variadic-functions

问题 I am trying to write a server class with a method that takes a function as an argument and pass in a value (generated inside the server class) to the function for being run inside a thread. The following is just an example to illustrate what I am trying to do. My goal is to have the Server::runFunctionInThread general enough that works for methods of different classes as well as for functions that do not belong to any classes. Is this a possible thing to do? #include <iostream> #include

问题 Here is my minimal example: #include <stdio.h> #include <stdarg.h> #include <string.h> void print_strings_and_lengths(int count, ...) { va_list ap; /* Print strings */ va_start(ap, count); for (int i = 0; i < count; i++) { char *s = va_arg(ap, char *); printf("%d - %s\n", i, s); } /* Print string lengths */ va_start(ap, count); /* Is it okay to call va_start() again without calling va_end()? */ for (int i = 0; i < count; i++) { char *s = va_arg(ap, char *); printf("%d - %zu\n", i, strlen(s));

问题 Spark API RelationalGroupedDataset has a function agg : @scala.annotation.varargs def agg(expr: Column, exprs: Column*): DataFrame = { toDF((expr +: exprs).map { case typed: TypedColumn[_, _] => typed.withInputType(df.exprEnc, df.logicalPlan.output).expr case c => c.expr }) } Why does it take two separate arguments? Why can't it take just exprs: Column* ? Has someone an implicit function that takes one argument? 回答1: This is to make sure that you specify at least one argument. Pure varargs

问题 Spark API RelationalGroupedDataset has a function agg : @scala.annotation.varargs def agg(expr: Column, exprs: Column*): DataFrame = { toDF((expr +: exprs).map { case typed: TypedColumn[_, _] => typed.withInputType(df.exprEnc, df.logicalPlan.output).expr case c => c.expr }) } Why does it take two separate arguments? Why can't it take just exprs: Column* ? Has someone an implicit function that takes one argument? 回答1: This is to make sure that you specify at least one argument. Pure varargs

问题 I want to implement a macro named PRINT which gets zero or more parameters, and does the following: If it gets zero parameters - do nothing. If it gets one or more arguments - act like printf. I succeed in implementing it as you can see in my code below, but only at the cost of calling to printf with an empty string in the case we get zero arguments. Is there a way I can handle the zero arguments case without calling to printf (it's not efficient to print something when you just want to do

问题 I want to implement a macro named PRINT which gets zero or more parameters, and does the following: If it gets zero parameters - do nothing. If it gets one or more arguments - act like printf. I succeed in implementing it as you can see in my code below, but only at the cost of calling to printf with an empty string in the case we get zero arguments. Is there a way I can handle the zero arguments case without calling to printf (it's not efficient to print something when you just want to do

问题 I want to implement a macro named PRINT which gets zero or more parameters, and does the following: If it gets zero parameters - do nothing. If it gets one or more arguments - act like printf. I succeed in implementing it as you can see in my code below, but only at the cost of calling to printf with an empty string in the case we get zero arguments. Is there a way I can handle the zero arguments case without calling to printf (it's not efficient to print something when you just want to do

问题 I want to implement a macro named PRINT which gets zero or more parameters, and does the following: If it gets zero parameters - do nothing. If it gets one or more arguments - act like printf. I succeed in implementing it as you can see in my code below, but only at the cost of calling to printf with an empty string in the case we get zero arguments. Is there a way I can handle the zero arguments case without calling to printf (it's not efficient to print something when you just want to do

问题 I want to implement a macro named PRINT which gets zero or more parameters, and does the following: If it gets zero parameters - do nothing. If it gets one or more arguments - act like printf. I succeed in implementing it as you can see in my code below, but only at the cost of calling to printf with an empty string in the case we get zero arguments. Is there a way I can handle the zero arguments case without calling to printf (it's not efficient to print something when you just want to do

问题 I want to implement a macro named PRINT which gets zero or more parameters, and does the following: If it gets zero parameters - do nothing. If it gets one or more arguments - act like printf. I succeed in implementing it as you can see in my code below, but only at the cost of calling to printf with an empty string in the case we get zero arguments. Is there a way I can handle the zero arguments case without calling to printf (it's not efficient to print something when you just want to do