NSDecimalNumberBehaviors Protocol Reference

Adopted by
Framework
/System/Library/Frameworks/Foundation.framework
Availability
Available in OS X v10.0 and later.
Companion guide
Declared in
NSDecimal.h
NSDecimalNumber.h

Overview

The NSDecimalBehaviors protocol declares three methods that control the discretionary aspects of working with NSDecimalNumber objects.

The scale and roundingMode methods determine the precision of NSDecimalNumber’s return values and the way in which those values should be rounded to fit that precision. The exceptionDuringOperation:error:leftOperand:rightOperand: method determines the way in which an NSDecimalNumber object should handle different calculation errors.

For an example of a class that adopts the NSDecimalBehaviors protocol, see the specification for NSDecimalNumberHandler.

Tasks

Rounding

Handling Errors

Instance Methods

exceptionDuringOperation:error:leftOperand:rightOperand:

Specifies what an NSDecimalNumber object will do when it encounters an error. (required)

- (NSDecimalNumber *)exceptionDuringOperation:(SEL)method error:(NSCalculationError)error leftOperand:(NSDecimalNumber *)leftOperand rightOperand:(NSDecimalNumber *)rightOperand
Parameters
method

The method that was being executed when the error occurred.

error

The type of error that was generated.

leftOperand

The left operand.

rightOperand

The right operand.

Discussion

There are four possible values for error, described in NSCalculationError. The first three have to do with limits on the ability of NSDecimalNumber to represent decimal numbers. An NSDecimalNumber object can represent any number that can be expressed as mantissa x 10^exponent, where mantissa is a decimal integer up to 38 digits long, and exponent is between –256 and 256. The fourth results from the caller trying to divide by 0.

In implementing exceptionDuringOperation:error:leftOperand:rightOperand:, you can handle each of these errors in several ways:

  • Raise an exception. For an explanation of exceptions, see Exception Programming Topics.

  • Return nil. The calling method will return its value as though no error had occurred. If error is NSCalculationLossOfPrecision, method will return an imprecise value—that is, one constrained to 38 significant digits. If error is NSCalculationUnderflow or NSCalculationOverflow, method will return NSDecimalNumber's notANumber. You shouldn’t return nil if error is NSDivideByZero.

  • Correct the error and return a valid NSDecimalNumber object. The calling method will use this as its own return value.

Availability
  • Available in OS X v10.0 and later.
Declared In
NSDecimalNumber.h

roundingMode

Returns the way that NSDecimalNumber's decimalNumberBy... methods round their return values. (required)

- (NSRoundingMode)roundingMode
Return Value

Returns the current rounding mode. See “NSRoundingMode” for possible values.

Availability
  • Available in OS X v10.0 and later.
Declared In
NSDecimalNumber.h

scale

Returns the number of digits allowed after the decimal separator. (required)

- (short)scale
Return Value

The number of digits allowed after the decimal separator.

Discussion

This method limits the precision of the values returned by NSDecimalNumber’s decimalNumberBy... methods. If scale returns a negative value, it affects the digits before the decimal separator as well. If scale returns NSDecimalNoScale, the number of digits is unlimited.

Assuming that roundingMode returns NSRoundPlain, different values of scale have the following effects on the number 123.456:

Scale

Return Value

NSDecimalNoScale

123.456

2

123.45

0

123

–2

100

Availability
  • Available in OS X v10.0 and later.
Declared In
NSDecimalNumber.h

Constants

NSRoundingMode

These constants specify rounding behaviors.

enum {
   NSRoundPlain,
   NSRoundDown,
   NSRoundUp,
   NSRoundBankers
};
typedef NSUInteger NSRoundingMode;
Constants
NSRoundPlain

Round to the closest possible return value; when caught halfway between two positive numbers, round up; when caught between two negative numbers, round down.

Available in OS X v10.0 and later.

Declared in NSDecimal.h.

NSRoundDown

Round return values down.

Available in OS X v10.0 and later.

Declared in NSDecimal.h.

NSRoundUp

Round return values up.

Available in OS X v10.0 and later.

Declared in NSDecimal.h.

NSRoundBankers

Round to the closest possible return value; when halfway between two possibilities, return the possibility whose last digit is even.

In practice, this means that, over the long run, numbers will be rounded up as often as they are rounded down; there will be no systematic bias.

Available in OS X v10.0 and later.

Declared in NSDecimal.h.

Discussion

The rounding mode matters only if the scale method sets a limit on the precision of NSDecimalNumber return values. It has no effect if scale returns NSDecimalNoScale. Assuming that scale returns 1, the rounding mode has the following effects on various original values:

Original Value

NSRoundPlain

NSRoundDown

NSRoundUp

NSRoundBankers

1.24

1.2

1.2

1.3

1.2

1.26

1.3

1.2

1.3

1.3

1.25

1.3

1.2

1.3

1.2

1.35

1.4

1.3

1.4

1.4

–1.35

–1.4

–1.4

–1.3

–1.4

NSCalculationError

Calculation error constants used to describe an error in exceptionDuringOperation:error:leftOperand:rightOperand:.

enum {
   NSCalculationNoError = 0,
   NSCalculationLossOfPrecision,
   NSCalculationUnderflow,
   NSCalculationOverflow,
   NSCalculationDivideByZero
};
typedef NSUInteger NSCalculationError;
Constants
NSCalculationNoError

No error occurred.

Available in OS X v10.0 and later.

Declared in NSDecimal.h.

NSCalculationLossOfPrecision

The number can’t be represented in 38 significant digits.

Available in OS X v10.0 and later.

Declared in NSDecimal.h.

NSCalculationOverflow

The number is too large to represent.

Available in OS X v10.0 and later.

Declared in NSDecimal.h.

NSCalculationUnderflow

The number is too small to represent.

Available in OS X v10.0 and later.

Declared in NSDecimal.h.

NSCalculationDivideByZero

The caller tried to divide by 0.

Available in OS X v10.0 and later.

Declared in NSDecimal.h.