Classes Joe Meehean

Complex Data Types What if we need more complex data types? not simple primitives like ints, float, and chars data types that are made of other smaller data types e.g., Song name album artist duration Do we need to store all these things separately? Pass 4 parameters to each Song function? 2

C++ Classes Classes a set of data types stored in one object a set of functions that manipulate that data Glue data together into more complex, abstract data types Member data/variables: variable contained in a class Data is not independent of functions that modify it e.g., what are integers without addition and substraction? Attach functions that work over data types to the data types Methods/member functions: functions that are attached to a class 3

C++ Classes Syntax for defining a C++ class 4 class ClassName{ member_type1 member_variable1; member_type2 member_variable2; member_type3 member_variable3; member_function_prototype(param1, param2,…)... };

C++ Classes Syntax for defining a C++ class 5 class ClassName{ member_type1 member_variable1; member_type2 member_variable2; member_type3 member_variable3; member_function_prototype(param1, param2,…)... };

C++ Classes Syntax for defining a C++ class 6 class ClassName{ member_type1 member_variable1; member_type2 member_variable2; member_type3 member_variable3; member_function_prototype(param1, param2,…)... };

C++ Classes Syntax for defining a C++ class 7 class ClassName{ member_type1 member_variable1; member_type2 member_variable2; member_type3 member_variable3; member_function_prototype(param1, param2,…)... };

Example: Song Class 8 class Song{ char* name_; char* artist_; int duration_s_; void changeName(const char* new_name); void changeArtist(const char* new_artist); void changeDuration(int duration_s); int compare(const Song& other_song); }; Style Point: Member variables should end in ‘_’

Example: Song Class 9 class Song{ char* name_; char* artist_; int duration_s_; void changeName(const char* new_name); void changeArtist(const char* new_artist); void changeDuration(int duration_s); int compare(const Song& other_song); }; Best Practice: Don’t define functions in class definition

C++ Classes Member functions must be declared in the class definition (prototype) can be defined elsewhere (more on this later) have access to all of the member variables const member functions cannot modify member variables if the method doesn’t modify the member variables, make it constant increases flexibility of function can be called on const objects 10

C++ Classes Defining class class in defined between class class_name and }; methods are declared (prototyped) in class definition class defined in header file class_name.h Defining member methods methods are often defined in another file i.e., method bodies written in another file called cpp file: class_name.cpp Why? splits what a class is and what it does, e.g., Song from how it actually does it should only need to read header to figure out how it works 11

Example: Song Class 12 #include “Song.h” void Song::changeName(const char* new_name){ delete name_; int size = strlen(new_name) + 1; name_ = new char[size]; strncpy(name_, new_name, size); }... Must include the header

Example: Song Class 13 #include “Song.h” void Song::changeName(const char* new_name){ delete name_; int size = strlen(new_name) + 1; name_ = new char[size]; strncpy(name_, new_name, size); }... Must state change name is a member function of Song

Example: Song Class 14 #include “Song.h” void Song::changeName(const char* new_name){ delete name_; int size = strlen(new_name) + 1; name_ = new char[size]; strncpy(name_, new_name, size); }... can access member data

C++ Classes Type description of data it stores size allowable operators can be primitives (built-ins) or classes e.g., float, int, Song defining a type does not allocate memory for it Instance an object of a specific type memory allocated to store that type can be many instances of a class 15

C++ Classes Getting an instance of a class Syntax: class_name variable_name; Song song1; Can access member variables using ‘.’ Syntax: instance.mvariable cout << song1.artist_ << endl; Each instance of a class has its own copy of the member variables Song song1; Song song2; song1.duration_s_ = 124; song2.duration_s_ = 145; 16

C++ Classes Each instance of a class has its own copy of the member variables Song song1; Song song2; song1.duration_s_ = 124; song2.duration_s_ = 145; 17 song1 name artist duration_s 124 song2 name artist duration_s 145

C++ Classes Can call methods on instances of class using ‘.’ too Song song1; song1.changeName(“Jump Around”); song1.duration_s_ = 124; 18 “Jump Around” song1 name artist duration_s 124

Let’s write a Integer Matrix class called IntMatrix 19

Questions? 20

C++ Class Scoping Scope the portion of code that a variable can be referenced in Nested scopes scope within a scope in C++ a new scope starts with { and ends with } (called a block) Figuring out which variable a name refers to called name resolution or lookup C++ name resolution (without classes) it is illegal to reference a variable outside of its scope first look for the name in block where it was used then, check enclosing scopes 21

