How to handle C++ class construct failure in Lua - c++

Question

I use SWIG to bind C++ classes so I can use them in Lua.
I wonder if it's possible to handle construction failure of C++ class in Lua.
For example, I have the following Test class which tries to get the Userdata when it's constructed.
void *getUserdata(lua_State *L, const char *key)
{
lua_pushstring(L, key);
lua_gettable(L, LUA_REGISTRYINDEX);
return lua_touserdata(L, -1);
}
class Test
{
public:
Test(lua_State *L)
:data(static_cast<MyData *>(getUserdata(L, "my_name"))){};
void setDataNum(int num)
{
data->num = num;
}
private:
MyData *data;
};
However, if getUserdata() returns nullptr, calling setDataNum() crashes my application.
I wonder if there's any way to detect and handle the construction failure(data becoming nullptr in this case) so the class won't create in Lua.

Answer 1

The first step to error handling would be to check for the error. From the Lua reference manual:
void *lua_touserdata (lua_State *L, int index);
If the value at the given index is a full userdata, returns its block address. If the value is a light userdata, returns its pointer. Otherwise, returns NULL.
That means we can easily check whether the call succeeded by checking data for being NULL. Then we can act accordingly, I choose to throw an exception.
test.hpp
#pragma once
#include <stdexcept>
#include <lua.hpp>
struct MyData {
int num;
};
void *getUserdata(lua_State *L, const char *key) {
lua_pushstring(L, key);
lua_gettable(L, LUA_REGISTRYINDEX);
return lua_touserdata(L, -1);
}
class Test {
public:
Test(lua_State *L)
: data(static_cast<MyData *>(getUserdata(L, "my_name"))) {
if (data == nullptr) {
throw std::runtime_error("invalid userdata at \"my_name\"");
}
};
void setDataNum(int num) { data->num = num; }
private:
MyData *data;
};
This exception is not digestible by Lua and by default the interpreter will just crash with terminate called after throwing an instance of 'std::runtime_error'. This is not so nice and we rather want to have the exception converted to a Lua error. SWIG comes with support for that.
test.i
%module example
%{
#include "test.hpp"
%}
%include <exception.i>
%exception {
try {
$action
} catch (std::exception const &e) {
SWIG_exception(SWIG_RuntimeError, e.what());
}
}
%typemap(default) (lua_State *L) { $1 = L; }
%include "test.hpp"
Lua doesn't have exceptions and thus no try-catch blocks. Instead Lua has the concept of a protected call with pcall. This will return a flag whether the call was successful and the result of the call or an error.
local example = require("example")
local success, c = pcall(example.Test)
if (success) then
c:setDataNum(1)
else
print(c)
end
Example invocation:
$ swig -c++ -lua test.i
$ clang++ -Wall -Wextra -Wpedantic -I /usr/include/lua5.2 -fPIC -shared test_wrap.cxx -o example.so -llua5.2
$ lua5.2 test.lua
SWIG_RuntimeError:invalid userdata at "my_name"