binding structs and ctor/dtor with tolua++

Let's say I want to bind a piece of code to Lua that looks like this:

typedef struct bar {
  void * some_data;
} bar;
bar * bar_create(void);
void bar_do_something(bar * baz);
void bar_free(bar * baz);

I want to create these objects from a Lua script, and not explicitly manage their lifetime. Prefereably, I would like my script to write

require "foo"
local baz = foo:bar()
baz:do_something()
baz = nil

Problem: For that to work as expected, I need to somehow tell tolua++ about bar_create and bar_free being the constructor/destructor for bar. How do I do that? For classes, tolua++ claims to automatically use their ctor/dtor, but for structs?

The best thing I can come up with is this definition of foo.pkg:

module foo {
  struct bar {
    static tolua_outside bar_create @ create();
    tolua_outside bar_do_something @ do_something();
    tolua_outside bar_free @ free();
  };
}

which would mean I have to call create() and free() explicitly.

Solution

The bar functions can be imported into Lua using tolua++ and wrapped to yield an object-style interface, garbage collection included.

To demonstrate the passing of arguments, I have changed the bar interface to

bar * bar_create(int x);
int bar_do_something(bar * baz, int y);
void bar_free(bar * baz);

and written a test implementation that prints out x, y, etc. when the functions are called.

The bar_create() Lua function returns a userdata value. Lua deallocates such user data by calling the __gc method stored in the metatable of the data. Given a userdata value and a destructor gc, the __gc method is overwritten such that it first calls gc and then calls the original gc method:

function wrap_garbage_collector(userdata, gc)
    local mt = getmetatable(userdata)
    local old_gc = mt.__gc
    function mt.__gc (data)
        gc(data)
        old_gc(data)
    end
end

Userdata of the same type share the same metatable; therefore the wrap_garbage_collector() function should be called only once for each class (assuming that tolua++'s metatables are constructed once and deallocated only at exit).

At the bottom of this answer is a complete bar.pkg file that imports the bar functions and adds a bar class to a Lua module named foo. The foo module is loaded into the interpreter (see for example my SO tolua++ example) and used like this:

bars = {}

for i = 1, 3 do
    bars[i] = foo.bar(i)
end

for i = 1, 3 do
    local result = bars[i]:do_something(i * i)
    print("result:", result)
end

The test implementation prints out what happens:

bar(1)
bar(2)
bar(3)
bar(1)::do_something(1)
result: 1
bar(2)::do_something(4)
result: 8
bar(3)::do_something(9)
result: 27
~bar(3)
~bar(2)
~bar(1)

The construction of the bar class below is a little elaborate: the build_class() utility returns a class (a Lua table) given the constructor, destructor, and the class methods. Adjustments will no doubt be needed, but as a prototype demonstration the example should be OK.

$#include "bar.hpp"

// The bar class functions.
bar * bar_create(int x);
int bar_do_something(bar * baz, int y);
void bar_free(bar * baz);

$[
    -- Wrapping of the garbage collector of a user data value.
    function wrap_garbage_collector(userdata, gc)
        local mt = getmetatable(userdata)
        local old_gc = mt.__gc
        function mt.__gc (data)
            gc(data)
            old_gc(data)
        end
    end

    -- Construction of a class.
    --
    -- Arguments:
    --
    --   cons : constructor of the user data
    --   gc : destructor of the user data
    --   methods : a table of pairs { method = method_fun }
    --
    -- Every 'method_fun' of 'methods' is passed the user data 
    -- as the first argument.
    --
    function build_class(cons, gc, methods)
        local is_wrapped = false
        function class (args)
            -- Call the constructor.
            local value = cons(args)

            -- Adjust the garbage collector of the class (once only).
            if not is_wrapped then
                wrap_garbage_collector(value, gc)
                is_wrapped = true
            end

            -- Return a table with the methods added.
            local t = {}
            for name, method in pairs(methods) do
                t[name] =
                    function (self, ...)
                        -- Pass data and arguments to the method.
                        return (method(value, ...))
                    end
            end

            return t
        end
        return class
    end

    -- The Lua module that contains our classes.
    foo = foo or {}

    -- Build and assign the classes.
    foo.bar =
        build_class(bar_create, bar_free,
                    { do_something = bar_do_something })

    -- Clear global functions that shouldn't be visible.
    bar_create = nil
    bar_free = nil
    bar_do_something = nil
$]