Lua Glue Function Template

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Cower	Created : 23 February 2013 Edited : 23 February 2013 Language : Monkey Lua Glue Function Template A template for pushing glue functions for arbitrary functions onto Lua's stack.
This snippet is basically just a header with some templates that allow you to generate a glue function for any arbitrary function and push it onto a Lua stack. So, you just call lb_push_function with your function and it'll take care of the rest. Or the compiler will. // lbind.hh -- Noel Cower -- Licensed under CC BY-SA 2.5 // Attribution to Johannes Schaub for his answer on parameter packs on // Stack Overflow: https://stackoverflow.com/a/7858971/457812 // For license information, see: https://creativecommons.org/licenses/by-sa/2.5/ #ifndef __SNOW_LBIND_HH__ #define __SNOW_LBIND_HH__ #include <stdexcept> #include <string> extern "C" { #include <lua.h> } #ifndef NULL #define NULL (0) #endif /============================================================================== See lb_push_function below for the important bit. To provide push options for for non-integral/floating point/string types, you need to specialize on lub_push_value and lb_to_value. lb_push_value may take both a const reference or a non-const reference, so one or the other needs to be provided, but generally you'll likely only need to specialize the const reference version. ==============================================================================/ typedef int (lfunc_t)(lua_State ); /* Parameter pack shamelessly stolen from Johannes Schaub's awesome answer on Stack Overflow: https://stackoverflow.com/a/7858971/457812 / template<int ...> struct lb_pseq__ { }; template<int N, int ...S> struct lb_gens__ : lb_gens__<N-1, N-1, S...> { }; template<int ...S> struct lb_gens__<0, S...> { typedef lb_pseq__<S...> type; }; template <typename T> T lb_to_value(lua_State L, int index) { throw std::runtime_error("Cannot convert type from Lua value"); } template <typename T> void lb_push_value(lua_State L, T &value) { throw std::runtime_error("Cannot convert type to Lua value"); } template <typename T> void lb_push_value(lua_State L, const T &value) { throw std::runtime_error("Cannot convert type to Lua value"); } template <typename T> void lb_glue_from_(lua_State L, int index, T &out) { if (std::is_integral<T>::value) out = static_cast<T>(lua_tointeger(L, index)); else if (std::is_floating_point<T>::value) out = static_cast<T>(lua_tonumber(L, index)); else out = lb_to_value<T>(L, index); } template <> void lb_glue_from_<std::string>(lua_State L, int index, std::string &out) { size_t length; const char cstring = lua_tolstring(L, index, &length); if (cstring == NULL) throw std::runtime_error("Non-string value passed to string parameter of C function"); out = std::string(cstring, length); } template <int index, typename... T> void lb_unpack_values_into__(lua_State L, std::tuple<T...> &values, const int max_index) { typedef std::tuple<T...> tuple_type; if (index < max_index) lb_glue_from_(L, index + 1, std::get<index>(values)); if (index > 0) lb_unpack_values_into__<(index <= 0 ? 0 : index - 1), T...>(L, values, max_index); } template <typename T> void lb_push_value__(lua_State L, T &value) { if (std::is_integral<T>::value) lua_pushinteger(L, static_cast<lua_Integer>(value)); else if (std::is_floating_point<T>::value) lua_pushnumber(L, static_cast<lua_Number>(value)); else lb_push_value(L, value); } template <typename T> void lb_push_value__(lua_State L, const T &value) { if (std::is_integral<T>::value) lua_pushinteger(L, static_cast<lua_Integer>(value)); else if (std::is_floating_point<T>::value) lua_pushnumber(L, static_cast<lua_Number>(value)); else lb_push_value(L, value); } template <> void lb_push_value__<std::string>(lua_State L, const std::string &value) { lua_pushlstring(L, value.c_str(), value.length()); } template <typename F, typename R, typename... T, int... S> R lb_call_func__(F f, std::tuple<T...> &params, lb_pseq__<S...>) { return f(std::get<S>(params) ...); } template <typename F, typename R, typename... T> int lb_glue_func__(lua_State L) { typedef std::tuple<T...> values_t; F func = (F)lua_touserdata(L, lua_upvalueindex(1)); const int top = lua_gettop(L); const int num_values = sizeof...(T); const int num_returns = static_cast<int>(!std::is_void<R>::value); values_t values; lb_unpack_values_into__<sizeof...(T) - 1, T...>(L, values, top); if (num_returns); lb_push_value__<R>(L, lb_call_func__<F, R, T...>(func, values, typename lb_gens__<sizeof...(T)>::type())); return num_returns; } // For functions with no arguments - includes dummy template argument to make // use easier. template <int D, typename F, typename R> int lb_glue_func__(lua_State L) { F func = (F)lua_touserdata(L, lua_upvalueindex(1)); const int num_returns = static_cast<int>(!std::is_void<R>::value); if (num_returns); lb_push_value__<R>(L, func()); return num_returns; } template <typename F, typename R, typename... T> lfunc_t lb_glue_binding_fn__(R (func)(T...)) { return lb_glue_func__<F, R, T...>; } // For functions with no arguments. template <typename F, typename R> lfunc_t lb_glue_binding_fn__(R (func)()) { return lb_glue_func__<1, F, R>; } /============================================================================== lb_glue_binding_fn Returns the Lua glue function for the given function (func). ==============================================================================/ template <class F> lfunc_t lb_glue_binding_fn(F func) { return lb_glue_binding_fn__<F>(func); } /============================================================================== lb_push_function Pushes the given function's wrapper-function onto the Lua stack. ==============================================================================/ template <class F> void lb_push_function(lua_State L, F f) { // Store the original function in an upvalue (necessary since I haven't found // any other way to provide the function with its original implementation) (F)lua_newuserdata(L, sizeof(F)) = f; lua_pushcclosure(L, lb_glue_binding_fn(f), 1); } #endif /* end __SNOW_LBIND_HH__ include guard / --v And for a quick example (sanity of code below not guaranteed): #include "lbind.hh" extern "C" { #include <lua.h> #include <lauxlib.h> #include <lualib.h> } #include <sstream> std::string foo(int bar, int baz, double quux) { double r = double(bar + baz) + quux; std::stringstream ss; ss << "<\"" << r << "\">"; return ss.str(); } int return_some_int() { return 50; } int main(int argc, char const argv[]) { lua_State *l = luaL_newstate(); luaopen_base(l); lb_push_function(l, foo); lua_setglobal(l, "foo"); lb_push_function(l, return_some_int); lua_setglobal(l, "some_int"); luaL_dostring(l, "print(foo(1, 2, 0.1234))"); luaL_dostring(l, "print(some_int())"); lua_close(l); l = NULL; return 0; } --v

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Lua Glue Function Template

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