Lua Libraries - Learn Lua for iOS Game Development - page 25

One way to manage this is as follows:
local cat_01 = animal.new("cat")
local mouse_01 = animal.new("mouse")
animal.move(cat_01)
animal.move(mouse_01)
This might work for some; however, an alternative method to manage this would be the following:
local cat_01 = animal.new("cat")
local mouse_01 = animal.new("mouse")
cat_01:move()
mouse_01:move()
Further, if you had this in an array of animals, you could just about as easily have something like this:
local i
for i=1,#aryAnimals do
aryAnimals[i]:move()
end
Or you could very well also use something like
aryAnimals[i].move(aryAnimals[i])
Notice here how the function is invoked using the dot ( . ) or the colon ( : ). This is something a lot
of beginning Lua developers come across, and they often have questions about the difference
between, say, cat_01:move() and cat_01.move() . The following section will describe the differences.
The Difference Between . and :
In the preceding example, there's no difference between calling cat_01.move() and cat_01:move() .
But let's consider that in the same function we also need to pass a parameter speed , which indicates
how fast the animal should move. So we simply pass the speed as a parameter as
cat_01:move(10)
or
cat_01.move(10)
This on the face of things seems right, but will produce two very different results. The reason is that
when we use the . , we are telling Lua to invoke the function that is a member of the object (table),
and when we use the : , we are telling Lua to do this and we're also passing the function an unseen
parameter of itself. So when we call the function with : , we are passing it two parameters: the first
being the object itself and the second being the speed. With the . method, on the other hand, we
pass it only one, which is the speed. Let's look at how it is defined in code (see Figure 2-1 ).