Iterating a va_list when passed as a parameter to a method in Objective-C - objective-c

Question

I would like to pass a variable argument list from one method (functionOne) to another (functionTwo). Everything works fine, except that I have not been able to figure out how to setup the va_list in functionTwo in a way where I can access the first parameter in the va_list. Using va_arg advances to the second parameter in the va_list. Thx.
- (void)functionOne:(NSString *)configFiles, ... {
va_list args;
va_start(args, configFiles);
[self functionTwo:args];
va_end(args);
}
- (void)functionTwo:(va_list)files {
NSString *file;
while ((file = va_arg(configFiles, NSString *))) {
...
}
}

Answer 1

The first variadic argument is not the argument passed to va_start – it's the one immediately following it. If you want functionTwo: to have access to the configFiles string, you'll need to pass it in explicitly.

Answer 2

See Technical Q&A QA1405: Variable arguments in Objective-C methods.
Methods that take variable arguments are known as variadic methods.
Keep in mind that the implementation of an Objective-C method is just
a block of code, like a C function. The variadic argument macros
described in the stdarg(3) manual page work the same way in a method
as they do in an ordinary function.
Here's an example of an Objective-C category, containing a variadic
method that appends all the objects in a nil-terminated list of
arguments to an NSMutableArray instance:
#import <Cocoa/Cocoa.h>
#interface NSMutableArray (variadicMethodExample)
// This method takes a nil-terminated list of objects.
- (void)appendObjects:(id)firstObject, ...;
#end
#implementation NSMutableArray (variadicMethodExample)
- (void)appendObjects:(id)firstObject, ... {
id eachObject;
va_list argumentList;
if (firstObject) // The first argument isn't part of the varargs list,
{ // so we'll handle it separately.
[self addObject: firstObject];
// Start scanning for arguments after firstObject.
va_start(argumentList, firstObject);
while (eachObject = va_arg(argumentList, id)) // As many times as we can get an argument of type "id"
[self addObject: eachObject]; // that isn't nil, add it to self's contents.
va_end(argumentList);
}
}
#end

Answer 3

A solution that I use for debugging purposes is like
-(void) debug:(NSString*)format, ... {
if (level < MXMLogLevelDebug) return;
if(format == nil) return;
va_list args, args_copy;
va_start(args, format);
va_copy(args_copy, args);
va_end(args);
NSString *logString = [[NSString alloc] initWithFormat:format
arguments:args_copy];
NSString *funcCaller = #"";
NSArray *syms = [NSThread callStackSymbols];
if ([syms count] > 1) {
funcCaller = [syms objectAtIndex:1];
}
NSString *logMessage = [NSString stringWithFormat:#"%# DEBUG: %#", funcCaller, logString];
NSLog(#"%#",logMessage);
}
The side-effect that this can have is that you have to add a guard on the args to be sure is not NULL.