The first time around, the templated version of f is called, with a string as the first argument type, and the character and double are packed off as the variablesize argument.
首先调用 f 的模板化版本，第一个参数的类型是字符串，作为大小可变的参数提供字符和双精度值。
2
Variable-length character string having maximum length size bytes or characters.