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Variable length argument lists

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Submitted on: 1/3/2015 6:59:00 AM
By: Jason Beighel (from psc cd)  
Level: Intermediate
User Rating: By 5 Users
Compatibility: C, C++ (general), Microsoft Visual C++, UNIX C++
Views: 612
 
     After looking at functions like printf() and scanf() and their ability to accept any number of arguments I got curious to see how its done. This article explains how to create a function that will allow any number of parameters to be passed to it, and ho to accept those extra parameters.

 
				

In C it is possible to create a function that will accept a variable amount of arguments. The parameter must have at least one defined parameter followed by an ellipsis, three periods . For example printf()s prototype has one defined parameter and an ellipses:

printf(char *, );

However, just an ellipses is not allowed:

function();

You may use as many defined arguments as you would like, but the ellipses must be the last, for example:

function(int, char *, float, );

There are several functions used to access the arguments passed through the ellipses, to use them you must include cstdarg.

#include

Note that cstdarg is not a header file, but it is included like one.

In the function that is receiving the variable arguments you must have a va_list variable that is used by the functions that retrieve the arguments passed through the ellipses. The function va_start() will setup the va_list variable so you can retrieve arguments. The va_start() function takes two arguments. The first is the va_list variable, and the second is the argument that immediately precedes the variable arguments.

The function used to actually retrieve the arguments is va_arg(). This function also takes two arguments. The first is a va_list variable that has previously established with va_start(), and the second is the type of variable you want to retrieve. For the type you use any variable type; for example int or char. The va_arg() function will return the value of the variable you are retrieving.

When you are done you must call va_end() to clean up the va_list variable. It only takes a va_list variable.

Here is an example of all that put together:

#include<cstarg>

#include<stdio.h>

void VarArg(int Num, );

void main(void)

{

VarArg(4, 1, 2, 3, 4);

VarArg(3, 6, 7, 8);

return;

}

void VarArg(int Num, )

{

va_list VList;

va_start(VList, Num);

printf(\nPassed parameters: );

for (; Num > 0; Num--)

{

printf( %d,, va_arg(VList, int);

}

return;

}


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