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On Fri, Dec 16, 2005 at 08:17:40AM +0900, Ron Jeffries wrote:
On Thu, 15 Dec 2005 14:50:59 +0900, "Daniel Sheppard" <daniels@pronto.com.au> > wrote:
>I might be wrong - but I'm pretty sure that substrings in ruby are
>created with copy-on-write. That is, when you take a substring, a new
>block of memory isn't allocated to the new String, it references the
>same block of memory as the original string - the allocation of a new
>block of memory only occurs when one of the strings is modified.
I asked about this a week or two ago, as part of my Extended Set Theory thing,
and the impression I got from the answers then was that sub /arrays/ are copy on
write but that sub /strings/ were not.
I wonder how one might confirm this, other than by asking matz or reading the
compiler and libraries ...
-----------------------------------------------------------------------------
VALUE
rb_str_substr(str, beg, len)
VALUE str;
long beg, len;
{
VALUE str2;
if (len < 0) return Qnil;
if (beg > RSTRING(str)->len) return Qnil;
if (beg < 0) {
beg += RSTRING(str)->len;
if (beg < 0) return Qnil;
}
if (beg + len > RSTRING(str)->len) {
len = RSTRING(str)->len - beg;
}
if (len < 0) {
len = 0;
}
if (len == 0) {
str2 = rb_str_new5(str,0,0);
}
else if (len > sizeof(struct RString)/2 &&
beg + len == RSTRING(str)->len && !FL_TEST(str, STR_ASSOC)) {
/* ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ */
str2 = rb_str_new3(rb_str_new4(str));
RSTRING(str2)->ptr += RSTRING(str2)->len - len;
RSTRING(str2)->len = len;
}
else {
str2 = rb_str_new5(str, RSTRING(str)->ptr+beg, len);
}
OBJ_INFECT(str2, str);
return str2;
}
-----------------------------------------------------------------------------
... (optionally) ...
-----------------------------------------------------------------------------
static VALUE
str_new3(klass, str)
VALUE klass, str;
{
VALUE str2 = str_alloc(klass);
RSTRING(str2)->len = RSTRING(str)->len;
RSTRING(str2)->ptr = RSTRING(str)->ptr;
RSTRING(str2)->aux.shared = str;
FL_SET(str2, ELTS_SHARED);
OBJ_INFECT(str2, str);
return str2;
}
VALUE
rb_str_new3(str)
VALUE str;
{
return str_new3(rb_obj_class(str), str);
}
static VALUE
str_new4(klass, str)
VALUE klass, str;
{
VALUE str2 = str_alloc(klass);
RSTRING(str2)->len = RSTRING(str)->len;
RSTRING(str2)->ptr = RSTRING(str)->ptr;
if (FL_TEST(str, ELTS_SHARED)) {
FL_SET(str2, ELTS_SHARED);
RSTRING(str2)->aux.shared = RSTRING(str)->aux.shared;
}
else {
FL_SET(str, ELTS_SHARED);
RSTRING(str)->aux.shared = str2;
}
return str2;
}
VALUE
rb_str_new4(orig)
VALUE orig;
{
VALUE klass, str;
if (OBJ_FROZEN(orig)) return orig;
klass = rb_obj_class(orig);
if (FL_TEST(orig, ELTS_SHARED) && (str = RSTRING(orig)->aux.shared) && klass == RBASIC(str)->klass) {
long ofs;
ofs = RSTRING(str)->len - RSTRING(orig)->len;
if (ofs > 0) {
str = str_new3(klass, str);
RSTRING(str)->ptr += ofs;
RSTRING(str)->len -= ofs;
}
}
else if (FL_TEST(orig, STR_ASSOC)) {
str = str_new(klass, RSTRING(orig)->ptr, RSTRING(orig)->len);
}
else {
str = str_new4(klass, orig);
}
OBJ_INFECT(str, orig);
OBJ_FREEZE(str);
return str;
}
VALUE
rb_str_new5(obj, ptr, len)
VALUE obj;
const char *ptr;
long len;
{
return str_new(rb_obj_class(obj), ptr, len);
}
-----------------------------------------------------------------------------
--
Mauricio Fernandez
