tutorial: quick ‘n’ dirty

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You’ll need to #include <sol.hpp>/#include "sol.hpp" somewhere in your code. Sol is header-only, so you don’t need to compile anything.

opening a state

int main (int argc, char* argv[]) {
        sol::state lua;
        // open some common libraries
        lua.open_libraries(sol::lib::base, sol::lib::package);
        lua.script( "print('bark bark bark!')" );
}

sol::state on lua_State*

For your system/game that already has lua, but you’d like something nice:

int pre_existing_system( lua_State* L ) {
        sol::state_view lua(L);
        lua.script( "print('bark bark bark!')" );
        return 0;
}

running lua code

sol::state lua;
// load and execute from string
lua.script("a = 'test'");
// load and execute from file
lua.script_file("path/to/luascript.lua");

// run a script, get the result
int value = lua.script("return 54");
// value == 54

To run Lua code but have an error handler in case things go wrong:

sol::state lua;

// the default handler panics or throws, depending on your settings
auto result1 = lua.script("bad.code", &sol::default_on_error);

auto result2 = lua.script("123 herp.derp", [](lua_State* L, sol::protected_function_result pfr) {
        // pfr will contain things that went wrong, for either loading or executing the script
        // Can throw your own custom error
        // You can also just return it, and let the call-site handle the error if necessary.
        return pfr;
});

To check the success of a loading operation:

// load file without execute
sol::load_result script1 = lua.load_file("path/to/luascript.lua");
script1(); //execute

// load string without execute
sol::load_result script2 = lua.load("a = 'test'");
sol::protected_function_result script2result = script2(); //execute
// optionally, check if it worked
if (script2result.valid()) {
        // yay!
}
else {
        // aww
}

sol::load_result script3 = lua.load("return 24");
int value2 = script3(); // execute, get return value
// value2 == 24

To check whether a script was successfully run or not (if the actual loading is successful):

// execute and return result
sol::protected_function_result result1 = lua.do_string("return 24");
if (result1.valid()) {
        int value = result1;
        // value == 24
        // yay!
}
else {
        // ahhh :c
}

There is also lua.do_file("path/to/luascript.lua");.

set and get variables

You can set/get everything.

sol::lua_state lua;

lua.open_libraries(sol::lib::base);

// integer types
lua.set("number", 24);

// floating point numbers
lua["number2"] = 24.5;

// string types
lua["important_string"] = "woof woof";

// non-recognized types is stored as userdata
// is callable, therefore gets stored as a function
lua["a_function"] = [](){ return 100; };

// make a table
lua["some_table"] = lua.create_table_with("value", 24);

Equivalent to loading a lua file with:

number = 24
number2 = 24.5
important_string = "woof woof"
a_function = function () return 100 end
some_table = { value = 24 }

Retrieve these variables using this syntax:

// implicit conversion
int number = lua["number"];

// explicit get
auto number2 = lua.get<double>("number2");

// strings too
std::string important_string = lua["important_string"];

// dig into a table
int value = lua["some_table"]["value"];

// get a function
sol::function a_function = lua["a_function"];
int value_is_100 = a_function();

// get a std::function
std::function<int()> a_std_function = lua["a_function"];
int value_is_still_100 = a_std_function();

Retrieve Lua types using object and other sol:: types.

sol::state lua;

// ... everything from before

sol::object number_obj = lua.get<sol::object>( "number" );
// sol::type::number
sol::type t1 = number_obj.get_type();

sol::object function_obj = lua[ "a_function" ];
// sol::type::function
sol::type t2 = function_obj.get_type();
bool is_it_really = function_obj.is<std::function<int()>>(); // true

// will not contain data
sol::optional<int> check_for_me = lua["a_function"];

You can erase things by setting it to nullptr or sol::nil.

sol::state lua;

lua.script("exists = 250");

int first_try = lua.get_or( "exists", 322 );
// first_try == 250

lua.set("exists", sol::nil);
int second_try = lua.get_or( "exists", 322 );
// second_try == 322

Note that if its a userdata/usertype for a C++ type, the destructor will run only when the garbage collector deems it appropriate to destroy the memory. If you are relying on the destructor being run when its set to sol::nil, you’re probably committing a mistake.

tables

sol::state is a table too.

sol::state lua;

// Raw string literal for easy multiline
lua.script( R"(
        abc = { [0] = 24 }
        def = {
                ghi = {
                        bark = 50,
                        woof = abc
                }
        }
)"
);

sol::table abc = lua["abc"];
sol::table def = lua["def"];
sol::table ghi = lua["def"]["ghi"];

int bark1 = def["ghi"]["bark"];
int bark2 = lua["def"]["ghi"]["bark"];
// bark1 == bark2 == 50

int abcval1 = abc[0];
int abcval2 = ghi["woof"][0];
// abcval1 == abcval2 == 24

If you’re going deep, be safe:

sol::state lua;

sol::optional<int> will_not_error = lua["abc"]["DOESNOTEXIST"]["ghi"];
// will_not_error == sol::nullopt
int also_will_not_error = lua["abc"]["def"]["ghi"]["jklm"].get_or(25);
// is 25

