Difference between NSLog and NSLogv

Question

Can anyone explain the difference between NSLog and NSLogv? I know NSLog is used to print data in the console. But what is NSLogv?

Answer 1

Suppose you want to write a function similar to NSLog, but which also saves the message to an array in addition to logging it. How would you implement this?

If you write a variadic function void MySpecialLog(NSString *format, ...), someone can call your function just like NSLog — MySpecialLog(@"Hello %@!", name); — but the only way to access the extra arguments beyond format is with a va_list. There's no splat operator in C or Obj-C allowing you to pass them directly to NSLog inside the function.

NSLogv solves this problem by accepting all the additional arguments at once via a va_list. Its signature is void NSLogv(NSString *format, va_list args). You can use it to build your own NSLog wrappers.

Obj-C

void MySpecialLog(NSString *format, ...)
  NS_FORMAT_FUNCTION(1, 2)
    // The NS_FORMAT_FUNCTION attribute tells the compiler to treat the 1st argument like
    // a format string, with values starting from the 2nd argument. This way, you'll
    // get the proper warnings if format specifiers and arguments don't match.
{
    va_list args;
    va_start(args, format);

    // Do something slightly more interesting than just passing format & args through...
    NSString *newFormat = [@"You've called MySpecialLog()! " stringByAppendingString:format];

    NSLogv(newFormat, args);

    va_end(args);
}

You can even use the same technique to wrap NSLog with an Obj-C method. (And since -[NSString initWithFormat:] has a similar variant called -initWithFormat:arguments:, you can wrap it too.)

- (void)log:(NSString *)format, ... NS_FORMAT_FUNCTION(1, 2)
{
    // Similarly to the above, we can pass all the arguments to -initWithFormat:arguments:.
    va_list args;
    va_start(args, format);
    NSString *message = [[NSString alloc] initWithFormat:format arguments:args];
    va_end(args);

    // Why not both?
    va_start(args, format);
    NSLogv(format, args);
    va_end(args);
}

---

Swift

In Swift, you can do this with a variadic function accepting CVarArg...:

func mySpecialLog(_ format: String, _ args: CVarArg...) {
    withVaList(args) {
        NSLogv("You've called mySpecialLog()! " + format, $0)
    }
}