Simple example of Lua integration in C++

main.cpp

//
//  main.cpp
//  LuaTests
//
//  Created by Marco Mastropaolo on 08/06/16.
//  Copyright © 2016 Marco Mastropaolo. All rights reserved.
//

#include <iostream>
#include "../include/lua.hpp"


extern "C" int api_getVal(lua_State* lua)
{
    lua_pushinteger(lua, 5);
    return 1;
}

extern "C" int api_mul(lua_State* lua)
{
    lua_settop(lua, 2); // set the size of the stack to 2 and crop useless args

    int isnum;
    auto num = lua_tointegerx(lua, 1, &isnum);
    auto num2 = lua_tointegerx(lua, 2, &isnum);
    
    lua_pushinteger(lua, num * num2);
    return 1;
}



void print_error(lua_State* state) {
    // The error message is on top of the stack.
    // Fetch it, print it and then pop it off the stack.
    const char* message = lua_tostring(state, -1);
    puts(message);
    lua_pop(state, 1);
}

void execute(const char* code)
{
    lua_State *state = luaL_newstate();
    
    // Make standard libraries available in the Lua object
    luaL_openlibs(state);
    
    lua_register(state, "getVal", api_getVal);
    lua_register(state, "mul", api_mul);
    
    int result;
    
    // Load the program; this supports both source code and bytecode files.
    result = luaL_loadstring(state, code);
    
    if ( result != LUA_OK ) {
        print_error(state);
        return;
    }
    
    // Finally, execute the program by calling into it.
    // Change the arguments if you're not running vanilla Lua code.
    
    result = lua_pcall(state, 0, LUA_MULTRET, 0);
    
    if ( result != LUA_OK ) {
        print_error(state);
        return;
    }
}

int main(int argc, char** argv)
{
    execute( R"####(
            
            function fact(n)
                if (n == 0) then
                    return 1
                else
                    return mul(n, fact(n-1))
                end
            end
            
            print(fact(getVal()))
            
        )####");
    
    return 0;
}

Runs the calculation of 5! using C++ calling Lua calling C++.
The factorial is implemented in a Lua script, which calls a getVal() function exposed from C++ which returns 5. It also calls a mul(a,b) function instead of using the multiplication operator to show how to read arguments in C++.