// if you don't go safe,
// will throw (or do at_panic if no exceptions)
int aaaahhh = lua["boom"]["the_dynamite"];

make tables

Make some:

sol::state lua;

lua["abc"] = lua.create_table_with(
        0, 24
);

lua.create_named_table("def",
        "ghi", lua.create_table_with(
                "bark", 50,
                // can reference other existing stuff too
                "woof", lua["abc"]
        )
);

Equivalent Lua code:

abc = { [0] = 24 }
def = {
        ghi = {
                bark = 50,
                woof = abc
        }
}

You can put anything you want in tables as values or keys, including strings, numbers, functions, other tables.

Note that this idea that things can be nested is important and will help later when you get into namespacing.

functions

They’re great. Use them:

sol::state lua;

lua.script("function f (a, b, c, d) return 1 end");
lua.script("function g (a, b) return a + b end");

// fixed signature std::function<...>
std::function<int(int, double, int, std::string)> stdfx = lua["f"];
// sol::function is often easier:
// takes a variable number/types of arguments...
sol::function fx = lua["f"];

int is_one = stdfx(1, 34.5, 3, "bark");
int is_also_one = fx(1, "boop", 3, "bark");

// call through operator[]
int is_three = lua["g"](1, 2);
// is_three == 3
double is_4_8 = lua["g"](2.4, 2.4);
// is_4_8 == 4.8

If you need to protect against errors and parser problems and you’re not ready to deal with Lua’s longjmp problems (if you compiled with C), use sol::protected_function.

You can bind member variables as functions too, as well as all KINDS of function-like things:

void some_function () {
        std::cout << "some function!" << std::endl;
}

void some_other_function () {
        std::cout << "some other function!" << std::endl;
}

struct some_class {
        int variable = 30;

        double member_function () {
                return 24.5;
        }
};

sol::state lua;
lua.open_libraries(sol::lib::base);

// put an instance of "some_class" into lua
// (we'll go into more detail about this later
// just know here that it works and is
// put into lua as a userdata
lua.set("sc", some_class());

// binds a plain function
lua["f1"] = some_function;
lua.set_function("f2", &some_other_function);

// binds just the member function
lua["m1"] = &some_class::member_function;

// binds the class to the type
lua.set_function("m2", &some_class::member_function, some_class{});

// binds just the member variable as a function
lua["v1"] = &some_class::variable;

// binds class with member variable as function
lua.set_function("v2", &some_class::variable, some_class{});

The lua code to call these things is:

f1() -- some function!
f2() -- some other function!

-- need class instance if you don't bind it with the function
print(m1(sc)) -- 24.5
-- does not need class instance: was bound to lua with one
print(m2()) -- 24.5

-- need class instance if you
-- don't bind it with the function
print(v1(sc)) -- 30
-- does not need class instance:
-- it was bound with one
print(v2()) -- 30

-- can set, still
-- requires instance
v1(sc, 212)
-- can set, does not need
-- class instance: was bound with one
v2(254)

print(v1(sc)) -- 212
print(v2()) -- 254

Can use sol::readonly( &some_class::variable ) to make a variable readonly and error if someone tries to write to it.

self call

You can pass the ‘self’ argument through C++ to emulate ‘member function’ calls in Lua.

sol::state lua;

lua.open_libraries(sol::lib::base, sol::lib::package, sol::lib::table);

// a small script using 'self' syntax
lua.script(R"(
some_table = { some_val = 100 }

function some_table:add_to_some_val(value)
    self.some_val = self.some_val + value
end

function print_some_val()
    print("some_table.some_val = " .. some_table.some_val)
end
)");

// do some printing
lua["print_some_val"]();
// 100

sol::table self = lua["some_table"];
self["add_to_some_val"](self, 10);
lua["print_some_val"]();

multiple returns from lua

sol::state lua;

lua.script("function f (a, b, c) return a, b, c end");

std::tuple<int, int, int> result;
result = lua["f"](100, 200, 300);
// result == { 100, 200, 300 }
int a;
int b;
std::string c;
sol::tie( a, b, c ) = lua["f"](100, 200, "bark");
// a == 100
// b == 200
// c == "bark"

multiple returns to lua

sol::state lua;

lua["f"] = [](int a, int b, sol::object c) {
        // sol::object can be anything here: just pass it through
        return std::make_tuple( a, b, c );
};

std::tuple<int, int, int> result = lua["f"](100, 200, 300);
// result == { 100, 200, 300 }

std::tuple<int, int, std::string> result2;
result2 = lua["f"](100, 200, "BARK BARK BARK!");
// result2 == { 100, 200, "BARK BARK BARK!" }

int a, b;
std::string c;
sol::tie( a, b, c ) = lua["f"](100, 200, "bark");
// a == 100
// b == 200
// c == "bark"

