proxy, (protectedunsafe)_function_result - proxy_base derivatives

``table[x]`` and ``function(…)`` conversion struct

template <typename Recurring>
struct proxy_base;

template <typename Table, typename Key>
struct proxy : proxy_base<...>;

struct stack_proxy: proxy_base<...>;

struct unsafe_function_result : proxy_base<...>;

struct protected_function_result: proxy_base<...>;

These classes provide implicit assignment operator operator= (for set) and an implicit conversion operator operator T (for get) to support items retrieved from the underlying Lua implementation, specifically sol::table and the results of function calls on sol::function and sol::protected_function.

proxy

proxy is returned by lookups into sol::table and table-like entities. Because it is templated on key and table type, it would be hard to spell: you can capture it using the word auto if you feel like you need to carry it around for some reason before using it. proxy evaluates its arguments lazily, when you finally call get or set on it. Here are some examples given the following lua script:

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			[2] = "arf!" 
		} 
	}
	)";

After loading that file in or putting it in a string and reading the string directly in lua (see state), you can start kicking around with it in C++ like so:

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#define SOL_ALL_SAFETIES_ON 1
#include <sol/sol.hpp>

#include "assert.hpp"

int main() {

	const auto& code = R"(
	lua.script(code);

	// produces table_proxy, implicitly converts to std::string, quietly destroys table_proxy
	std::string arf_string = lua["bark"]["woof"][2];

	// lazy-evaluation of tables
	auto x = lua["bark"];
	auto y = x["woof"];
	auto z = y[2];

	// retrivies value inside of lua table above
	std::string value = z;
	c_assert(value == "arf!");

	// Can change the value later...
	z = 20;

	// Yay, lazy-evaluation!
	int changed_value = z; // now it's 20!
	c_assert(changed_value == 20);
	lua.script("assert(bark.woof[2] == 20)");

	lua["a_new_value"] = 24;

We don’t recommend using proxy lazy evaluation the above to be used across classes or between function: it’s more of something you can do to save a reference to a value you like, call a script or run a lua function, and then get it afterwards. You can also set functions (and function objects) this way, and retrieve them as well:

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	lua["chase_tail"] = [](int chasing) {
		int r = 2;
		for (int i = 0; i < chasing; ++i) {
			r *= r;
		}
		return r;
	};

	lua.script("assert(a_new_value == 24)");
	lua.script("assert(chase_tail(2) == 16)");

	return 0;
}

members

functions: [overloaded] implicit conversion get
requires( sol::is_primitive_type<T>::value == true )
template <typename T>
operator T() const;

requires( sol::is_primitive_type<T>::value == false )
template <typename T>
operator T&() const;

Gets the value associated with the keys the proxy was generated and convers it to the type T. Note that this function will always return T&, a non-const reference, to types which are not based on sol::reference and not a primitive lua type

function: get a value
template <typename T>
decltype(auto) get( ) const;

Gets the value associated with the keys and converts it to the type T.

function: optionally get a value
template <typename T, typename Otherwise>
optional<T> get_or( Otherwise&& otherise ) const;

Gets the value associated with the keys and converts it to the type T. If it is not of the proper type, it will return a sol::nullopt instead.

function: [overloaded] optionally get or create a value
template <typename T>
decltype(auto) get_or_create();
template <typename T, typename Otherwise>
decltype(auto) get_or_create( Otherwise&& other );

Gets the value associated with the keys if it exists. If it does not, it will set it with the value and return the result.

operator[] proxy-only members

function: valid
bool valid () const;

Returns whether this proxy actually refers to a valid object. It uses sol::stack::probe_get_field to determine whether or not its valid.

functions: [overloaded] implicit set
requires( sol::detail::Function<Fx> == false )
template <typename T>
proxy& operator=( T&& value );

requires( sol::detail::Function<Fx> == true )
template <typename Fx>
proxy& operator=( Fx&& function );

Sets the value associated with the keys the proxy was generated with to value. If this is a function, calls set_function. If it is not, just calls set. Does not exist on unsage_function_result or protected_function_result.

function: set a callable
template <typename Fx>
proxy& set_function( Fx&& fx );

Sets the value associated with the keys the proxy was generated with to a function fx. Does not exist on unsafe_function_result or protected_function_result.

function: set a value
template <typename T>
proxy& set( T&& value );

Sets the value associated with the keys the proxy was generated with to value. Does not exist on unsafe_function_result or protected_function_result.

stack_proxy

sol::stack_proxy is what gets returned by sol::variadic_args and other parts of the framework. It is similar to proxy, but is meant to alias a stack index and not a named variable.

unsafe_function_result

unsafe_function_result is a temporary-only, intermediate-only implicit conversion worker for when function is called. It is NOT meant to be stored or captured with auto. It provides fast access to the desired underlying value. It does not implement set / set_function / templated operator=, as is present on proxy.

This type does, however, allow access to multiple underlying values. Use result.get<Type>(index_offset) to retrieve an object of Type at an offset of index_offset in the results. Offset is 0 based. Not specifying an argument defaults the value to 0.

unsafe_function_result also has begin() and end() functions that return (almost) “random-acess” iterators. These return a proxy type that can be implicitly converted to stack_proxy.

protected_function_result

protected_function_result is a nicer version of unsafe_function_result that can be used to detect errors. Its gives safe access to the desired underlying value. It does not implement set / set_function / templated operator= as is present on proxy.

This type does, however, allow access to multiple underlying values. Use result.get<Type>(index_offset) to retrieve an object of Type at an offset of index_offset in the results. Offset is 0 based. Not specifying an argument defaults the value to 0.

unsafe_function_result also has begin() and end() functions that return (almost) “random-acess” iterators. These return a proxy type that can be implicitly converted to stack_proxy.

on function objects and proxies

Note

As of recent versions of sol3 (2.18.2 and above), this is no longer an issue, as even bound classes will have any detectable function call operator automatically bound to the object, to allow this to work without having to use .set or .set_function. The note here is kept for posterity and information for older versions. There are only some small caveats, see: this note here.