sol2/docs/source/api/readonly.rst

readonly
========
routine to mark a member variable as read-only
----------------------------------------------

.. code-block:: cpp
	
	template <typename T>
	auto readonly( T&& value );

The goal of read-only is to protect a variable set on a usertype or a function. Simply wrap it around a member variable, e.g. ``sol::readonly( &my_class::my_member_variable )`` in the appropriate place to use it. If someone tries to set it, it will throw an error. This can ONLY work on :doc:`usertypes<usertype>` and when you specifically set a member variable as a function and wrap it with this. It will NOT work anywhere else: doing so will invoke compiler errors.

If you are looking to make a read-only table, you need to go through a bit of a complicated song and dance by overriding the ``__index`` metamethod. Here's a complete example on the way to do that using ``sol``:


.. code-block:: cpp
	:caption: read-only.cpp

	#define SOL_CHECK_ARGUMENTS
	#include <sol.hpp>

	#include <iostream>

	struct object {
	    void my_func() {
	        std::cout << "hello\n";
	    }
	};

	int deny(lua_State* L) {
	    return luaL_error(L, "HAH! Deniiiiied!");
	}

	int main() {
	    sol::state lua;
	    lua.open_libraries(sol::lib::base);

	    object my_obj;

	    sol::table obj_table = lua.create_named_table("object");

	    sol::table obj_metatable = lua.create_table_with();
	    obj_metatable.set_function("my_func", &object::my_func, &my_obj);
	    // Set whatever else you need to
	    // on the obj_metatable, 
	    // not on the obj_table itself!

	    // Properly self-index metatable to block things
	    obj_metatable[sol::meta_function::new_index] = deny;
	    obj_metatable[sol::meta_function::index] = obj_metatable;

	    // Set it on the actual table
	    obj_table[sol::metatable_key] = obj_metatable;

	    try {
	        lua.script(R"(
	print(object.my_func)
	object["my_func"] = 24
	print(object.my_func)
	        )");
	    }
	    catch (const std::exception& e) {
	        std::cout << e.what() << std::endl;
	    }
	    return 0;
	}

It is a verbose example, but it explains everything. Perhaps in the future ``sol2`` may feature a ``create_readonly_table`` abstraction for users!