Rerouting Lua print in C++ with default print behavior

Question

I am using the answer to this question to redirect lua's print to a stringstream. My function code is below.

My issue is that the code doesn't always match what lua would have printed on it's own. Most notably, when I attempt to print a function, I just get the function, rather than the function and address. Example:

> --in lua
> print(os.exit)
function: 0xabcdef01

> --in my interpreter
> print(os.exit)
function

The obvious solution is to force my custom print function to call lua's tostring prior to writing to luaout (like the default print does). However, I can't really figure out how to make this work. If anyone can help me out, I'd appreciate it a lot.

Here's my custom print:

static int l_my_print(lua_State* L) {
    int nargs = lua_gettop(L);

    for (int i=1; i <= nargs; i++) {
        int t = lua_type(L, i);
        switch (t) {
            case LUA_TSTRING: { /* strings */
                luaout << lua_tostring(L, i);
                break;
            }
            case LUA_TBOOLEAN: { /* booleans */
                luaout << (lua_toboolean(L, i) ? "true" : "false");
                break;
            }
            case LUA_TNUMBER: { /* numbers */
                luaout << lua_tonumber(L, i);
                break;
            }
            default: { /* other values */
                luaout << lua_typename(L, t);
                break;
            }
        }
        if (i!=nargs){
            luaout << "\t";
        }
    }
    luaout << endl;

    return 0;
}

See my answer here: http://stackoverflow.com/a/40025297/774082 — Martin Gerhardy, Oct 13 '16 at 15:38

tozka · Accepted Answer · 2012-10-08T14:04:12.170

4

You can try on default this:

lua_pushfstring(L, "%s: %p", luaL_typename(L, i), lua_topointer(L, i));

or

 luaout << luaL_typename(L, i) << ": " << lua_topointer(L, i);

This will add the name of the function and the pointer.

You can call lua function from C++ (don't guarantee to work as I have not tested it, but it should aside from grammatical errors)

lua_getglobal(L, "tostring");
lua_pushvalue (L, i);

if (lua_pcall(L, 1, 1, 0) != 0) {
        printf("error running function `%s': %s\n", "tostring", lua_tostring(L, -1));
        return -1;
}
// get result
char *result = luaL_checkstring (L, -1);

edited Oct 08 '12 at 14:04

answered Oct 08 '12 at 13:26

tozka

3,211
19
23

this is partially helpful, as it will behave the expected way for things like functions, but the ideal solution is to simply output the result of the tostring function. What I need to do is force lua to call tostring and then push the resultant string to the stack, rather than pushing the object itself – ewok Oct 08 '12 at 13:28
one note: in line 2, `t` should be `i`. Other than that, it works great. Thanks! – ewok Oct 08 '12 at 13:49

Rerouting Lua print in C++ with default print behavior

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