C++ classes from C++

Everything that is not a:

Is set as a userdata + usertype.

struct Doge {
        int tailwag = 50;
};

Doge dog{};

// Copy into lua: destroyed by Lua VM during garbage collection
lua["dog"] = dog;
// OR: move semantics - will call move constructor if present instead
// Again, owned by Lua
lua["dog"] = std::move( dog );
lua["dog"] = Doge{};
lua["dog"] = std::make_unique<Doge>();
lua["dog"] = std::make_shared<Doge>();
// Identical to above

Doge dog2{};

lua.set("dog", dog2);
lua.set("dog", std::move(dog2));
lua.set("dog", Doge{});
lua.set("dog", std::unique_ptr<Doge>(new Doge()));
lua.set("dog", std::shared_ptr<Doge>(new Doge()));

std::unique_ptr/std::shared_ptr‘s reference counts / deleters will be respected. If you want it to refer to something, whose memory you know won’t die in C++, do the following:

struct Doge {
        int tailwag = 50;
};

sol::state lua;
lua.open_libraries(sol::lib::base);

Doge dog{}; // Kept alive somehow

// Later...
// The following stores a reference, and does not copy/move
// lifetime is same as dog in C++
// (access after it is destroyed is bad)
lua["dog"] = &dog;
// Same as above: respects std::reference_wrapper
lua["dog"] = std::ref(dog);
// These two are identical to above
lua.set( "dog", &dog );
lua.set( "dog", std::ref( dog ) );

Get userdata in the same way as everything else:

struct Doge {
        int tailwag = 50;
};

sol::state lua;
lua.open_libraries(sol::lib::base);

Doge& dog = lua["dog"]; // References Lua memory
Doge* dog_pointer = lua["dog"]; // References Lua memory
Doge dog_copy = lua["dog"]; // Copies, will not affect lua

Note that you can change the data of usertype variables and it will affect things in lua if you get a pointer or a reference from Sol:

struct Doge {
        int tailwag = 50;
};

sol::state lua;
lua.open_libraries(sol::lib::base);

Doge& dog = lua["dog"]; // References Lua memory
Doge* dog_pointer = lua["dog"]; // References Lua memory
Doge dog_copy = lua["dog"]; // Copies, will not affect lua

dog_copy.tailwag = 525;
// Still 50
lua.script("assert(dog.tailwag == 50)");

dog.tailwag = 100;
// Now 100
lua.script("assert(dog.tailwag == 100)");

C++ classes put into Lua

See this section here and after perhaps see if simple usertypes suit your needs. Also check out some a basic example, special functions and initializers,

namespacing

You can emulate namespacing by having a table and giving it the namespace names you want before registering enums or usertypes:

struct my_class {
        int b = 24;

        int f () const {
                return 24;
        }

        void g () {
                ++b;
        }
};

sol::state lua;
lua.open_libraries();

// set up table
sol::table bark = lua.create_named_table("bark");

bark.new_usertype<my_class>( "my_class",
        "f", &my_class::f,
        "g", &my_class::g
); // the usual

// 'bark' namespace
lua.script("obj = bark.my_class.new()" );
lua.script("obj:g()");
my_class& obj = lua["obj"];
// obj.b == 25

This technique can be used to register namespace-like functions and classes. It can be as deep as you want. Just make a table and name it appropriately, in either Lua script or using the equivalent Sol code. As long as the table FIRST exists (e.g., make it using a script or with one of Sol’s methods or whatever you like), you can put anything you want specifically into that table using sol::table’s abstractions.

advanced

Some more advanced things you can do/read about:
  • metatable manipulations allow a user to change how indexing, function calls, and other things work on a single type.
  • ownership semantics are described for how Lua deals with its own internal references and (raw) pointers.
  • stack manipulation to safely play with the stack. You can also define customization points for stack::get/stack::check/stack::push for your type.
  • make_reference/make_object convenience function to get the same benefits and conveniences as the low-level stack API but put into objects you can specify.
  • stack references to have zero-overhead Sol abstractions while not copying to the Lua registry.
  • unique usertype traits allows you to specialize handle/RAII types from other frameworks, like boost and Unreal, to work with Sol.
  • variadic arguments in functions with sol::variadic_args.
  • this_state to get the current lua_State*.
  • resolve overloads in case you have overloaded functions; a cleaner casting utility. You must use this to emulate default parameters.