D2 which has such errors :) Without a const
system, D1 cannot distinguish between char[] that originated from a
literal (i.e. in the static data segment) or which originated from the
heap. At the point where you see the line:
s[$-1] = '!';
You don't have the entire history of where s came from. It can be even
harder to detect than you think. For instance, should you allow the
following code to compile?
int main(char[][] args)
{
char[] s;
if(args[1] == "y")
s = "immutable literal?";
else
s = "mutable literal?".dup;
s[$-1] = "!";
return 0;
}
This program runs fine unless you pass the exact argument 'y' to the
program, and then it crashes. How does the compiler know at the line
where s is modified that it could have possibly come from a literal?
If you still think it's possible, what about this?
void exclaim(char[] s)
{
s[$-1] = '!';
}
If this is in its own module, how can the compiler tell whether s is
immutable or not?
-Steve
