# Luabind - 'abort' when trying to call object method which contains lua errors

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Silverlan    662

I've used the example from http://www.rasterbar.com/products/luabind/docs.html#deriving-in-lua to define a class in c++ that I can derive from in lua:

class base
{
public:
base(const char* s)
{ std::cout << s << "\n"; }

virtual void f(int a)
{ std::cout << "f(" << a << ")\n"; }
};

struct base_wrapper : base, luabind::wrap_base
{
base_wrapper(const char* s)
: base(s)
{}

virtual void f(int a)
{
call<void>("f", a);
}

static void default_f(base* ptr, int a)
{
return ptr->base::f(a);
}
};

...

module(L)
[
class_<base, base_wrapper>("base")
.def(constructor<const char*>())
.def("f", &base::f, &base_wrapper::default_f)
];



I've then created a derived class in lua:

class 'base_derived' (base)

function base_derived:__init(str)
base.__init(self,str)
end

function base_derived:f()
this_function_doesnt_exist()
end


Any call to 'f' is supposed to throw a lua error, which works fine if I do it in lua:

local x = base_derived("Test")
x:f() -- Throws "attempt to call a nil value..." error


I'd like to do the equivalent of that, but in c++:

auto g = luabind::globals(l);
auto r = g["base_derived"];
if(r)
{
luabind::object o = r("Test");
auto gm = luabind::object_cast<base_wrapper*>(o);
if(gm != nullptr)
{
try
{
luabind::call_member<void>(o,"f",5);
}
catch(luabind::error &e)
{
std::cout<<"[LUA] Error: "<<e.what()<<std::endl;
}
}
o.push(l);
}


However the 'luabind::call_member'-call causes an abort in 'luabind/detail/call_member.hpp', line 258:

// Code snippet of luabind/detail/call_member.hpp
~proxy_member_void_caller()
{
if (m_called) return;

m_called = true;

// don't count the function and self-reference
// since those will be popped by pcall
int top = lua_gettop(L) - 2;

// pcall will pop the function and self reference
// and all the parameters

push_args_from_tuple<1>::apply(L, m_args);
if (pcall(L, boost::tuples::length<Tuple>::value + 1, 0))
{
assert(lua_gettop(L) == top + 1);
#ifndef LUABIND_NO_EXCEPTIONS
////////////////////////////////////////////
throw luabind::error(L); // LINE 258
////////////////////////////////////////////
#else
error_callback_fun e = get_error_callback();
if (e) e(L);

assert(0 && "the lua function threw an error and exceptions are disabled."
"If you want to handle this error use luabind::set_error_callback()");
std::terminate();
#endif
}
// pops the return values from the function
stack_pop pop(L, lua_gettop(L) - top);
}


The exception in that line isn't actually thrown, but it is what causes the abort.

However, the abort only happens if the lua-functions causes a lua error. If I comment the 'this_function_doesnt_exist()'-call, both the lua- and c++-versions run just fine.

Why is the 'throw luabind::error(L);' causing an abort and what can I do to safely call the function from c++ even with potential lua errors?

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BitMaster    8651
Are you absolutely certain LUABIND_NO_EXCEPTIONS is not defined? If the line is definitely reached, is it possible the error happens because something bad happens when luabind::error tries to read extra information from the Lua state? I believe the default behavior of Lua is to call exit when it needs to raise an error (see also Lua's lua_atpanic.

Additionally, if you try to throw an exception out of any function which is marked as noexcept you will get terminate called. Note that none of the things I listed actually call abort(), but it might depend on your exact compiler.

The only thing I can think of which explicitly calls abort() would be a failed assert. With a decent debugger you should be able to find the exact line where things go wrong.

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Silverlan    662

Additionally, if you try to throw an exception out of any function which is marked as noexcept you will get terminate called. Note that none of the things I listed actually call abort(), but it might depend on your exact compiler.

Actually, the solution was to add noexcept to the destructor. Someone on [URL=http://stackoverflow.com/a/33099844/2482983]stack overflow[/URL] posted this idea, I'll just paste his response here for better visibility, in case anyone else has the same problem:

See line 254:

if (pcall(L, boost::tuples::length<Tuple>::value + 1, 0))

Here, pcall is asking lua to execute your x:f(5) equivalent call. Apparently this code returns an error because pcall() returns something different than zero. This is expected because you are _indeed_ creating an error in lua by calling this_function_doesnt_exist(). This also explains why the abort does not happen when you comment the this_function_doesnt_exist() call.

Then, to the C++ code:

throw luabind::error(L);

This error is thrown from a destructor: ~proxy_member_void_caller(), and it turns out that [throwing an exception from a destructor is bad practice][1]. This problem is known for luabind ([see this question][2]) to trigger the call of abort without even throwing.

The solution is to add noexcept(false)to the signature of ~proxy_member_void_caller(), like this:

~proxy_member_void_caller() noexcept(false)

[1]: http://stackoverflow.com/questions/130117/throwing-exceptions-out-of-a-destructor
[2]: http://stackoverflow.com/questions/23574323/why-cant-i-catch-a-luabinderror-exception-when-my-lua-code-throws-an-error