How to use va_args to pass arguments on (variadic parameters, elipsis)

Question

I can't get my head around the syntax for multiple arguments in Objective-C. I have seen this question, but the answer hasn't helped me (yet).

Here is my code (actually I will want to eventually pass to NSString stringWithFormat, but getting an NSLog to work would be good enough for now):

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
 // Insert code here to initialize your application 
 [self log:@"blah blah %d", 32];
}


- (void)log:(NSString *)text, ... {
 va_list args;
 va_start(args, text);
 NSLog(text, args);
}

The argument (or some argument) comes through, but it's got some weird value (output is blah blah 1606412704). How should I pass the values that come in via ...?

Answer 1

You could use -[NSString initWithFormat:arguments:]:

- (void)log:(NSString *)text, ...
{
    va_list args;
    va_start(args, text);
    NSString *log_msg = [[[NSString alloc] initWithFormat:text arguments:args] autorelease];
    NSLog(@"%@", log_msg);
}

Answer 2

There's a variant of NSLog that accepts a va_list called NSLogv:

- (void) log:(NSString *)text, ... {
  va_list args;
  va_start(args, text);
  NSLogv(text, args);
  va_end(args);
}

The only way to forward the actual ... (not the va_list) is to use a macro. For example:

#define MyLog(f,...) { \
NSLog(f,##__VA_ARGS); \
[someObject doSomething:f, ##__VA_ARGS]; \
}

However, this should be used very sparingly, since macros can make code really obfuscated.