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objective-cxcodeformatvariadic-functionsnull
Chris Rutkowski
Chris Rutkowski

Reputation: 1814

How to create variable argument methods in Objective-C

Maybe this will be obviously simple for most of you, but could you please give an example how to create similar methods (in Objective-C) and functions in C to create functions like NSString's stringWithFormat:, or NSLog().

Just to remind:

[NSString stringWithFormat:@"example tekst %i %@ %.2f", 122, @"sth", 3.1415"];
NSLog(@"account ID %i email %@", accountID, email);

I'd like to create the similar to NSString's method stringWithFormat:, NSURL - urlWithFormat.

Upvotes: 69

Views: 46888

Answers (4)

Jinwoo Kim
Jinwoo Kim

Reputation: 607

Here is GNUStep's implementation

#import <Foundation/Foundation.h>

#define    GS_MAX_OBJECTS_FROM_STACK    128

#define GS_USEIDLIST(firstObject, code...) ({\
  va_list    __ap; \
  unsigned int    __max = GS_MAX_OBJECTS_FROM_STACK; \
  unsigned int    __count = 0; \
  id        __buf[__max]; \
  id        *__objects = __buf; \
  id        __obj = firstObject; \
  va_start(__ap, firstObject); \
  while (__obj != nil && __count < __max) \
    { \
      __objects[__count] = __obj; \
      __obj = va_arg(__ap, id); \
      if (++__count == __max) \
    { \
      while (__obj != nil) \
        { \
          __count++; \
          __obj = va_arg(__ap, id); \
        } \
    } \
    } \
  va_end(__ap); \
  if (__count > __max) \
    { \
      unsigned int    __tmp; \
      __objects = (id*)NSZoneMalloc(NSDefaultMallocZone(),__count*sizeof(id)); \
      va_start(__ap, firstObject); \
      __objects[0] = firstObject; \
      for (__tmp = 1; __tmp < __count; __tmp++) \
    { \
      __objects[__tmp] = va_arg(__ap, id); \
    } \
      va_end(__ap); \
    } \
  code; \
  if (__objects != __buf) NSZoneFree (NSDefaultMallocZone(),__objects); \
})

@interface MyObject : NSObject
- (void)printWithObjects:(id)firstObject, ... NS_REQUIRES_NIL_TERMINATION;
@end

@implementation MyObject

- (void)printWithObjects:(id)firstObject, ... {
    GS_USEIDLIST(firstObject, {
        for (unsigned int i = 0; i < __count; i++) {
            NSLog(@"%@", __objects[i]);
        }
    });
}

@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        [[MyObject new] printWithObjects:@"1", @"2", @"3", nil];
    }
    return 0;
}

Upvotes: 0

karim
karim

Reputation: 15589

One thing to mention here is that, the first NSString parameter here comes as format, and the other are passed in the variable argument. right? So before entering the for loop, you have one parameter to handle.

- (NSString *) append:(NSString *)list, ...
{
    NSMutableString * res = [NSMutableString string];
    [res appendString:list];

    va_list args;
    va_start(args, list);
    id arg = nil;

    while(( arg = va_arg(args, id))){
        [res appendString:arg];
    }
    va_end(args);
    return res;
}

- (void) test_va_arg
{
    NSString * t = [self append:@"a", @"b", @"c", nil];
    STAssertEqualObjects(@"abc", t, @"");
}

Upvotes: 12

Williham Totland
Williham Totland

Reputation: 29009

What these are called, generally, is "variadic functions" (or methods, as it were).

To create this, simply end your method declartion with , ..., as in 

- (void)logMessage:(NSString *)message, ...;

At this point you probably want to wrap it in a printf-like function, as implementing one of those from scratch is trying, at best.

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

Note the use of NSLogv and not NSLog; consider NSLog(NSString *, ...); vs NSLogv(NSString *, va_list);, or if you want a string; initWithFormat:arguments: on NSString *.

If, on the other hand, you are not working with strings, but rather something like

+ (NSArray *)arrayWithObjects:(id)object, ... NS_REQUIRES_NIL_TERMINATION;

things get a lot easier.

In that case, instead of a vprintf-style function, use a loop going through args, assuming id as you go, and parse them as you would in any loop.

- (void)logMessage:(NSString *)format, ... {
  va_list args;
  va_start(args, format);

  id arg = nil;
  while ((arg = va_arg(args,id))) {
  /// Do your thing with arg here
  }

  va_end(args);
}

This last sample, of course, assumes that the va_args list is nil-terminated.

Note: In order to make this work you might have to include <stdarg.h>; but if memory serves, this gets included in connection with NSLogv, meaning it comes down by way of "Foundation.h", therefore also "AppKit.h" and "Cocoa.h", as well as a number of others; so this should work out of the box.

Upvotes: 134

M-frankied
M-frankied

Reputation: 370

- (void)methodWithFormat:(NSString*)format, ... {
  va_list args;
  va_start(args,format);
  //loop, get every next arg by calling va_arg(args,<type>)
  // e.g. NSString *arg=va_arg(args,NSString*) or int arg=(args,int)
  va_end(args);
}

If you want to pass the variable arguments to stringWithFormat:, use something like:

NSString *s=[[[NSString alloc] initWithFormat:format arguments:args] autorelease];

Upvotes: 23

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