---
name: "perlmroapi"
section: "1"
platform: "linux"
package: "perl-doc"
version: "5.40.1-6+deb13u1"
release: "Debian 13 (Trixie)"
platforms: ["linux", "openbsd"]
url: "https://new.carta.tech/perlmroapi"
---

# perlmroapi(1)

Perl method resolution plugin interface

## 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.