man > PERLMROAPI(1)

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 v5.38.2                                 2026-08-18                                PERLMROAPI(1)