How to print Lua table using the redefined print function?

Question

I learned how to redefine the Lua's print() in C++ from this post. (https://stackoverflow.com/a/4514193/5224286)

Here's the redefined print function that prints variables to my host program's console. (through the functions named post..)

int l_my_print(lua_State *L)
{
    int nargs = lua_gettop(L);
    for (int i = 1; i <= nargs; ++i)
    {
        if (lua_isnil(L, i))
            poststring("nil");
        else if (lua_isboolean(L, i))
            lua_toboolean(L, i) ? poststring("true") : poststring("false");
        else if (lua_isnumber(L, i))
            postfloat(static_cast<t_float>(lua_tonumber(L, i)));
        else if (lua_isstring(L, i))
            poststring(lua_tostring(L, i));
        else if (lua_istable(L, i))
            poststring("table: "); //how to print like Lua's built-in print()?
    }
    endpost();
    return 0;
}

This code works fine except when I print tables. (it just prints table: now)

I wonder how to print tables just like how Lua's print() works.

For example, when I run the following code in Lua: (before the redefine)

print({1,2,3});

I get this result: (which seems to change constantly)

table: 0x23b8660

Is this a hex representation of pointer to the Lua-table?

What should I do with my l_my_print() function so it can work just like Lua's print()?

See lua_topointer: lua.org/manual/5.3/manual.html#lua_topointer — John5342
– John5342, Commented Sep 1, 2018 at 11:13
"Is this a hex representation of a pointer to the Lua-table?" Not by definition. It's not even guaranteed to be unique. (People do find it somewhat useful when debugging when it is, though.) Why do you need it? — Tom Blodget
– Tom Blodget, Commented Sep 3, 2018 at 19:15
@TomBlodget I don't need it specifically for anything. I just wanted to make my custom print to work just like the original one. — Zack Lee
– Zack Lee, Commented Sep 4, 2018 at 15:22

Henri Menke · Accepted Answer · 2018-09-02 03:08:57Z

Just use luaL_tolstring to get the string representation of anything. This also respects the __tostring metamethod. The example below uses std::string_view from C++17 for zero-copy read-only string arguments.

#include <iostream>
#include <string_view>

#include <lua.hpp>

void poststring(std::string_view sv) { std::cout << sv << '\n'; }

void endpost() { std::cout << "---\n"; }

int l_my_print(lua_State *L) {
    int nargs = lua_gettop(L);
    for (int i = 1; i <= nargs; ++i) {
        poststring(luaL_tolstring(L, i, nullptr));
        lua_pop(L, 1); // remove the string
    }
    endpost();
    return 0;
}

int main() {
    lua_State *L = luaL_newstate();
    luaL_openlibs(L);

    lua_pushcfunction(L, l_my_print);
    lua_setglobal(L, "my_print");

    int i = 0;
    lua_pushlightuserdata(L, &i);
    lua_setglobal(L, "udata");

    luaL_dostring(L, "my_print(1, 3.14, \"Hello World\")\n"
                     "my_print(false, udata, {})\n");

    lua_close(L);
}

Example invocation:

$ clang++ -Wall -Wextra -Wpedantic -std=c++17 -I/usr/include/lua5.3 test.cpp -llua5.3
$ ./a.out 
1
3.14
Hello World
---
false
userdata: 0x7fff4685993c
table: 0x883300
---

G. B. · Accepted Answer · 2018-09-05 14:45:58Z

0

Or you could do the conversion in Lua:

    --Produces a compact, uncluttered representation of a table. Mutual recursion is employed
    --source: http://lua-users.org/wiki/TableUtils
    function table.tostring( tbl )
      local result, done = {}, {}
      for k, v in ipairs( tbl ) do
        table.insert( result, table.val_to_str( v ) )
        done[ k ] = true
      end
      for k, v in pairs( tbl ) do
        if not done[ k ] then
          table.insert( result,
            table.key_to_str( k ) .. "=" .. table.val_to_str( v ) )
        end
      end
      return "{" .. table.concat( result, "," ) .. "}"
    end

answered Sep 5, 2018 at 14:45

G. B.

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