Return an NSMutableString as NSString Avoiding “Uncaught Error 11″ with Cocoa

By Michael Robinson on October 29, 2009, Filed in: Cocoa | Objective C | Rapidweaver | Slider

This post was originally published in 2009
It may contain stale & outdated information. Or it may have grown more awesome with age, like the author.

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Another stumbling block on the road to Slider completion was this:

NSUncaughtSystemExceptionException — Uncaught system exception: signal 11

This vague and unhelpful error message (in this case) was caused by my trying to return an NSMutableString in place of an NSString:

- (NSString *)generateHTML{
 
	NSMutableString *html = [NSMutableString string];
 
	@try{
 
	//Begin wrapper
	[html appendString:[NSString stringWithFormat:@"<div id=\"slider%@\" style=\"height:%ipx;width:%ipx;position:relative;z-index:0;margin-left:%ipx;%@\">",
						UID, (int)portalSize.height, (int)portalSize.width, leftMargin, [self generateBGCSS]]];
 
	//Begin other content
	[html appendString:[self someYetToBeWrittenFunction]];
 
	//Close wrapper
	[html appendString:@"</div>"];
 
	}
	@catch (NSException *e) {
		NSLog(@"generateHTML: %@", e);
	}
	return html;
}

Googling resulted in this solution, from the Cocoa Dev Archives (which have saved me from suicide more times than I can count).

The successful code is:

- (NSString *)generateHTML{
 
	NSMutableString *html = [NSMutableString string];
 
	@try{
 
	//Begin wrapper
	[html appendString:[NSString stringWithFormat:@"<div id=\"slider%@\" style=\"height:%ipx;width:%ipx;position:relative;z-index:0;margin-left:%ipx;%@\">",
						UID, (int)portalSize.height, (int)portalSize.width, leftMargin, [self generateBGCSS]]];
 
	//Begin other content
	[html appendString:[self someYetToBeWrittenFunction]];
 
	//Close wrapper
	[html appendString:@"</div>"];
 
	}
	@catch (NSException *e) {
		NSLog(@"generateHTML: %@", e);
	}
	return [NSString stringWithString:html];
}