How to Run Lua Files within C++

In this article we will see the reverse process that we saw in the video: How to Incorporate C/C++ Functions in Lua.

We will see how to run and obtain variable values from Lua code in C++. This concept is widely used in Game Development.

If you don’t have Lua installed, you can still test it, just download Lua from this address: http://www.lua.org/download.html and download the package: lua-5.4.6.tar .gz.

Unzip and use -I include for the src of the unzipped directory.

01. Basic code

For the first example, let’s create a code in C++ that prints a Hello World on the screen, but with the Lua print:

// Include Lua files
#include <lua5.1/lua.hpp>

int main() {
     // Initialize the Lua state
     lua_State *L = luaL_newstate();

     // Load the standard Lua libraries
     luaL_openlibs(L);

     // Run a Lua script, it doesn't show what the error was
     luaL_dostring(L, "print('Hi, I'm being printed in Lua')");

     // Close the Lua state
     lua_close(L);

     return 0;
}

To compile use:

g++ main.cpp -llua -ldl

02. Handling errors

What if I have a Lua syntax error?

if (luaL_dostring(L, "print('Hi, I'm being printed in Lua')") != 0) {
   fprintf(stderr, "Error executing Lua script: %s\n", lua_tostring(L, -1));
   return 1;
}

03. Running external files

file.lua

x = 36
y = 90
print("x + y = " .. x + y)

Replace luaL_dostring with luaL_dofile

if(luaL_dofile(L, "./file.lua") != 0) {
   fprintf(stderr, "Error executing Lua script: %s\n", lua_tostring(L, -1));
   return 1;
};

04. Get variables from Lua

...
if(luaL_dofile(L, "./file.lua") != 0) {
   fprintf(stderr, "Error executing Lua script: %s\n", lua_tostring(L, -1));
   return 1;
};

lua_getglobal(L, "x");
int val = (int)lua_tointeger(L, -1);
lua_pop(L, 1);
printf("The value of x is: %d\n", val);
...

If you have a variable screen = "1920 x 1080" in the file file.lua and want to know its contents, just use: lua_tostring(L., -1):

lua_getglobal(L, "screen");
const char* val = (const char*) lua_tostring(L, -1);
lua_pop(L, 1);
printf("The screen resolution is: %s\n", val);

lua cpp

YouTube channel