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perlmroapi - Perl method resolution plugin interface


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);

Pointer to the linearisation function, described below.


Name of the MRO, either in ISO-8859-1 or UTF-8.


Length of the name.


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().


A precomputed hash value for the MRO's name, or 0.


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 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.


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.


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


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.