Question

I’m trying to pass a variadic argument (NSString *) from one method to another method like the following example:

- (NSURL *)urlForId:(NSString *)aId params:(NSDictionary *)aParams parts:(NSString *)aPart, ... {
    // ... do something with parts
}

- (NSURL *)specialUrlForId:(NSString *)aId params:(NSDictionary *)aParams parts:(NSString *)aPart, ... {
    va_list arg;
    va_start(args, aPart);
    [self urlForId:aId params:aParam parts:args];
    va_end(args);
}

The problems start when trying to pass args along. ARC complains about an implicit conversion from va_list (char *) to NSString *. I’ve tried everything to get this to work.

This same technique will work if i pass the va_list into [NSString stringWithFormat:…] so I don’t see why it isn’t working here.

Any help appreciated.

Answer 1

Your urlForId:params:parts: method expects the first argument after the parts: keyboard to be an NSString*, but you’re passing it a va_list. Those are different types. Passing a va_list variable as a function parameter doesn’t magically expand it into all of the original arguments. Under the covers a va_list is really just a pointer into your stack frame, and that’s all that gets passed.

What you really need to do here is factor out the section of urlForId:params:parts: that walks the parts list, and call that factored-out section directly from specialUrlForId:params:parts:. Presumably your urlForId:params:parts: method looks something like this:

- (NSURL *)urlForId:(NSString *)aId params:(NSDictionary *)aParams parts:(NSString *)aPart, ...
{
    va_list ap;
    va_start(ap, aPart);

    // loop over the variable arguments, starting with aPart, perhaps
    // stopping when you find nil.
    while (aPart != nil) {
        doSomething(aPart);
        aPart = va_arg(ap, NSString *);
    }

    va_end(ap);

    return someUrl;
}

So what you’d do here is move the part after va_start and before va_end to a method that takes a va_list parameter:

- (NSURL *)urlForId:(NSString *)aId params:(NSDictionary *)aParams arguments:(va_list)ap
{
    // loop over the variable arguments, starting with aPart, perhaps
    // stopping when you find nil.
    while (aPart != nil) {
        doSomething(aPart);
        aPart = va_arg(ap, NSString *);
    }
    return someUrl;
}

Then you make both urlForId:params:parts: and specialUrlForId:params:parts: call this new urlForId:params:arguments: method:

- (NSURL *)urlForId:(NSString *)aId params:(NSDictionary *)aParams parts:(NSString *)aPart, ...
{
    va_list ap;
    va_start(ap, aPart);
    NSURL *url = [self urlForId:aId params:aParams arguments:ap];
    va_end(ap);
    return url;
}

- (NSURL *)specialUrlForId:(NSString *)aId params:(NSDictionary *)aParams parts:(NSString *)aPart, ...
{
    va_list ap;
    va_start(ap, aPart);
    NSURL *url = [self urlForId:aId params:aParams arguments:ap];
    va_end(ap);
    return url;
}