LuaWrapperCpp

Lua Wrapper for C/C++ written from scratch

This wrapper is still work-in-progress.

Wrapper logic

Wrapper is writtent from scratch using C++11 without the in-depth knowledge of existing wrappers. If there is a similarity with an existing solution, its purely coincidental.

The main part of the wrapper consist of two classes located in the files with the same name - LuaWrapper and LuaScript.

LuaWrapper is a singleton class. It initialize Lua virtual machine, create new scripts (LuaScript) and store them in a map. In Lua callbacks, this is used to get LuaScript instances from lua_State *.

LuaScript represents a single script that can be started.

Description of other files:

LuaWrapperCpp.h is the main file to include the wrapper.

LuaCallbacks.h contains main part of the wrapper. It contains templated meethods that are called from Lua, methods binded to Lua metamethods like __gc, __index, __newindex, __tostring etc. and constructors for Lua classes.

LuaFunctionWrapper.h contains a static class wrapping Lua methods with a templated methods. This way, you can call "generic" get / set methods and in background, correct Lua methods are called (lua_tounsigned, lua_tonumber etc.).

LuaClassBind.h contains a helper class for binding C++ class to Lua. Its content is used to build C++ class representation that can be directly binded to Lua callbacks.

LuaUtils.h contains a helper class with static helper methods.

LuaTypes.h contains helper typedefs

LuaMacros.h contains helper macros to reduce size of the code. Some of these macros can be user-defined to change default behaviour.

How to use it

You just have to include LuaWrapperCpp.h. There are no external dependencies, except Lua library (version 5.2 is included in the project)

If you want, you may change some settings in LuaMacros.h

#define name	meaning	default value
SAFE_PTR_CHECKS	Safe checks for pointers during retrieval of raw pointers from Lua (see Benchmarks)	1
LUA_STRING	String class used for LuaStrings. Must have .c_str() and .length() methods	std::string

First, you have to start with LuaWrapper initialization, which is a singleton class:

Lua::LuaWrapper::Initialize([](const LuaString & filePath) -> LuaString {
		return Lua::LuaUtils::LoadFromFile(filePath);
	});	

You must provide script loading functor. This is usefull, if you have a virtual file system and you need to handle loading in a different way. Function must return loaded string with a script. You can also pass nullptr, in that case default loading Lua::LuaUtils::LoadFromFile(filePath) will be used.

In some cases, you may want to add some global variables or register classes / functions for all scripts, that are loaded with the wrapper. You can do this using AddRegisterCallback. This will add callback that is called after the script has been created via AddScript. You can have more of these registered from different places of your code.

Lua::LuaWrapper::GetInstance()->AddRegisterCallback(
	[&](std::shared_ptr<Lua::LuaScript> script) -> void {
		script->SetGlobalVarClass<Account>("global_account", someAccount);
	});	

Binding C methods to wrapper

We can bind classic C methods.

void cfunc1(int v)
{
	printf("Hello method Param: %i\n", v);
}

void cfunc(int v)
{
	printf("Hello method Param: %i\n", v);
}

void cfunc(int v1, int v2)
{
	printf("Hello method Param: %i %i\n", v1, v2);
}

And bind to Lua

std::shared_ptr<Lua::LuaScript> ls = ....;

ls->RegisterLuaFunction("cfunc_1", METHOD_OVERLOAD(cfunc, int));	
ls->RegisterLuaFunction("cfunc_2", METHOD_OVERLOAD(cfunc, int, int));	
ls->RegisterLuaFunction("cfunc", METHOD(cfunc1));	

Binding C++ class to wrapper

Class can be binded for use within Lua (for now, only instance methods and public members are supported). Each class is wrapped with LuaClassBind.

We have simple C++ test class:

struct TestStruct 
{
	float m_x = 0.0f;
	float m_y = 0.0f;
};

class TestClass
{
public:
    double m_val = 148;
	TestStruct m_tc;
		
	TestClass(double val) { m_val = val; }
	TestClass(double v1, double v2) { m_val = v1 * v2; }
				
	void Print0() { printf("PRINTF_0\n"); }

	void Print0(double vl, double v2) { printf("PRINTF_0: overload %f %f\n", vl, v2); }

	void Print1(const short& v) { printf("PRINTF_1 %i\n", v); }
        
	int Print2() { printf("PRINTF_2\n"); return 5;}
		
	void Print3(TestClass * t) { printf("PRINTF_3 % f\n", t->m_val);}
		
	void Print4(TestClass & t) { printf("PRINTF_4 % f\n", t.m_val);}
		
	void Print5() const { printf("PRINTF_5\n"); }
};	

And bind it to LuaClassBind

Lua::LuaClassBind<TestStruct> sb("TestStruct");	
sb.SetDefaultCtor<>();
	
sb.AddAttribute("m_x", CLASS_ATTRIBUTE(TestClass, m_x));
sb.AddAttribute("m_y", CLASS_ATTRIBUTE(TestClass, m_y));
	
sb.SetToString([](TestStruct * a) -> LuaString {		
	return "TestStruct...";
});
	
Lua::LuaClassBind<TestClass> cb("TestClass");	
cb.SetDefaultCtor<double>();
cb.AddCtor<double, double>("TestClass_ctor2");

cb.AddMethod("Print0", CLASS_METHOD_OVERLOAD(TestClass, Print0));
cb.AddMethod("Print0_args", CLASS_METHOD_OVERLOAD(TestClass, Print0, double, double));
			
cb.AddMethod("Print1", CLASS_METHOD(TestClass, Print1));
cb.AddMethod("Print2", CLASS_METHOD(TestClass, Print2));
cb.AddMethod("Print3", CLASS_METHOD(TestClass, Print3));
cb.AddMethod("Print4", CLASS_METHOD(TestClass, Print4));
cb.AddMethod("Print5", CLASS_METHOD(TestClass, Print5));

cb.AddAttribute("m_val", CLASS_ATTRIBUTE(TestClass, m_val));
cb.AddAttribute("m_tc", CLASS_ATTRIBUTE(TestClass, m_tc));
	
cb.SetToString([](TestClass * a) -> LuaString {		
	return "TestClass...";
});
			

Each class must have one binded ctor via SetDefaultCtor<...>. This ctor name is same as the name of the "class". However, you can specifiy additional ctors via AddCtor<...> and for each of them, unique name must be set.
All ctors behave like Factory pattern. In Lua, you call an ordinary method that returns a new class instance.

