FFI: Add ctype metamethods and ffi.metatype().

doc/ext_ffi_api.html

@@ -238,6 +238,31 @@ This functions is mainly useful to override the pointer compatibility
 checks or to convert pointers to addresses or vice versa.
 </p>
 
+<h3 id="ffi_metatype"><tt>ctype = ffi.metatype(ct, metatable)</tt></h3>
+<p>
+Creates a ctype object for the given <tt>ct</tt> and associates it with
+a metatable. Only <tt>struct</tt>/<tt>union</tt> types, complex numbers
+and vectors are allowed. Other types may be wrapped in a
+<tt>struct</tt>, if needed.
+</p>
+<p>
+The association with a metatable is permanent and cannot be changed
+afterwards. Neither the contents of the <tt>metatable</tt> nor the
+contents of an <tt>__index</tt> table (if any) may be modified
+afterwards. The associated metatable automatically applies to all uses
+of this type, no matter how the objects are created or where they
+originate from. Note that pre-defined operations on types have
+precedence (e.g. declared field names cannot be overriden).
+</p>
+<p>
+All standard Lua metamethods are implemented. These are called directly,
+without shortcuts and on any mix of types. For binary operations, the
+left operand is checked first for a valid ctype metamethod. The
+<tt>__gc</tt> metamethod only applies to <tt>struct</tt>/<tt>union</tt>
+types and performs an implicit <a href="#ffi_gc"><tt>ffi.gc()</tt></a>
+call during creation of an instance.
+</p>
+
 <h3 id="ffi_gc"><tt>cdata = ffi.gc(cdata, finalizer)</tt></h3>
 <p>
 Associates a finalizer with a pointer or aggregate cdata object. The

doc/ext_ffi_semantics.html

@@ -582,6 +582,10 @@ Reference types are dereferenced <em>before</em> performing each of
 the operations below &mdash; the operation is applied to the
 C&nbsp;type pointed to by the reference.
 </p>
+<p>
+The pre-defined operations are always tried first before deferring to a
+metamethod for a ctype (if defined).
+</p>
 
 <h3 id="cdata_array">Indexing a cdata object</h3>
 <ul>
@@ -803,9 +807,10 @@ vararg functions</a>.
 </p>
 <p>
 Memory areas returned by C functions (e.g. from <tt>malloc()</tt>)
-must be manually managed, of course. Pointers to cdata objects are
-indistinguishable from pointers returned by C functions (which is one
-of the reasons why the GC cannot follow them).
+must be manually managed, of course (or use
+<a href="ext_ffi_api.html#ffi_gc"><tt>ffi.gc()</tt></a>)). Pointers to
+cdata objects are indistinguishable from pointers returned by C
+functions (which is one of the reasons why the GC cannot follow them).
 </p>
 
 <h2 id="clib">C Library Namespaces</h2>
@@ -977,6 +982,9 @@ two.</li>
 value.</li>
 <li>Calls to C&nbsp;functions with 64 bit arguments or return values
 on 32 bit CPUs.</li>
+<li>Calls to ctype metamethods which are not plain functions.</li>
+<li>ctype <tt>__newindex</tt> tables and non-string lookups in ctype
+<tt>__index</tt> tables.</li>
 <li>Accesses to external variables in C&nbsp;library namespaces.</li>
 <li><tt>tostring()</tt> for cdata types.</li>
 <li>The following <a href="ext_ffi_api.html">ffi.* API</a> functions:
@@ -988,7 +996,6 @@ Other missing features:
 <ul>
 <li>Bit operations for 64&nbsp;bit types.</li>
 <li>Arithmetic for <tt>complex</tt> numbers.</li>
-<li>User-defined metamethods for C&nbsp;types.</li>
 <li>Callbacks from C&nbsp;code to Lua functions.</li>
 <li>Atomic handling of <tt>errno</tt>.</li>
 <li>Passing structs by value to vararg C&nbsp;functions.</li>

doc/ext_ffi_tutorial.html

@@ -386,6 +386,99 @@ application might work on some systems, but would fail in a POSIX/x64
 environment.
 </p>
 
+<h2 id="metatype">Defining Metamethods for a C&nbsp;Type</h2>
+<p>
+The following code explains how to define metamethods for a C type.
+We define a simple point type and add some operations to it:
+</p>
+<pre class="code mark">
+<span class="codemark">&nbsp;
+&#9312;
+
+
+
+&#9313;
+
+&#9314;
+
+&#9315;
+
+
+
+&#9316;
+
+&#9317;</span>local ffi = require("ffi")
+ffi.cdef[[
+<span style="color:#00a000;">typedef struct { double x, y; } point_t;</span>
+]]
+
+local point
+local mt = {
+  __add = function(a, b) return point(a.x+b.x, a.y+b.y) end,
+  __len = function(a) return math.sqrt(a.x*a.x + a.y*a.y) end,
+  __index = {
+    area = function(a) return a.x*a.x + a.y*a.y end,
+  },
+}
+point = ffi.metatype("point_t", mt)
+
+local a = point(3, 4)
+print(a.x, a.y)  --> 3  4
+print(#a)        --> 5
+print(a:area())  --> 25
+local b = a + point(0.5, 8)
+print(#b)        --> 12.5
+</pre>
+<p>
+Here's the step-by-step explanation:
+</p>
+<p>
+<span class="mark">&#9312;</span> This defines the C&nbsp;type for a
+two-dimensional point object.
+</p>
+<p>
+<span class="mark">&#9313;</span> We have to declare the variable
+holding the point constructor first, because it's used inside of a
+metamethod.
+</p>
+<p>
+<span class="mark">&#9314;</span> Let's define an <tt>__add</tt>
+metamethod which adds the coordinates of two points and creates a new
+point object. For simplicity, this function assumes that both arguments
+are points. But it could be any mix of objects, if at least one operand
+is of the required type (e.g. adding a point plus a number or vice
+versa). Our <tt>__len</tt> metamethod returns the distance of a point to
+the origin.
+</p>
+<p>
+<span class="mark">&#9315;</span> If we run out of operators, we can
+define named methods, too. Here the <tt>__index</tt> table defines an
+<tt>area</tt> function. For custom indexing needs, one might want to
+define <tt>__index</tt> and <tt>__newindex</tt> functions instead.
+</p>
+<p>
+<span class="mark">&#9316;</span> This associates the metamethods with
+our C&nbsp;type. This only needs to be done once. For convenience, a
+constructor is returned by
+<a href="ffi_ext_api.html#ffi_metatype"><tt>ffi.metatype()</tt></a>.
+We're not required to use it, though. The original C&nbsp;type can still
+be used e.g. to create an array of points. The metamethods automatically
+apply to any and all uses of this type.
+</p>
+<p>
+Please note that the association with a metatable is permanent and
+<b>the metatable must not be modified afterwards!</b> Ditto for the
+<tt>__index</tt> table.
+</p>
+<p>
+<span class="mark">&#9317;</span> Here are some simple usage examples
+for the point type and their expected results. The pre-defined
+operations (such as <tt>a.x</tt>) can be freely mixed with the newly
+defined metamethods. Note that <tt>area</tt> is a method and must be
+called with the Lua syntax for methods: <tt>a:area()</tt>, not
+<tt>a.area()</tt>.
+</p>
+
 <h2 id="idioms">Translating C&nbsp;Idioms</h2>
 <p>
 Here's a list of common C&nbsp;idioms and their translation to the