NAME
perlmroapi - Perl method resolution plugin interface
DESCRIPTION
As of Perl 5.10.1 there is a new interface for plugging and using
method resolution orders other than the default (linear depth first
search). The C3 method resolution order added in 5.10.0 has been re-
implemented as a plugin, without changing its Perl-space interface.
Each plugin should register itself with "Perl_mro_register" by
providing the following structure
struct mro_alg {
AV *(*resolve)(pTHX_ HV *stash, U32 level);
const char *name;
U16 length;
U16 kflags;
U32 hash;
};
resolve
Pointer to the linearisation function, described below.
name
Name of the MRO, either in ISO-8859-1 or UTF-8.
length
Length of the name.
kflags
If the name is given in UTF-8, set this to "HVhek_UTF8". The value
is passed direct as the parameter kflags to "hv_common()".
hash
A precomputed hash value for the MRO’s name, or 0.
Callbacks
The "resolve" function is called to generate a linearised ISA for the
given stash, using this MRO. It is called with a pointer to the stash,
and a level of 0. The core always sets level to 0 when it calls your
function - the parameter is provided to allow your implementation to
track depth if it needs to recurse.
The function should return a reference to an array containing the
parent classes in order. The caller is responsible for incrementing the
reference count if it wants to keep the structure. Hence if you have
created a temporary value that you keep no pointer to, "sv_2mortal()"
to ensure that it is disposed of correctly. If you have cached your
return value, then return a pointer to it without changing the
reference count.
Caching
Computing MROs can be expensive. The implementation provides a cache,
in which you can store a single "SV *", or anything that can be cast to
"SV *", such as "AV *". To read your private value, use the macro
"MRO_GET_PRIVATE_DATA()", passing it the "mro_meta" structure from the
stash, and a pointer to your "mro_alg" structure:
meta = HvMROMETA(stash);
private_sv = MRO_GET_PRIVATE_DATA(meta, &my_mro_alg);
To set your private value, call "Perl_mro_set_private_data()":
Perl_mro_set_private_data(aTHX_ meta, &c3_alg, private_sv);
The private data cache will take ownership of a reference to
private_sv, much the same way that "hv_store()" takes ownership of a
reference to the value that you pass it.
Examples
For examples of MRO implementations, see "S_mro_get_linear_isa_c3()"
and the "BOOT:" section of mro/mro.xs, and "S_mro_get_linear_isa_dfs()"
in mro.c
AUTHORS
The implementation of the C3 MRO and switchable MROs within the perl
core was written by Brandon L Black. Nicholas Clark created the
pluggable interface, refactored Brandon’s implementation to work with
it, and wrote this document.