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# perlmroapi

Perl 5 version 12.2 documentation
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# perlmroapi

- 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 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.

Page index

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