PERLMROAPI(1) Perl Programmers Reference Guide PERLMROAPI(1)
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 by providing the following structure
struct mro_alg {
AV *(*resolve)(pTHX_ HV *stash, U32 level);
const char *name;
U16 length;
U16 kflags;
U32 hash;
};
and calling "Perl_mro_register":
Perl_mro_register(aTHX_ &my_mro_alg);
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 string SVs
giving the names of parent classes in order. The names of the classes
should be the result of calling HvENAME() on the stash. In those cases
where HvENAME() returns null, HvNAME() should be used instead.
The caller is responsible for incrementing the reference count of the
array returned 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 ext/mro/mro.xs, and S_mro_get_linear_isa_dfs() in
mro_core.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.
perl v5.40.1 2026-08-30 PERLMROAPI(1)
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