Constants Objectives Describe ways to create constants const - - PDF document

Constants

Objectives

• Describe ways to create constants

– const – readonly – enum

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Motivation

• Idea of constant is useful

– makes programs more readable – allows more compile time error checking

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Const

• Keyword const used to indicate compile time constant

– applicable to local variables – applicable to fields

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Const local variable

• Local variable can be declared constant

– must be initialized inline – value cannot be changed later

double Area(double radius) { const double pi = 3.14; return pi * radius * radius; }

constant local variable

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Const field

• Field can be declared constant

– must be initialized inline – value cannot be changed later

public class Math { public const double PI = 3.14159265358979323846; public const double E = 2.7182818284590452354; ... }

constant fields

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Const and static

• Field cannot be both declared both const and static

– not needed – const field automatically static – saves memory by generating single copy

public class Math { public const double PI = 3.14159265358979323846; public const double E = 2.7182818284590452354; ... }

implicitly static

double radius = 1.5; double area = Math.PI * radius * radius;

access using type name

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Compile time const

• const provides restrictive notion of constant

– value must be computable at compile time

void Process(int width, int height) { const int area = width * height; ... }

error, value must be compile time constant

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Limited const references

• Value for const reference must be compile time constant

– references can only be null – too limited to be truly useful

• Works for strings since compile time literals exist

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const Person p = null; const Person ann = new Person("Ann");

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error

const string greeting = "hello";

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No const parameters

• Parameters cannot be const

– not value type parameters – not reference type parameters – parameter value is not determined at compile time

error, const parameters not supported

double Average(const int a, const int b) { ... }

error, const parameters not supported

double Process(const Person p) { ... }

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Readonly

• Keyword readonly used to indicate runtime constant

– applicable only to fields

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Readonly field

• Field can be declared readonly

– can only be initialized using variable initializer or in constructor – value cannot be changed later – compiler warning if not set – value used for initialization can be determined at runtime

class Person { readonly DateTime dob; public Person(DateTime dob) { this.dob = dob; } ... }

readonly field

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Readonly and static

• Fields often made both readonly and static

– more flexible than const – prevents write access – allows runtime determination of value – saves memory by generating single copy

13 struct Point { public static readonly Point origin = new Point(0, 0); ... }

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Enum

• Keyword enum used to create new type

– with corresponding set of symbolic constants

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Enum definition

• Can create enumeration value type

– use keyword enum – specify type name – give list of named constants

enum Day { Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday }

define enum

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Enum use

• Can create variables of enum type

– use enum name as type name

• Can use values listed in enum

– values are scoped inside enum – prefix with enum name for access

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Day d; d = Day.Monday; if (d == Day.Saturday || d == Day.Sunday) Console.WriteLine("weekend");

create variable

Enum underlying type

• Enum uses an underlying type for representation

– default is int – can use any integral type except char

represented using int

enum ErrorCode : byte { DomainError, RangeError }

represented using short

enum Day { Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday }

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Enum values

• Enum constants have values

– default is sequential starting at 0 – can explicitly specify any value and others follow in sequence

enum Day { Sunday, // 0 Monday, // 1 Tuesday, // 2 Wednesday, // 3 Thursday, // 4 Friday, // 5 Saturday // 6 } default values enum Day { Sunday = 1, Monday, // 2 Tuesday, // 3 Wednesday, // 4 Thursday = 10, Friday, // 11 Saturday // 12 } specify value

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Enum and casting

• Must cast to convert between enum and underlying type

– cast needed in either direction – helps reduce chance of assigning invalid value

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Day d = (Day)3; int i = (int)d;

convert to Day convert to int

Enum inheritance hierarchy

• enum is part of type hierarchy

– ultimately descends from Object like all types – automatically derived from the value type Enum – Enum class provides many convenient utility methods

ValueType Object Enum user enum

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Boxing

• Enums are value types

– boxed when used as object – value copied into box

Day d = Day.Monday;

• bject o = d;

...

boxed

• Monday

Monday d

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Unboxing

• Can extract enum from box

– cast required – System.InvalidCastException thrown if cast fails

Day d = Day.Monday;

• bject o = d;

Day e = (Day)o; ...

unbox

• Monday

Monday e

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Summary

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• Three different techniques to create constants

– const – readonly – enum

• static often combined with readonly on field

– gives more flexible notion of constant than const keyword