- NAME
- DESCRIPTION
- Callbacks
- Caching
- Examples
- AUTHORS

# 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 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.

# 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.