Open Radar

 
Area:
Foundation

Summary:
`NSNumberFormatter.generatesDecimalNumbers = YES` makes `-[NSNumberFormatter numberFromString:]` producing `NSDecimalNumber` instead of `NSNumber`. Unfortunately this is merely a class rewrapping, does not change actual number parsing, so decimal numbers are affected by binary approximation error.

In case of numbers parsing, `-[NSNumberFormatter numberFromString:]` uses `CFNumberFormatterCreateNumberFromString` which in turn uses `cficu_unum_parseDecimal`, then `strtod` and then `CFNumberCreate` to create `CFNumber` wrapped double type. At the end, when `NSNumberFormatter.generatesDecimalNumbers == YES`, `NSNumberFormatter` executes `[NSDecimalNumber decimalNumberWithDecimal:number.decimalValue]` which is merely a class rewrapping – NSDecimalNumber stores decimal representation of binary approximation, not a true decimal representation of actual number.

Steps to Reproduce:
```
NSNumberFormatter *fmt = [[NSNumberFormatter alloc] init];
fmt.generatesDecimalNumbers = YES;
fmt.locale = [NSLocale localeWithLocaleIdentifier:@"en_US"];
NSDecimalNumber *number = (NSDecimalNumber *)[fmt numberFromString:@"99.9"];
NSString *value = [number stringValue]; // "99.90000000000001", should be "99.9"
NSDecimal decimal = [number decimalValue];
int exponent = decimal._exponent; // -14, should be -1
int mantissa = decimal._mantissa[0]; // should be 999
```

Expected Results:
When `NSNumberFormatter.generatesDecimalNumbers == YES`, `-[NSNumberFormatter numberFromString:]` should produce `NSDecimalNumber` with true decimal representation. 

Actual Results:
When `NSNumberFormatter.generatesDecimalNumbers == YES`, `-[NSNumberFormatter numberFromString:]` should produces `NSDecimalNumber` with decimal representation of rewrapped number binary approximation. Produced number is affected by binary approximation error.

Version:
iOS 10.2, macOS 10.11.6

Notes:
Optionally `NSNumberFormatter.generatesDecimalNumbers` could be deprecated, and `-[NSNumberFormatter numberFromString:]` could always produce `NSDecimalNumber`.

Configuration:
iPhone 6 Plus, MacBook Pro (Retina, 13-inch, Early 2015)

Attachments: