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.IX Title "PERLMROAPI 1"
.TH PERLMROAPI 1 "2021-09-24" "perl v5.32.1" "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 \s-1MRO,\s0 either in \s-1ISO\-8859\-1\s0 or \s-1UTF\-8.\s0
.IP "length" 4
.IX Item "length"
Length of the name.
.IP "kflags" 4
.IX Item "kflags"
If the name is given in \s-1UTF\-8,\s0 set this to \f(CW\*(C`HVhek_UTF8\*(C'\fR. The value is passed
direct as the parameter \fIkflags\fR to \f(CW\*(C`hv_common()\*(C'\fR.
.IP "hash" 4
.IX Item "hash"
A precomputed hash value for the \s-1MRO\s0's name, or 0.
.SH "Callbacks"
.IX Header "Callbacks"
The \f(CW\*(C`resolve\*(C'\fR function is called to generate a linearised \s-1ISA\s0 for the
given stash, using this \s-1MRO.\s0 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 the parent
classes in order. The names of the classes should be the result of calling
\&\f(CW\*(C`HvENAME()\*(C'\fR on the stash. In those cases where \f(CW\*(C`HvENAME()\*(C'\fR returns null,
\&\f(CW\*(C`HvNAME()\*(C'\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(CW\*(C`sv_2mortal()\*(C'\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(CW\*(C`MRO_GET_PRIVATE_DATA()\*(C'\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(CW\*(C`Perl_mro_set_private_data()\*(C'\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(CW\*(C`hv_store()\*(C'\fR takes ownership of a reference to the
value that you pass it.
.SH "Examples"
.IX Header "Examples"
For examples of \s-1MRO\s0 implementations, see \f(CW\*(C`S_mro_get_linear_isa_c3()\*(C'\fR
and the \f(CW\*(C`BOOT:\*(C'\fR section of \fIext/mro/mro.xs\fR, and
\&\f(CW\*(C`S_mro_get_linear_isa_dfs()\*(C'\fR in \fImro_core.c\fR
.SH "AUTHORS"
.IX Header "AUTHORS"
The implementation of the C3 \s-1MRO\s0 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.