How do I define a macro with variadic method in objective-C?

Question

The method I am trying to call is;

- (void)addLogWithLevel:(MDCLogLevel)logLevel logContent:(NSString *)logContent, ...
{
    va_list args;
    va_start(args, logContent);
    NSString *message = [[NSString alloc] initWithFormat:logContent
                                               arguments:args];
    va_end(args);
    MDCLog *log = [MDCLog logWithContent:message content:logLevel];
    [self.deviceLogs addObject:log];
}

I have defined the macro as;

#define MDCLogDebug(format, ...) [[MDCLogController sharedController] addLogWithLevel:MDCLogLevelDebug logContent:(__VA_ARGS__)];

I have tried various formats of this macro, but nothing seems to work.

If I am to call;

MDCLogDebug(@"Test:%@", @"Hey");

All I see in the console is;

Hey

Where am I going wrong? I'm new to using Variadic methods and my C isn't so great!

Answer 1

Actually, your problem is not really related to Objective-C directly, but to C itself, as macros are plain C preprocessor directives.

In a macro, __VA_ARGS__ represents the arguments that are placed instead of the ....

So in your call to MDCLogDebug(@"Test:%@", @"Hey"), the format argument is @"Test:%@" and __VA_ARGS__ represents the rest of the arguments afterwards, namely simply @"Hey" in your case.

If you want to pass both the @"Test:%@" and @"Hey" as arguments to logContent:, you have to explicitly tell it so, using:

#define MDCLogDebug(format, ...) [[MDCLogController sharedController] addLogWithLevel:MDCLogLevelDebug logContent:format, __VA_ARGS__]

---

Note: An even better solution would be to use the ## prefix before __VA_ARGS__ so that the comma won't be added if __VA_ARGS__ is empty (namely if you only pass a format argument but nothing afterwards, like MDCLogDebug(@"Foo")):

#define MDCLogDebug(format, ...) [[MDCLogController sharedController] \
                                  addLogWithLevel:MDCLogLevelDebug \
                                       logContent:format, ## __VA_ARGS__]

(Note: I use backslashes in this last macro definition above to allow the macro to be written on multiple lines, instead of writing it on one single big long line)

For more information, see the official GCC documentation about Variadic Macros here.