package.preload alternative in Lua? - c++

Question

I learned that package.preload can be used to expose a script to other scripts.
Here's my example code.
lua_State *L = luaL_newstate();
luaL_openlibs(L);
lua_settop(L, 0);
//Script A
luaL_dostring(L, "local A = {} package.preload['A'] = function () return A end A.num = 3");
//Script B
luaL_dostring(L, "local A = require 'A' print(A.num)");
lua_close(L);
The result: 3
Although this works fine, I wonder if Script A's code can be more simplified or if there's other alternative solution to expose a script to other scripts.
ADDED: The main reason I'm asking this is because I think package.preload['A'] = function () return A end is quite long and boring to write.

Answer 1

In this case, where you have some set of in-C strings that represent Lua modules, package.preload is exactly the tool to use. Though your specific use of it leaves something to be desired.
Generally speaking, the modules themselves do not define their names. So hard-coding the module's name into the string is not the correct move. Similarly, modules do not register themselves; they should be registered by the environment around the module.
What you really want is to take an array of name+Lua code strings and register them as module preloads in a loop. So you'd have something like this. I'll be using Lua 5.3; you can translate it to older version of Lua pretty easily.
Also, be warned: this code is untested.
const char *lua_preloads[] =
{
"A", "local A = {}\n"
"A.num = 3\n"
"return A)\n", //Modules are usually tables, not functions.
...
NULL //Null-terminated list.
};
//Loader function
int lua_preloader_func(lua_State *L)
{
int nargs = lua_gettop(L);
int lua_func_ix = lua_upvalueindex(1);
lua_pushvalue(L, lua_func_ix);
//Move the function to the bottom of the stack
lua_insert(lua_func_ix, 1);
//Call with all of the given arguments.
lua_call(L, nargs, LUA_MULTRET);
return lua_gettop(L);
}
int top = lua_gettop(L);
//Get the package.preload table.
lua_getglobal(L, "package");
lua_getfield(L, -1, "preload");
int preload_ix = lua_gettop();
for(const char **position = lua_preloads;
*position;
position += 2)
{
const char *module_name = position[0];
const char *module = position[1];
//Compile the preload script into a Lua function.
int err = luaL_loadbufferx(L, module, strlen(module), module_name, "t");
//Check for errors in `err`.
//Create a Lua C-function with the script as an upvalue.
lua_pushcclosure(L, lua_preloader_func, 1);
//Stick that Lua C-function inside of package.preload[preload.first].
lua_setfield(L, preload_ix, module_name);
}
lua_settop(L, top);

Answer 2

It seems as if you want to prefix local A = {} package.preload['A'] = function () return A end to every chunk defining a module (where A is the module name). I think it would be much easier to just use string concatenation for that.
#include <string>
#include <lua.hpp>
int preload(lua_State *L, std::string const &modname,
std::string const &modcode) {
std::string code = "package.preload['" + modname + "'] = function()\n" +
"local " + modname + " = {}\n" + modcode + "\n"
"return " + modname + " end";
return luaL_dostring(L, code.c_str());
}
int main() {
lua_State *L = luaL_newstate();
luaL_openlibs(L);
// Script A
preload(L, "A", "A.num = 3");
// Script B
luaL_dostring(L, "local A = require 'A' print(A.num)");
lua_close(L);
}