C++ Class Scoping 22 void func1(int a, int b){ int c; c += a; } void func2(int e, int f){ c += e; // ERROR, c is not in scope }

C++ Class Scoping 23 file scope nested in disjoint 1 int g; 2 void func(int a, int b){ 3 int d = 7; 4 if( a < b){ 5 int g = 15; 6 d += g; 7 } 8 if( b == d ){ 9 int c = 2 10 g = d + c; 11 } 12}

C++ Class Scoping 24 1 int g; 2 void func(int a, int b){ 3 int d = 7; 4 if( a < b){ 5 int g = 15; 6 d += g; 7 } 8 if( b == d ){ 9 int c = 2 10 g = d + c; 11 } 12} g = d + c; g -> g from 1 d -> d from 3 c -> c from 9 d += g; g -> g from 5 d -> d from 3

C++ Class Scoping Classes have their own scope Member functions are in their class’s scope they can reference member variables C++ name resolution in a class scope 1.declarations in the member function body (including parameters) 2.declarations of member data 3.declarations in the scope of that encloses the block (either method definition block or class block) 25

C++ Class Scoping What if we have a parameter with the same name as a member variable? parameter overload the name name will always resolve to parameter 26 // does nothing void Song::changeDuration(int duration_s_){ duration_s_ = duration_s_; }

C++ Class Scoping The this variable is a pointer to the instance a method was called on Give direct access to the instance Gives access to class scope 27 // sets duration void Song::changeDuration(int duration_s_){ this->duration_s_ = durations_s_; }

C++ Class Scoping Accessing member data outside the class scope if you have an instance of the class use the. operator for instances of the class e.g., cout << song1.name_ << endl; use the -> operator for pointers to instances of the class e.g., Song* pSong = new Song(); cout name_ << endl; 28

C++ Class Scoping Accessing class scope outside the header file use the :: operator (don’t use with an instance) we use this to define member functions e.g., void Song::changeName(const char* new_name){... } states that changeName is in the class Song’s scope 29

C++ Class Scoping Overloading member functions several member functions can have the same name as long as they have either different number or different type of parameters 30 void IntMatrix::init(int rows, int columns){... } void IntMatrix::init(int rowsAndColumns){ // assumes matrix is square... }

C++ Class Scoping Overloading member functions can have the same function in const and non-const varieties 31 // caller can change artist char* Song::getArtist(){ return artist_; } // returns an immutable artist const char* Song::getArtist() const{ return artist_; }

Questions? 32

Constructors Recall we wrote an init method for TicTacToe allocated and initialized matrix We should use Constructors instead special kind of initialized function called when an object is created should set member variables to their initial values Syntax: ClassName(type1 name1, type2 name2, …) does not specify a return type can take from 0 to N parameters 33

Example: Song Class 34 class Song{ char* name_; char* artist_; int duration_s_; Song(const char* name, const char* artist, int duration); void changeName(const char* new_name); void changeArtist(const char* new_artist); void changeDuration(int duration_s); int compare(const Song& other_song); };

Example: Song Class 35 Song::Song( const char* name, const char* artist, int duration){ // initialize the name this->name_ = new char[strlen(name)+1]; strcpy(name_, name); // initialize the artist this->artist_ = new char[strlen(artist)+1]; strcpy(artist_, artist); // initialize the duration this->duration_s_ = duration; }

Constructors A single class may have many constructors overload just like any other method must have different number of parameters or parameter types 36 class IntMatrix{ int **matrix_; int rows_; int columns_; IntMatrix(int rows, int columns); // makes a square matrix IntMatrix(int rows_and_columns); };

Example: Song Class 37 IntMatrix::IntMatrix(int rows, int columns){ // copy the rows and columns this->rows_ = rows; this->columns_ = columns; // allocate and initialize the matrix this->matrix_ = new int*[rows_]; for(size_t i = 0; i < rows_; i++){ this->matrix[i] = new int[columns_]; for(size_t j = 0; j < columns_; j++){ matrix[i][j] = 0; }

Example: Song Class 38 IntMatrix::IntMatrix(int rows_and_columns){ // copy the rows and columns this->rows_ = rows_and_columns; this->columns_ = rows_and_columns; // allocate and initialize the matrix this->matrix_ = new int*[rows_]; for(size_t i = 0; i < rows_; i++){ this->matrix[i] = new int[columns_]; for(size_t j = 0; j < columns_; j++){ matrix[i][j] = 0; }

