© Copyright Eliyahu Brutman Programming Techniques Course Version 1.0

© Copyright Eliyahu Brutman Chapter 4 – Copy Constructor, Overloading Operators Version 1.0

© Copyright Eliyahu Brutman Overloading & Operators - 3 String copy-constructor char* String::getString(){return chars;} String::String(const String& other){ length = other.length; chars = new char[length+1]; strcpy(other.chars, chars); } String.cpp

© Copyright Eliyahu Brutman Overloading & Operators - 4 Copy-constructors Single parameter – reference to the copied object class X{ public X(const X& x); } Called by the compiler When an object is initialized using another object of the same class  X x; X x1(x); When an object is passed by value  void foo(X xparam); foo(x); If no copy-constructor defined, compiler adds one Bitwise copy of the copied object Not always fits

© Copyright Eliyahu Brutman Overloading & Operators - 5 Using constructors: String void main(){ String s1(“My String”); String s2(s1); String s3=s1; } What’s the difference? void main(){ String s1(“My String”); String s2(s1); String s3; s3=s1; }

© Copyright Eliyahu Brutman Overloading & Operators - 6 Operators Definition We want User-defined types to behave the same way as built-in types. We want operators to be supported, for the uniform convention The language allows this And the usage: class String { char* m_str; int m_len; public: String(); String(char*); ~String(); int operator>(const String& s); String operator+(const String& s); char* GetString(); void SetString(char* newStr); };

© Copyright Eliyahu Brutman Overloading & Operators - 7 Operators - Usage Who is the caller? Who is the parameter? void main() { String s1= “ Computer ” ; String s2= “ Techniques ” String s3 = s1+s2;// not operator=, but CCtor } Invokes “function” s1.operator+(s2)

© Copyright Eliyahu Brutman Overloading & Operators - 8 Operators - Implementation int String::operator>(const string& s) { return strcmp(m_str, s.m_str); }

© Copyright Eliyahu Brutman Overloading & Operators - 9 Operators - Implementation String String::operator+(const string& s) { char* old_str = m_str; m_str = new char[m_len+s.m_len+1]; m_len+=s.m_len; strcpy(m_str, old_str); strcat(m_str, s.m_str); delete [] old_str; return this; } Within a member function: explicit name for the object on which the function is called

© Copyright Eliyahu Brutman Overloading & Operators - 10 Operators: Good news Can be defined and re-defined Defined by default: & (unary), =,, Rules: At least one operand must be class Cannot define new operators Cannot change precedence/associativity Cannot redefine., ?:, sizeof, ::,.,*

© Copyright Eliyahu Brutman Overloading & Operators - 11 Example: Class Vector class Vector2D{ private: double x, y; public: Vector2D(double x_init=0, double y_init=0): x(x_init), y(y_init){} double getX() const {return x;} double getY() const {return y;} Vector2D add(const Vector2D& other) const {return Vector2D(x+other.x, y+other.y);} } //usage: Vector2D a(3,2), b(1); Vector2D c = a.add(b); cout << “(“ << c.getX() << “,” << c.getY() << ‘)’;

© Copyright Eliyahu Brutman Overloading & Operators - 12 Example: Class Vector class Vector2D{ private: double x, y; public: Vector2D(double x_init=0, double y_init=0): x(x_init), y(y_init){} double getX() const {return x;} double getY() const {return y;} Vector2D operator+(const Vector2D& other) const {return Vector2D(x+other.x, y+other.y);} } //usage: Vector2D a(3,2), b(1); Vector2D c = a+b; cout << “(“ << c.getX() << “,” << c.getY() << ‘)’;

© Copyright Eliyahu Brutman Overloading & Operators - 13 Overloading ++ and -- Pre- and post-increment x = 0; cout << x++; cout <<++x; Pre-increment and pre-decrement – usual unary operators Vector2D operator++(); Post-increment and post-decrement – use dummy parameter Vector2D operator++(int); Rule of thumb: for performance reasons, use the “ pre ” versions

© Copyright Eliyahu Brutman Overloading & Operators - 14 Friend access modifier The following will work: c=a+b; c=a+1; The following will not work: c=1+b; The first argument cannot be built-in! Solution: define the operator not as a member function But what about encapsulation? Access modifier friend : allows to access private fields and methods of the class

© Copyright Eliyahu Brutman Overloading & Operators - 15 Example: Class Vector class Vector2D{ private: double x, y; public: Vector2D(double x_init=0, double y_init=0): x(x_init), y(y_init){} double getX() const {return x;} double getY() const {return y;} friend Vector2D operator+( const Vector2D& left, const Vector2D& right); } Vector2D operator+(const Vector2D& left, const Vector2D& right){ return Vector2D(left.x+right.x, left.y+right.y);} //usage: Vector2D a(3,2), b(1); Vector2D c = a+b; cout << “(“ << c.getX() << “,” << c.getY() << ‘)’;

© Copyright Eliyahu Brutman Overloading & Operators - 16 Class Vector: more operators class Vector2D{ … Vector2D operator-() const{return Vector2D(-x, -y);} double operator[](int index) const {return (index == 0)?x:y;} friend ostream& operator<<( ostream& ostr, const Vector2D& v); } ostream& operator<<(ostream& ostr, const Vector2D& v){ return ostr << “(“ << v.x << “,” << v.y << ‘)’; } //usage: Vector2D a(3,2), b(1); Vector2D c = -a; cout << “(“ << c[0] << “,” << c[1] << ‘)’; cout << c << b << a;

© Copyright Eliyahu Brutman Overloading & Operators - 17 operator= By default: bitwise copy Will this work for Vector2D? Will this work for String? Rule of thumb: either you need all of {destructor, copy- constructor, operator=} or you need none Rules: MyClass& MyClass::operator=(const MyClass& other) Return *this Define as member to ensure target object is assignable

© Copyright Eliyahu Brutman Overloading & Operators - 18 operator= : example class String{ … public: String& operator=(const String& other){ if (this == &other) return *this; if (m_str != NULL) delete[] m_str; m_len = other.m_len; m_str = new char[m_len+1]; strcpy(m_str, other.m_str); return *this; }