strlen 返回纯C字符串的字符串长度。

strlen returns the length of string of a plain C string.

AC字符串与字符串开头和终止空字符之间的字符数量一样长。

A C string is as long as the amount of characters between the beginning of the string and the terminating null character.

如果你使用String对象，你可以使用 length 或

If you're using the String object you can use the length or size method of the object:

http://www.cplusplus.com/reference/string/string/length/