Constructors How to use a constructor use it when creating an instance of a class syntax: Type varname(arg1, arg2, …) e.g., Song aSong(“O.P.P.”, “Naughty By Nature”, 271) What if we don’t know the duration? can provide default values for constructor parameters 39

Example: Song Class 40 class Song{ char* name_; char* artist_; int duration_s_; Song(const char* name, const char* artist, int duration = 0); void changeName(const char* new_name); void changeArtist(const char* new_artist); void changeDuration(int duration_s); int compare(const Song& other_song); };

Example: Song Class 41 Song::Song( const char* name, const char* artist, int duration = 0){ // initialize the name this->name_ = new char[strlen(name)+1]; strcpy(name_, name); // initialize the artist this->artist_ = new char[strlen(artist)+1]; strcpy(artist_, artist); // initialize the duration this->duration_s_ = duration; }

Constructors What happens if we don’t call the constructor? e.g., Song aSong; it calls the Default Constructor Default constructor takes no parameters syntax: Type() e.g., Song() should initialize the member data with sensible empty values 42

Example: Song Class 43 class Song{ char* name_; char* artist_; int duration_s_; Song(); Song(const char* name, const char* artist, int duration = 0); void changeName(const char* new_name); void changeArtist(const char* new_artist); void changeDuration(int duration_s); int compare(const Song& other_song); };

Example: Song Class 44 Song::Song(){ // initialize the name this->name_ = NULL; // initialize the artist this->artist_ = NULL; // initialize the duration this->duration_s_ = 0; }

Constructors What happens if we don’t define the default constructor? If you define no constructors the compiler makes a default constructor for you synthesized default constructor primitive member data is initialized using same rules for uninitialized variables member data that are classes are initialized using their default constructors 45

Constructors What happens if we don’t define the default constructor? If you define other constructors it is a compiler error e.g., Song aSong; // ERROR 46

Questions? 47

Constructors Are our member data items implicitly initialized using their default constructors? 48 class HitSong{ Song hit_song_; int chart_rank_; int singles_sold_; HitSong( Song& song, int chart_rank, int singles_sold);... };

Constructors Are our member data items implicitly initialized using their default constructors? it depends Constructors have 2 phases 1.initialization phase 2.general computation phase If member data is not initialized after initialization phase, they are constructed using the default constructor initialization phase ends at { 49

Example: Hit Song Class 50 HitSong::HitSong( Song& song, int chart_rank, int singles_sold){ // at this point // this->hit_song_ == Song() // this->chart_rank_ == undefined // this->singles_sold_ == undefined this->hit_song_ = song; // overwrites memory this->chart_rank_ = chart_rank; // overwrites this->singles_sold_ = singles_sold; }

Constructors How can we initialize member data before the { ? Use the construction initializer list Syntax: Type(T1 parm1, T2 parm2) : member1(parm1), member2(parm2){ initializes the member data just like you had written: T1 member1(parm1); T2 member2(parm2); sometimes called member initialization list 51

Example: Hit Song Class 52 HitSong::HitSong( Song& song, int chart_rank, int singles_sold) : hit_song(song), chart_rank_(chart_rank), singles_sold_(singles_sold) { // nothing left to do }

Constructors You must use the construction initializer list if member data doesn’t provide a default constructor Initializers may be an expression e.g., chart_rank_(chart_rank * 7 + 5) Member data initialized in order of definition, NOT in order of construction initialize list 53

Example 54 class TwoInts{ int a_; int b_; TwoInts(int c); }; // ERROR TwoInts::TwoInts(int c) : b_(c), a(b_){} // OK TwoInts::TwoInts(int c) : a_(c), b_(a_){} // Also OK TwoInts::TwoInts(int c) : a_(c), b_(c){}

Constructors Why you should always define a default constructor? If you don’t define a default constructor for class A: 1.Every constructor of every class that stores an A must explicitly initialize it in construction initializer list 2.The compiler won’t synthesize default constructors for classes with A as member data 3.Cannot dynamically allocate an array of As 4.Statically allocated arrays of As must be explicitly initialized 5.Other sometimes unexpected problems 55

