Add Complex Lua Binding

This skill helps you create comprehensive Lua bindings for complex C++ classes including:

• Classes with methods and properties
• Inheritance hierarchies
• Constructors and factory methods
• Custom operators

Planning Your Binding

Before you start, determine:

1. Class name and Lua name
2. Base classes (if any)
3. Constructors to expose
4. Methods to expose (public API)
5. Properties to expose
6. Target file (catalua_bindings_*.cpp - group by domain)

Step-by-Step Process

1. Add Luna Documentation Macro

In src/catalua_luna_doc.h:

// For regular class
LUNA_DOC( my_class, "MyClass" )

// For class that can be detached (owned by Lua)
LUNA_PTR_VAL( my_class, "MyClass" )

2. Choose/Create Binding File

Files are organized by domain:

• catalua_bindings_creature.cpp - Creature, Character, Player, Avatar, NPC
• catalua_bindings_item.cpp - Item and related
• catalua_bindings_map.cpp - Map, terrain, furniture
• catalua_bindings_*.cpp - Create new if needed

3. Include Required Headers

At top of binding file:

#include "catalua_bindings.h"
#include "catalua_bindings_utils.h"
#include "catalua_luna.h"
#include "catalua_luna_doc.h"

#include "my_class.h"  // Your class header
// ... other needed headers

4. Create Registration Function

auto cata::detail::reg_my_domain( sol::state &lua ) -> void
{
#define UT_CLASS my_class
    {
        DOC( "Description of MyClass" );
        DOC( "Additional documentation lines" );

        auto ut = luna::new_usertype<UT_CLASS>(
            lua,
            // Specify base classes
            luna::bases<base_class>(),  // or luna::no_bases
            // Specify constructors
            luna::constructors<
                UT_CLASS(),                    // Default
                UT_CLASS( const UT_CLASS & ),  // Copy
                UT_CLASS( int, std::string )   // Custom
            >()
            // or luna::no_constructor
        );

        // === MEMBER VARIABLES ===
        SET_MEMB( my_field );           // Read/write
        SET_MEMB_RO( readonly_field );  // Read-only

        // === MEMBER FUNCTIONS ===
        SET_FX( simple_method );

        // Method with overloads (specify signature)
        SET_FX_T( overloaded_method, int( std::string ) const );

        // Method with lambda wrapper
        luna::set_fx( ut, "custom_method",
            []( UT_CLASS &self, int arg ) -> std::string { return self.internal_method( arg ); }
        );

        // === STATIC FUNCTIONS ===
        luna::set_fx( ut, "static_factory", &UT_CLASS::create );

        // === OPERATORS ===
        luna::set_fx( ut, sol::meta_function::equal_to, &UT_CLASS::operator== );
        luna::set_fx( ut, sol::meta_function::less_than, &UT_CLASS::operator< );
        luna::set_fx( ut, sol::meta_function::to_string,
            []( const UT_CLASS &obj ) -> std::string { return obj.to_string(); }
        );
    }
#undef UT_CLASS
}

5. Register in Main Binding Function

In src/catalua_bindings.h:

namespace cata::detail {
    void reg_my_domain( sol::state &lua );
}

In src/catalua_bindings.cpp, in reg_all_bindings():

void cata::detail::reg_all_bindings( sol::state &lua ) {
    // ... existing registrations
    reg_my_domain( lua );
}

Advanced Patterns

Handling Inheritance

// Base class
luna::new_usertype<base_class>(
    lua,
    luna::no_bases,
    luna::no_constructor
);

// Derived class
luna::new_usertype<derived_class>(
    lua,
    luna::bases<base_class>(),  // Specify base
    luna::no_constructor
);

Detached Pointers (Lua ownership)

For objects that Lua can own:

// Return detached pointer from C++
luna::set_fx( ut, "remove_item",
    []( character &ch, item &it ) -> detached_ptr<item> {
        return detached_ptr<item>( ch.remove_item( it ) );
    }
);

// Accept detached pointer as parameter
luna::set_fx( ut, "add_item",
    []( character &ch, detached_ptr<item> it ) {
        ch.add_item( std::move( it ) );
    }
);

Optional Parameters and Return Values

// Optional parameters (sol::optional)
luna::set_fx( ut, "method_with_optional",
    []( UT_CLASS &self, int required, sol::optional<std::string> opt ) {
        if( opt ) {
            self.do_something( required, *opt );
        } else {
            self.do_something( required );
        }
    }
);

