lua中print

reference:

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

 

print (···)

Receives any number of arguments and prints their values to stdout, using the tostring 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 string.format andio.write.

 

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, usestring.format.)

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來作Metatable
c = {}
c.__tostring =function(set)
    local s = "{" 
    local sep = ""
    for _,e in pairs(set) do
        s = s .. sep .. e
        sep = ", "
    end 
return s .. "}" 
end

setmetatable(a, c)
print(a)

 

執行結果：

table: 0x1041150
{5, 6}

 

註釋：

1. 調用第一個打印print(a)，表示a爲一個table，以及對應的地址。即print 函數調用 tostring 來格式化的輸出。

2.調用第二個打印print(a)，結果爲__tostring的返回值。即當格式化一個對象的時候，tostring 會首先檢查對象是否存在一個帶有__tostring 域的 metatable。若是存在則以對象做爲參數調用對應的函數來完成格式化，返回的結果即爲 tostring 的結果。