When would initWithFormat:arguments: be used?

The NSString method initWithFormat:arguments: takes a va_list as an argument. I can't f开发者_Python百科igure out when it would be used (or even how to use it). Why would Apple add such a method when the regular initWithFormat: is much more user-friendly?

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You can't pass a dynamic list of format arguments to -initWithFormat:. For example, if you wanted to implement -stringByAppendingFormat: yourself without -initWithFormat:arguments:, you'd have a job of it. With the va_list version, you could do it:

- (NSString *)stringByAppendingFormat:(NSString *)format, ... {
    va_list args;
    va_start(args, format);
    NSString * result = [self stringByAppendingString:[NSString stringWithFormat:format arguments:args]];
    va_end(args);
    return result;
}

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It's useful when your own function or method uses variadic arguments, because in that case it is impossible to use the vanilla initWithFormat: method.

For instance, the following (useless) example snippet:

void log(NSString* format, ...)
{
    va_list arguments;
    va_start(arguments, format);

    // impossible:
    // NSString* formattedString = [[NSString alloc] initWithFormat: ???];

    // possible
    va_list argsCopy;
    va_copy(argsCopy, arguments);
    NSString* formattedString = [[NSString alloc] initWithFormat:format arguments:argsCopy];    

    // do something cool with your string
    NSLog(@"%@", formattedString);
    va_end(argsCopy);
    va_end(arguments);
}

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I would say without looking further into it that Apple provide NSString initWithFormat: as a utility method on top of NSString initWithFormat:arguements: meaning the short version just ends up calling the longer one.

There's also [NSString stringWithFormat:] that can return an autoreleased NSString, saving you the alloc call if you don't need the string around for long.