Method SetToString is optional and sets lambda function that is called, when __tostring metamethod is called in Lua. CLASS_METHOD, CLASS_ATTRIBUTE and CLASS_METHOD_OVERLOAD are macros to simplify binding. They are defined in LuaMacros.h.

After class is created, you must bind it a given script.

std::shared_ptr<Lua::LuaScript> ls = ....;

ls->RegisterLuaClass(cb);

The above binded class can be called from Lua as

t = TestClass(150)
t:Print0()
t:Print0_args(1, 2)
local r = t:Print2()
t:Print3(t)
t:Print4(t)
t:Print5()
	
	
v = TestStruct();
v.m_x = 15.0
print(v)
	
t.m_tc = v
print(t.m_tc)
	
t.m_val = 10
print(t.m_val)
	
t2 = TestClass_ctor2(150, 150)
	

Calling Lua from C++

You can also call Lua function from within C++. The only problem is, that script must be executed at least once before you can do this. So, if you want to use this, be aware of this and try not to add any functionality outside "functions".

function add(x, y)
	return x + y
end	
	
function mul(x, y)
	return x * y
end	

function my_print(x, y)
	print("Print " .. x .. " / " .. y)
end	

std::shared_ptr<Lua::LuaScript> script = //.... init LuaScript ....

Lua::LuaFunction lAdd(script, "add");	
int resAdd = lAdd.Call<int>(78, 45); //will return 123
	
Lua::LuaFunction lMull(script, "mul");	
int resMul = lMull.Call<int>(78, 45); //will return 3510

Lua::LuaFunction lPrint(script, "my_print");	
lPrint.Call<void>(78, 45); //will return anything

Benchmark & comparison

We have compared our solution with lua-intf

Our solution is compared in two modes: X (SAFE_PTR_CHECKS 1) / Y (SAFE_PTR_CHECKS 0) SAFE_PTR_CHECKS macro is located in LuaMacros.h

class Account {
public:
	double val = 148;
	Account(double balance) { m_balance = balance; }
	void deposit(double amount) { m_balance += amount; }
	double balance(void) { return m_balance; }
}

Test #1 Only class-methods exposed to Lua

a = Account(150)

for i=1, 10000000 do
	a:deposit(i)
end

print(a:balance())

Wrapper	Time [ms]
Ours	927 / 605
Lua-intf	1183

Test #2 Class-members exposed to Lua

a = Account(150)

for i=1, 10000000 do
	a.val = a.val + i
end

print(a.val)

Wrapper	Time [ms]
Ours	2688 / 2133
Lua-intf	4305

Test #3 Class-methods and members exposed to Lua

a = Account(150)

for i=1, 10000000 do
	a:deposit(i)
end

print(a:balance())

for i=1, 10000000 do
	a.val = a.val + i
end

print(a.val)

Wrapper	Time [ms]
Ours	4413 / 3241
Lua-intf	5467

Test #4 Class-methods and members exposed to Lua

a = Account(150)

for i=1, 10000000 do
	a:deposit(i)
end

print(a:balance())

Wrapper	Time [ms]
Ours	1511 / 1172
Lua-intf	1112

As you can see from above test, our wrapper in "unsafe" mode (SAFE_PTR_CHECKS 0) is faster, however, you are loosing type control. This solution is recomended for release builds and only if scripts have already been tested and are working.

If we bind class-methods and members together, but members are not used, our wrapper is slower (see Test #4). That is because each method call goes through __index metamethod and it slows thing down a bit. We should inspect Lua code before actual binding and see, if members are used and based on this disable __index call. However, this is in our TO-DO list and right now, other things are more important. So, if you are not using class-members in your code, do not bind them to Lua.

Test #5 Calling Lua from C++

function testSin(x)
	return math.sin(x)
end

function testSum(x, y)
	return x + y
end

Test runs 10,000,000.

Function	Time Lua [ms]	Time C++ [ms]
testSin	1077	76
testSum	651	12

References

Lua reference manual helper:

• http://pgl.yoyo.org/luai/i/_

Stackoverflow questions related to this project:

Many thanks to those, who helped. Without them, it would not be possible to create this

• http://stackoverflow.com/questions/40060728/c-make-tupple-from-variadic-template

• http://stackoverflow.com/questions/40224646/c-template-method-specialization-to-return-pointer-or-reference

• http://stackoverflow.com/questions/40135561/decltype-for-overloaded-member-function

• http://stackoverflow.com/questions/40116828/c-stddecayt-and-stdmake-tupled-cannot-convert-argument-from-d-to

• http://stackoverflow.com/questions/40082173/c11-template-stdenable-if-and-stdresult-of

• http://stackoverflow.com/questions/40072627/c-template-parameter-as-function-call-name

• http://stackoverflow.com/questions/40418367/c11-stddeclvalt-name-of-method

• http://stackoverflow.com/questions/40436966/c-const-vs-non-const-member-function-template-with-function-pointers