Scripting with Lua - Game Coding Complete - page 362

In Lua, tables are used for just about everything. A variable is really just a field in a
table indexed by a string. That string is the variable name. When you define a global
variable in Lua, what really happens is that Lua inserts it into a special table where all
global variables live. There ' s nothing special about this table; you can even access it

directly.

-- These two lines are equivalent
x=10
_G[ “ x ” ]=10

-- This will print out all global variables
for key, val in pairs(_G) do
print(key, val)
end
In fact, if you re feeling really crazy, you can even change the behavior of global vari-

ables by assigning a metatable to _G with __index or __newindex defined! You
could completely forbid new global variables or call a function whenever a global is
accessed or set.

'

Don ' t Do That

Just because you can modify the behavior of the globals table doesn t mean

you should. Leave it alone unless you have a really, really, really good
reason. I

'

ve shipped five professional games with Lua, and we never once
had to mess with the behavior of this table. Chances are, neither will you.

'

To access a global variable in LuaPlus, you grab the globals table from the LuaState
object and access it like any other member. Here ' s the missing code from the exam-
ple above that gets the positionVec global:

LuaObject globals = pLuaState->GetGlobals();
LuaObject positionTable = globals.GetByName( “ positionVec ” );

Once you have this globals table, you can assign values as well:

globals.SetString( );

This creates a global variable called programName and sets it to the value of
“Teapot Wars.”

“ programName ” , “ Teapot Wars ”

You can access it in Lua as normal:

print(programName) -- > prints “ Teapot Wars ”