reference:

http://www.lua.org/manual/5.3/manual.html

print (···)

Receives any number of arguments and prints their values to stdout, using the  function to convert each argument to a string. print is not intended for formatted output, but only as a quick way to show a value, for instance for debugging. For complete control over the output, use  and.

tostring (v)

Receives a value of any type and converts it to a string in a human-readable format. (For complete control of how numbers are converted, use.)

If the metatable of v has a __tostring field, then tostring calls the corresponding value with v as argument, and uses the result of the call as its result.

示例：

a = {5, 6}print(a)--用c来做Metatablec = {}c.__tostring =function(set)    local s = "{"     local sep = ""    for _,e in pairs(set) do        s = s .. sep .. e        sep = ", "    end return s .. "}" endsetmetatable(a, c)print(a)

执行结果：

table: 0x1041150{5, 6}

注释：

1. 调用第一个打印print(a)，表示a为一个table，以及对应的地址。即print 函数调用 tostring 来格式化的输出。

2.调用第二个打印print(a)，结果为__tostring的返回值。即当格式化一个对象的时候，tostring 会首先检查对象是否存在一个带有__tostring 域的 metatable。如果存在则以对象作为参数调用对应的函数来完成格式化，返回的结果即为 tostring 的结果。