Re: copy_string will never terminate - recursive function problem

Hi Markus,

this isn't by design, but it is a bug in the design of 
nsDependentConcatenation. We've tried to squash it in time for Mozilla 1.0 but 
unfortunately didn't make it. Right now it means two things:

1) you can't concatenate a concatenation if we can't tell it's a concatenation 
by the string's type e.g.:

   // XXX won't work :-(
   function f(const nsAString& inStr) {
     mStr = inStr + mStr;
   }

   f(str1 + str2);

This doesn't work because by passing the concatenation (str1+str2) into 
function f you lose the type information, and the concatenation (mStr+inStr) 
inside function f can't properly deal with not knowing that inStr is also a 
concatenation.

   // Suggested work-around:
   nsAutoString myStr(str1 + str2);
   f(myStr);

2) you can't have a concatenation as the right hand side of another 
concatenation, e.g.:

   // XXX won't work :-(
   function f(const nsDependentString& inStr) {
     mStr = mStr + inStr;
   }

   f(str1 + str2);


As a rule of thumb, don't pass a concatenation into a function.
Note that we do it in places in Mozilla code where we know we can get away 
with it (for now).

Your problem is a variation of #1. Another way to work around your problem:

void test(int iteration, const nsDependentConcatenation& prefix)
{
     if(iteration>10) {
         nsAutoString result;
         result=prefix;
         return;
     }
     test(iteration+1, prefix+NS_LITERAL_STRING(".x"));
}
void test(int iteration, const nsAString& prefix)
{
     test(iteration+1, prefix+NS_LITERAL_STRING(".x"));
}
test(0, NS_LITERAL_STRING("x"));

Or skip the nsAString version of test and use this call:

test(0, NS_LITERAL_STRING("")+NS_LITERAL_STRING("x"));

Yeah, those are kinda ugly, but it'll save you a few copies along the way.

I hope this helps,

   jag

Markus Essl wrote:
> Hi!
> 
> I hope this is the right newsgroup, and I hope you can answer my question:
> 
> I have a problem with an nsDependentConnection when called recursivly:
> 
> (result contains x.x\0\0\0\0 - but sometimes the copy itererator won't 
> finish because bytes_copied is 0, and first!=last will never get true.. 
> (nsAlgorithm.h:91)
> 
> void test_fails(int iteration, const nsAString& prefix)
> {
>     if(iteration>10) {
>         nsAutoString result;
>         result=prefix;
>         return;
>     }
>     test_fails(iteration+1, prefix+NS_LITERAL_STRING(".x"));
> }
> 
> But this works: (result.mUStr contains "x.x.x.x.x.x.x.x.x.x.x")
> 
> void test(int iteration, const nsAString& prefix)
> {
>     if(iteration>10) {
>         nsAutoString result;
>         result=prefix;
>         return;
>     }
>     nsAutoString string;
>     string=prefix;
>     test(iteration+1, string+NS_LITERAL_STRING(".x"));
> }
> test(0, NS_LITERAL_STRING("x"));
> 
> 
> Is this by design (i don't hope so!) or is this a bug? I couldn't find 
> anything in bugzilla. BTW, I use the RC3 codebase.
> 
> bye,
> Markus
>