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.IX Title "PERLMROAPI 1"
.TH PERLMROAPI 1 2024-04-17 "perl v5.38.2" "Perl Programmers Reference Guide"
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.SH NAME
perlmroapi \- Perl method resolution plugin interface
.SH DESCRIPTION
.IX Header "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.
.PP
Each plugin should register itself by providing
the following structure
.PP
.Vb 7
\&    struct mro_alg {
\&        AV *(*resolve)(pTHX_ HV *stash, U32 level);
\&        const char *name;
\&        U16 length;
\&        U16 kflags;
\&        U32 hash;
\&    };
.Ve
.PP
and calling \f(CW\*(C`Perl_mro_register\*(C'\fR:
.PP
.Vb 1
\&    Perl_mro_register(aTHX_ &my_mro_alg);
.Ve
.IP resolve 4
.IX Item "resolve"
Pointer to the linearisation function, described below.
.IP name 4
.IX Item "name"
Name of the MRO, either in ISO\-8859\-1 or UTF\-8.
.IP length 4
.IX Item "length"
Length of the name.
.IP kflags 4
.IX Item "kflags"
If the name is given in UTF\-8, set this to \f(CW\*(C`HVhek_UTF8\*(C'\fR. The value is passed
direct as the parameter \fIkflags\fR to \f(CWhv_common()\fR.
.IP hash 4
.IX Item "hash"
A precomputed hash value for the MRO's name, or 0.
.SH Callbacks
.IX Header "Callbacks"
The \f(CW\*(C`resolve\*(C'\fR 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 \fIlevel\fR of 0. The core always sets \fIlevel\fR to 0 when it calls your
function \- the parameter is provided to allow your implementation to track
depth if it needs to recurse.
.PP
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 \f(CWHvENAME()\fR on the stash. In those cases where
\&\f(CWHvENAME()\fR returns null, \f(CWHvNAME()\fR should be used instead.
.PP
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, \f(CWsv_2mortal()\fR 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.
.SH Caching
.IX Header "Caching"
Computing MROs can be expensive. The implementation provides a cache, in
which you can store a single \f(CW\*(C`SV *\*(C'\fR, or anything that can be cast to
\&\f(CW\*(C`SV *\*(C'\fR, such as \f(CW\*(C`AV *\*(C'\fR. To read your private value, use the macro
\&\f(CWMRO_GET_PRIVATE_DATA()\fR, passing it the \f(CW\*(C`mro_meta\*(C'\fR structure from the
stash, and a pointer to your \f(CW\*(C`mro_alg\*(C'\fR structure:
.PP
.Vb 2
\&    meta = HvMROMETA(stash);
\&    private_sv = MRO_GET_PRIVATE_DATA(meta, &my_mro_alg);
.Ve
.PP
To set your private value, call \f(CWPerl_mro_set_private_data()\fR:
.PP
.Vb 1
\&    Perl_mro_set_private_data(aTHX_ meta, &c3_alg, private_sv);
.Ve
.PP
The private data cache will take ownership of a reference to private_sv,
much the same way that \f(CWhv_store()\fR takes ownership of a reference to the
value that you pass it.
.SH Examples
.IX Header "Examples"
For examples of MRO implementations, see \f(CWS_mro_get_linear_isa_c3()\fR
and the \f(CW\*(C`BOOT:\*(C'\fR section of \fIext/mro/mro.xs\fR, and
\&\f(CWS_mro_get_linear_isa_dfs()\fR in \fImro_core.c\fR
.SH AUTHORS
.IX Header "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.