// Optional return (std::optional)
luna::set_fx( ut, "maybe_get_value",
    &UT_CLASS::try_get_value  // Returns std::optional<T>
);

Variadic Arguments

luna::set_fx( ut, "variadic_method",
    []( UT_CLASS &self, sol::variadic_args va ) {
        for( auto arg : va ) {
            // Process each argument
        }
    }
);

Exposing Collections

// Return vector/array as Lua table
luna::set_fx( ut, "get_items",
    &UT_CLASS::get_items  // Returns std::vector<item>
);

// Return map as Lua table
luna::set_fx( ut, "get_stats",
    &UT_CLASS::get_stats  // Returns std::map<std::string, int>
);

Documentation with Parameters

DOC( "Applies damage to a body part" );
DOC_PARAMS(
    "attacker: Creature - The attacking creature",
    "bp: BodyPartTypeIntId - Target body part", 
    "damage: DamageInstance - Damage to apply"
);
luna::set_fx( ut, "apply_damage", &UT_CLASS::apply_damage );

Method Signature Resolution

When a method has overloads, use sol::resolve:

// Method: int get_value() const
SET_FX_T( get_value, int() const );

// Method: int get_value(std::string) const  
SET_FX_T( get_value, int( std::string ) const );

// Or with sol::resolve directly:
luna::set_fx( ut, "get_value",
    sol::resolve<int() const>( &UT_CLASS::get_value )
);

Coding Conventions

Follow Cataclysm-BN C++23 style (see AGENTS.md):

/// Doc comments with triple slash
auto function_name( int param ) -> int { return param * 2; }

// Use auto for types
auto ut = luna::new_usertype<UT_CLASS>( /*...*/ );

// Use trailing return types in lambdas
luna::set_fx( ut, "method",
    []( UT_CLASS &self ) -> int { return self.value; }
);

// Use designated initializers
auto result = my_struct{ .a = 1, .b = 2 };

// Use std::ranges over for loops
auto values = items
    | std::views::filter( []( const auto &v ) { return v.is_valid(); } )
    | std::ranges::to<std::vector>();

// Use snake_case for functions and variables
auto my_variable = 42;

Testing Your Bindings

-- In Lua console (Debug menu -> Lua console)

-- Test constructor
local obj = MyClass.new()

-- Test method
local result = obj:some_method()

-- Test property
print(obj.some_field)

-- Test static function
local instance = MyClass.static_factory("arg")

-- Test inheritance
local derived = DerivedClass.new()
derived:base_method()  -- Should work if inheritance correct

Build and Verify

# Build
cmake --build --preset linux-full --target cataclysm-bn-tiles cata_test-tiles

# Format
cmake --build build --target format

# Test
./out/build/linux-full/tests/cata_test-tiles "[lua]"

Common Patterns Reference

Units (mass, volume, energy, angle)

// These are already bound, use as reference
luna::set_fx( ut, "from_gram", &units::from_gram<std::int64_t> );
luna::set_fx( ut, "to_gram", &units::to_gram<std::int64_t> );

Time (duration, time_point)

// Already bound, see catalua_bindings.cpp

Coordinates (tripoint, point)

// Already bound, see catalua_bindings_coords.cpp

Troubleshooting

Issue: Linker errors about undefined references

• Fix: Include the header file containing the class implementation

Issue: luna_traits<T>::impl is false

• Fix: Add LUNA_DOC or appropriate macro to catalua_luna_doc.h

Issue: Ambiguous function call

• Fix: Use SET_FX_T or sol::resolve to specify exact signature

Issue: Type not copyable/movable

• Fix: Use luna::no_constructor and provide factory functions

Issue: Method returns reference but Lua sees copy

• Fix: Consider if reference is correct, or use pointers

Files to Modify

1. src/catalua_luna_doc.h - Add LUNA_* macro
2. src/catalua_bindings_*.cpp - Implementation (choose or create)
3. src/catalua_bindings.h - Declare registration function
4. src/catalua_bindings.cpp - Call registration in reg_all_bindings()

Reference Files

• Example complex class: src/catalua_bindings_creature.cpp (Character, Creature)
• Example with items: src/catalua_bindings_item.cpp
• Luna system: src/catalua_luna.h
• Utility macros: src/catalua_bindings_utils.h
• Integration docs: docs/en/mod/lua/explanation/lua_integration.md