Setting up an 'environment' for an embedded Lua Script

I haven't heard of read-only variables in Lua but you can prevent modification by making the environment available via a function call instead.

If the C++ application is large, you will probably want to use a tool to generate an interface you can call from Lua. I have used tolua++ in the past with some luck:

Suppose demo.hpp is a header file of the C++ application:

#ifndef SO_DEMO_HPP
#define SO_DEMO_HPP

namespace demo
{
    class Foo
    {
        double x;

    public:
        Foo(double x) : x(x) {}
        double getX() const { return x; }
    };

    Foo* getFoo();
}

#endif

Implement demo::getFoo() in demo.cpp.

demo.pkg lists the things that should be callable from Lua:

$#include "demo.hpp"

namespace demo
{
    class Foo
    {
        double getX() const;
    };

    Foo* getFoo();
}

Generate demo_stub.cpp and demo_stub.hpp files containing our Lua module:

$ tolua++5.1 -o demo_stub.cpp -H demo_stub.hpp demo.pkg

main.cpp is a Lua interpreter that loads the demo module:

#include "demo.hpp"

extern "C" {
#include <lua.h>
#include <lualib.h>
#include <lauxlib.h>
#include <tolua++.h>
}
#include "demo_stub.hpp"

int main()
{
    lua_State *L = lua_open();

    luaL_openlibs(L);
    tolua_demo_open(L);

    if (luaL_dofile(L, NULL) != 0)
        fprintf(stderr, "%s\n", lua_tostring(L, -1));

    lua_close(L);
    return 0;
}

The tolua_demo_open() function was generated by tolua++ and is declared in demo_stub.hpp.

Build a Lua interpreter named demo:

$ g++ -I/usr/include/lua5.1 demo.cpp demo_stub.cpp main.cpp -ltolua++5.1 -llua5.1 -o demo

Construct a demo.lua script

print("Hello, world!")
print(tolua.type(demo.getFoo()))
print(demo.getFoo():getX())

and feed the script to the interpreter:

$ ./demo < demo.lua
Hello, world!
demo::Foo
42