Anno, that seems to me to be a reasonable guess .... but I'm looking at a '+=' overload XSub that neither increments the refcount nor (afaict) returns anything ... and I'm wondering "how the hell does that work ?". It's from the GMP module - which is not to be found on CPAN. (It ships with the gmp source - in the demos/perl folder.) I don't know if it's going to help the discussion, but here's that XSub: 
void
overload_addeq (x, y, o)
    mpz_assume   x
    mpz_coerce   y
    order_noswap o
ALIAS:
    GMP::Mpz::overload_subeq = 1
    GMP::Mpz::overload_muleq = 2
    GMP::Mpz::overload_diveq = 3
    GMP::Mpz::overload_remeq = 4
    GMP::Mpz::overload_andeq = 5
    GMP::Mpz::overload_ioreq = 6
    GMP::Mpz::overload_xoreq = 7
PREINIT:
    static_functable const struct {
      void (*op) (mpz_ptr, mpz_srcptr, mpz_srcptr);
    } table[] = {
      { mpz_add    }, /* 0 */
      { mpz_sub    }, /* 1 */
      { mpz_mul    }, /* 2 */
      { mpz_tdiv_q }, /* 3 */
      { mpz_tdiv_r }, /* 4 */
      { mpz_and    }, /* 5 */
      { mpz_ior    }, /* 6 */
      { mpz_xor    }, /* 7 */
    };
PPCODE:
    assert_table (ix);
    (*table[ix].op) (x->m, x->m, y);
    XPUSHs (ST(0));

[download]
First up - I don't really understand this XSub at all. I gather it's also handling -=, *=, /=, %=, &=, |= and ^= via some sort of lookup. The XPUSHs() also puzzles me. What does that achieve ?

Anyway, it seems to me that this particular module has overloaded '+=' and friends without any need to return anything, and without any need to increase the refcount. (I've searched the entire XS file and there's not a single SvREFCNT_inc() to be found.)

Cheers,
Rob
PS: I haven't actually built that GMP module for a while. I assume it works fine with perl-5.8.8, but that's something that's unverified. Last time I built this module, I also benchmarked the overloading and found it to be very efficient - not surprising, given that the author is a smart and talented programmer.

In reply to Re^2: Overload '+=' with XSub by syphilis
in thread Overload '+=' with XSub by syphilis

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