CODEWITHSUNDEEP
objective-cios
Dan2552
Dan2552

Reputation: 1264

Defining methods with NSString with format

How do you define a method in your own classes that accepts an NSString with format?

I see several different things using it like NSLog, [NSPredicate predicateWithFormat:(NSString *)predicateFormat, ...] and of course [NSString stringWithFormat:(NSString *)format, ...]

Also, in the header files, NSLog and stringWithFormat have the following after their declaration: NS_FORMAT_FUNCTION(1,2). Googling didn't help much with telling me what this meant.

Obviously the ellipsis is the format parameters, but I don't know how to deal with them in the method itself.

Upvotes: 4

Views: 2678

Answers (2)

rmaddy
rmaddy

Reputation: 318944

You need to make use of some C-code:

- (void)someMethod:(NSString *)format,... {
    va_list argList;
    va_start(argList, format);
    while (format) {
        // do something with format which now has next argument value
        format = va_arg(argList, id);
    }
    va_end(argList);
}

And it's possible to forward the args in to NSString as follows:

- (void)someMethod:(NSString *)format,... {
    va_list args;
    va_start(args, format);
    NSString *msg = [[NSString alloc] initWithFormat:format arguments:args];
    va_end(args);
}

Upvotes: 7

DrummerB
DrummerB

Reputation: 40221

You're looking for Objective-C methods with variable number of arguments. Here is how it's done:

Declaration in the header:

- (void)setContentByAppendingStrings:(NSString *)firstString, ... 
   NS_REQUIRES_NIL_TERMINATION;

Implementation:

- (void)setContentByAppendingStrings:(NSString *)firstArg, ...
{
    NSMutableString *newContentString = [NSMutableString string];

    va_list args;
    va_start(args, firstArg);
    for (NSString *arg = firstArg; arg != nil; arg = va_arg(args, NSString*))
    {
        [newContentString appendString:arg];
    }
    va_end(args);

    contents = newContentString;
}

Source: Cocoa With Love.

Upvotes: 2

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