How to I pass a table from Lua into C++?

Question

How would I pass a table of unknown length from Lua into a bound C++ function?

I want to be able to call the Lua function like this:

call_C_Func({1,1,2,3,5,8,13,21})

And copy the table contents into an array (preferably STL vector)?

Answer 1

If you use LuaBind it's as simple as one registered call. As for rolling up your own, you need to take a look at lua_next function.

Basically the code is as follows:

lua_pushnil(state); // first key
index = lua_gettop(state);
while ( lua_next(state,index) ) { // traverse keys
  something = lua_tosomething(state,-1); // tonumber for example
  results.push_back(something);
  lua_pop(state,1); // stack restore
}

Answer 2

This would be my attempt (without error checking):

int lua_test( lua_State *L ) {
    std::vector< int > v;
    const int len = lua_objlen( L, -1 );
    for ( int i = 1; i <= len; ++i ) {
        lua_pushinteger( L, i );
        lua_gettable( L, -2 );
        v.push_back( lua_tointeger( L, -1 ) );
        lua_pop( L, 1 );
    }
    for ( int i = 0; i < len; ++i ) {
        std::cout << v[ i ] << std::endl;
    }
    return 0;
}

Answer 3

You can also use lua_objlen:

Returns the "length" of the value at the given acceptable index: for strings, this is the string length; for tables, this is the result of the length operator ('#'); for userdata, this is the size of the block of memory allocated for the userdata; for other values, it is 0.