lua中的self

lua编程中，经常遇到函数的定义和调用。我们有两种函数定义和调用的方法。一种是用属性的方式，另外一种是通过冒号的形式（其实也是属性）。只不过用冒号形式申明的函数默认会有一个参数self。self指向本身（表）。下面举例说明：

shape = {side = 4}
function shape.set_side(shape, side)
    shape.side = side
end

function shape.print_area(shape)
    print(shape.side * shape.side)
end

print(shape.side)
shape.set_side(shape, 5)
print(shape.side)
shape.print_area(shape)

上述运行结果是：

4
5
25

上面是用“ . ”来定义和访问函数的方法。下面同样用“ ：”来实现同样功能的改写如下

shape = {side = 4}
function shape:set_side(side)
    self.side = side
end

function shape:print_area()
    print(self.side * self.side)
end

print(shape.side)
shape:set_side(5)
print(shape.side)
shape:print_area()

运行结果和上面例子一样：

4
5
25

从上面两个例子我们可以看出：冒号定义和冒号调用其实跟上面的效果一样，只是把第一个隐藏参数省略了。而self则是指向调用者自身。
当然，我们也可以用点号“ . ”来定义函数，冒号“ ：”来调用函数。或者冒号定义点号调用。如下：

shape = {side = 4}
function shape.set_side(shape, side)
    shape.side = side
end

function shape.print_area(shape)
    print(shape.side * shape.side)
end

print(shape.side)
shape:set_side(5)
print(shape.side)
shape:print_area()

或者

shape = {side = 4}
function shape:set_side(side)
    self.side = side
end

function shape:print_area()
    print(self.side * self.side)
end

print(shape.side)
shape.set_side(shape, 5)
print(shape.side)
shape.print_area(shape)

上述运行结果：

4
5
25