Protecting Member Data Member data shouldn’t be visible to everybody what if some user of our class screws up our member data What if we need methods to do internal work? stuff that users of our class shouldn’t be able to call Need a way to protect member data and some methods 56

Protecting Member Data Use access labels change visibility of methods and member data public: accessible by all parts of the program private: accessible only inside the class scope label in effect until another label encountered default label is private 57

Example: Song Class 58 class Song{ private: char* name_; char* artist_; int duration_s_; public: Song(char* name, char* artist, int duration = 0); void changeName(const char* new_name); void changeArtist(const char* new_artist); void changeDuration(int duration_s); int compare(const Song& other_song); };

Questions? 59

Static Members What if you have an object that every instance of a class needs to share? e.g., count of how many instances were created e.g., shared set of error messages Could make it a global variable then anyone can modify it Can we make an object that is shared by all instances of a class, but is not visible outside this class? 60

Static Members Class static member data and methods Advantages over global variables 1.Reduces namespace pollution name of static member is in scope of the class 2.Limits visibility static member data can be private 3.Clear association variable clearly associated with enclosing class 61

Static Members Declaration syntax static type varname e.g., static int default_value_; Usage syntax outside of declaring class: Class::varname e.g., IntMatrix::default_value_ inside declaring class: varname e.g., default_value_ 62

Static Members 63 class IntMatrix{ private: int **matrix_; int rows_; int columns_; public: static int default_value_; IntMatrix(int rows, int columns);... };

Static Members 64 IntMatrix::IntMatrix(int rows, int columns){ // copy the rows and columns this->rows_ = rows; this->columns_ = columns; // allocate and initialize the matrix this->matrix_ = new int*[rows_]; for(size_t i = 0; i < rows_; i++){ this->matrix[i] = new int[columns_]; for(size_t j = 0; j < columns_; j++){ matrix[i][j] = default_value_; }

Static Members 65 void main(){ IntMatrix i_matrix; int top_left = i_matrix.getCell(0,0) if( top_left == IntMatrix::default_value_ ){ cout << “Top left value unitialized”; }

Static Members Definition static data members must be defined exactly once outside of the class body Cannot be initialized in constructor constructor called many times static member data should be initialized once Ordinary convention is to tack initialization on at end of source file (.cpp) Syntax type Class::varname = init_value; 66

Static Members 67 IntMatrix::IntMatrix(int rows, int columns){... } int IntMatrix::default_value_ = 0; director’s cut

Static Members Static member data’s special features Constant static integrals can be initialized in the class body 68 class IntMatrix{ private: int **matrix_; int rows_; int columns_; const static int default_value_ = 0; public: IntMatrix(int rows, int columns);... };

Static Members Static member data’s special features Static member data is not a part of any class instance, so: a static data member can be the same class type of its enclosing class 69 class IntMatrix{ private: int **matrix_; int rows_; int columns_; static IntMatrix nested_matrix_;... };

Static Members Static member data’s special features Static member data is not a part of any class instance, so: a static data member can be used as a default argument 70 class IntMatrix{ private: int **matrix_; int rows_; int columns_; const static int DEF_ROWS_ = 3; public: IntMatrix(int rows = DEF_ROWS_, int columns); };

Static Methods Classes can also have static methods These methods have no this pointer Declaration syntax static return_type method(t1 parm1, t2 parm2) e.g., static int minuteToMillis(float minutes) Definition syntax return_type Class::method(t1 parm1, t2 parm2) e.g., int Time::minutesToMillis(float minutes) 71

Static Methods Useful for reasons similar to static member data 1.Reduces namespace pollution name of static method is in scope of the class 2.Limits visibility static method can be private 3.Clear association method clearly associated with enclosing class 4.Can be used to initialize static member data for non-integral complex member data 72

Static Methods Can initialize static member data with static method 73 class CircleMath{ private: const static float pi; static float initPi();... };

Static Methods Can initialize static member data with static method 74 float CircleMath::initPi(){ return f; } float CircleMath::pi = initPi(); director’s cut

Classes Discussion Data abstraction separate what something does from how it does it interface: what it does implementation: how it does it allows code to be broke into modules coding can be gluing modules together instead of rewriting same code again and again Class abstraction interface is public methods and public member data implementation includes method definitions, private methods, and member data 75

Classes Discussion Encapsulation don’t just separate interface from implementation hide implementation enforces good data abstraction Class encapsulation anything declared private classes also glue together related data so it can be modified together 76

Questions? 77