Corrected patch for libstdc++/4354

[Paolo Carlini]

Hi again,

I hope you will forgive me my very slow progress toward a working
solution: this is my first patch to gcc/libstdc++!

The fact is, my previous attempts were badly wrong.

I did'nt completely understand the reference counting issues in
_M_mutate and therefore I wrongly proposed a solution calling
_M_copy_chars *before* _M_mutate: this is not correct every time the
string is shared. In fact my patches badly broken the string class :( as
revealed by any simple test adding to Roberto's a string bux initialized
at the beginning as = aux, which became corrupted as an unwanted effect
of the aux.assign.

Now I believe that perhaps there is no simple escape to temporarily
copying the source data in a temporary string (we could avoid this if we
were sure that destination != source). Therefore the following patch is
my current proposal, which survives a make check and all the variants of
Roberto's testcase I could think of (also involving shared copies of
aux, of course)

Cheers,
Paolo.


--- basic_string.tcc.orig       Sun Sep 23 16:56:21 2001
+++ basic_string.tcc    Sun Sep 23 17:00:11 2001
@@ -453,10 +453,14 @@
        if (__dmax <= __dnew)
          __throw_length_error("basic_string::_M_replace");
        size_type __off = __i1 - _M_ibegin();
+       // Save concerned source string data in a temporary
+       const basic_string __temp = string(__k1, __k2);
        _M_mutate(__off, __dold, __dnew);
-       // Invalidated __i1, __i2
+       // Invalidated __i1, __i2 (and clobbered original source string
+       // data when destination string == source string and the string
+       // is unshared)
        if (__dnew)
-         _S_copy_chars(_M_data() + __off, __k1, __k2);
+         _S_copy_chars(_M_data() + __off, __temp.begin(),
__temp.end());

        return *this;
       }