Cocoa – Formatting URL parameters in an NSURL object

Introduced in iOS8 and OS X 10.10 is NSURLQueryItem, which can be used to build queries. From the docs on NSURLQueryItem:

An NSURLQueryItem object represents a single name/value pair for an item in the query portion of a URL. You use query items with the queryItems property of an NSURLComponents object.

To create one use the designated initializer queryItemWithName:value: and then add them to NSURLComponents to generate an NSURL. For example:

NSURLComponents *components = [NSURLComponents componentsWithString:@"http://stackoverflow.com"];
NSURLQueryItem *search = [NSURLQueryItem queryItemWithName:@"q" value:@"ios"];
NSURLQueryItem *count = [NSURLQueryItem queryItemWithName:@"count" value:@"10"];
components.queryItems = @[ search, count ];
NSURL *url = components.URL; // http://stackoverflow.com?q=ios&count=10

Notice that the question mark and ampersand are automatically handled. Creating an NSURL from a dictionary of parameters is as simple as:

NSDictionary *queryDictionary = @{ @"q": @"ios", @"count": @"10" };
NSMutableArray *queryItems = [NSMutableArray array];
for (NSString *key in queryDictionary) {
    [queryItems addObject:[NSURLQueryItem queryItemWithName:key value:queryDictionary[key]]];
}
components.queryItems = queryItems;

I've also written a blog post on how to build URLs with NSURLComponents and NSURLQueryItems.

NSURL objects are immutable so there is no way to change the base URL after creating it. Your best bet is creating a new NSURL instance with URLWithString:relativeToURL:. However, I still think what you want will not work. The docs for -[NSURL relativePath] state that if the receiver is an absolute URL (which your example is), this method returns the same value as path.