Operators & Overloading

Joe Meehean

Expressions• Expression composed of operands combined with operators• e.g., a + b

• Operands• variables and literals in an expression• e.g., a b

• Operators• symbol that combine the operands• e.g., + - / * %

• Result• what is left after evaluating expression• result of applying operators to operands

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Expressions• Operator types• Unary• takes only a single operand• e.g., !

• Binary• takes two operands• e.g., + * / %

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Expressions• How are expressions evaluated?• Depends on associativity and precedence• Precedence• if expression contains more than 1 operator• which operator to apply first• operators with higher precedence applied first• e.g., 7 + 3 * 5 == 7 + (3 * 5)

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Expressions• How are expressions evaluated?• Depends on associativity and precedence• Associativity• given multiple operators with the same precedence• which operator to apply first• depends on associativity

• Left associative• apply operators from left to right• e.g., 2 + 3 + 5 == (2 + 3) + 5

• Right associative• apply operators from right to left• e.g., 2 + 3 + 5 == 2 + (3 + 5) 5

Arithmetic Operators

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Operator Function Associativity Level *

+ unary plus R 1

− unary minus R 1

* multiplication L 6

/ division L 6

% modulo L 6

+ addition L 7

− subtraction L 7

* lower level == higher precedence. Level 1 has highest precedence

Arithmetic Operators• Dangers• Illegal operations• e.g., division by 0• causes program to crash

• Overflow• value goes outside of legal range• 2 byte signed short max value: 32767

• 2^15 (16th bit reserved for sign)• adding one pushes a 1 into the sign position• causes value to go negative: - 32768

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Relational & Logical Operators

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Operator Function Associativity Level *

! logical not R 3

<, <= less than (equal) L 9

>, >= greater than (equal) L 9

==, != (not) equals L 10

&& logical and L 14

|| logical or L 15

* lower level == higher precedence. Level 1 has highest precedence

Relational & Logical Operators• Interesting info• Relational operators do not chain• <, <=, =>, >• e.g., a < b < c < d will not compile• associativity is irrelevant

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Relational & Logical Operators• Interesting info• Short-circuit evaluation• given a series of conditions connected by ||• if any of the conditions is true, the expression is true• conditions only evaluated until the first true condition is found• e.g., a || b || c• if a is true, b and c won’t be evaluated• if a is false and b is true, only a and b will be evaluated

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Relational & Logical Operators• Interesting info• Short-circuit evaluation• given a series of conditions connected by &&• expression true only if all conditions are true• conditions only evaluated until the first false condition is found• e.g., a && b && c• if a is false, b and c won’t be evaluated• if a is true and b is false, only a and b will be evaluated• power (avoid going out of bounds):while( i < theVector.size() && theVector[i] != 7){ i++;} 11

Bitwise Operators• AND• combines two bit sets• bit in result only 1 if both bits in operands are 1• e.g., 0101 AND 1100 => 0100

• OR• combines two bit sets• bit in result 1 if bit in either operand is 1• e.g., 0101 OR 1010 => 1111

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Bitwise Operators• NOT• flips all of the bits• e.g., 0101 => 1010

• XOR• combines two bit sets• bit in results 1 only if exactly one bit in the operands was 1• e.g., 1010 XOR 1010 => 0000• e.g., 1110 XOR 1010 => 0100

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Bitwise Operators• left shift• shifts all bits to the left• e.g., 0011 => 0110• new bits are zero• left-most bit falls off the end

• right shift• shifts all bits to the right• e.g., 0111 => 0011• new bits are zero• right-most bit falls off the end

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Bitwise Operators

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Operator Function Associativity Level *

~ bitwise NOT R 3

<<, >> left (right) shift L 8

& bitwise AND L 11

^ bitwise XOR L 12

| bitwise OR L 13

* lower level == higher precedence. Level 1 has highest precedence

IO Operators• bitwise shift operators overloaded for IO• <<• insertion operator• inserts data into output stream• e.g., cout << a << endl;

• >>• extraction operator• extracts data from input stream• e.g., cin >> a ;

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Other Operators

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Operator Function Associativity Level *

() grouping L 2

++, -- postfix L 2

++, -- prefix R 3

* Lower level == higher precedence. Level 1 has highest precedence

Assignment Operators

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Operator Function Associativity Level *

= assignment R 17

+=, −=, *=, /=, %=

arithmetic assignment

R 17

>>=, <<=, &=, |=, ^=

bitwise assignment R 17

* Lower level == higher precedence. Level 1 has highest precedence

Assignment Operators• Assignment operator has low precedence and is right

associative• very important

• Right associativity • allows: a = b = c = 0

• Low precedence• a = b + c => a = (b + c)• makes logical sense

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Questions?

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Operator Overloading• Redefine operators for classes we write• Lets us make intuitive use of operators• like [] for vector and string• += for string append

• Powerful and easy to read• v[i] vs. v.get(i)• str += “Hello” vs. str.append(“Hello”)

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Operator Overloading• 2 different types of operator overloading• Member overloading• defined as a member method of a class• first operand is always this• if binary operator, method takes 1 parameter

• the other operand• if unary operator, method takes 0 parameters• generally best choice for assignment/modifier operators

• =• +=, -=, …• []• ->

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Operator Overloading• 2 different types of operator overloading• Nonmember overloading• defined as a global function• if binary operator, method takes 2 parameter• if unary operator, method takes 1 parameters• generally best choice for symmetric operators

• arithmetic• equality• relational• bitwise

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Operator Overloading• Syntax: member method• requires the operator keyword• type Class::operator +=(type)• e.g., bool UsedBook::operator ==(const UsedBook& rhs)

• Syntax: nonmember method• type1 operator +=(type, type)• e.g., bool operator==(const UsedBook& lhs, const UsedBook& rhs)

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Operator Overloading• Limitations• can overload all operators discussed in class except:

:: , .* , . , ?:• cannot invent new operators: operator **• at least one operand must be a class• precedence and associativity cannot be changed

• Short-circuit evaluations is not preserved• &&, ||• both operands are always evaluate• evaluation order is not guaranteed

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Operator Overloading• Operators you should define• if it makes sense

• For use with STL• < • ==• != : to go along with ==

• To print your class• <<

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Operator Overloading• Overloading the output operator <<• Non-member method• 1st parameter is always a reference to an ostream• cout is an instance of the ostream class• reference because ostream cannot be copied

• 2nd parameter is often const reference to class object• printing an object rarely changes it• reference prevents making unnecessary copies

• Always returns an ostream reference• so we can chain << operators together• e.g., cout << a << b << c << endl

• Do not add an end line when writing operator << 27

Operator Overloading

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ostream& operator<<(ostream& os, const Book& book){ os << book.author_ << “ “; os << book.title_ << “ “; os << book.retail_price_;}

Operator Overloading• Non-member function in previous example had access to

Book’s private member data. How?• Friends• class can call other classes or method friends• friends can access private member data and methods

• Syntax: friend classes• friend class class1• e.g., friend class Song

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Operator Overloading• Non-member function in previous example had access to

Book’s private member data. How?• Friends• class can call other classes or method friends• friends can access private member data and methods

• Syntax: friend methods• friend return_type name(type1 var1, type2 var2,…)• e.g., friend ostream& operator<<(ostream& os, const Book& b);• does not declare this method• just says this method exists and is a friend

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Operator Overloading

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class Song{ private: string title_; string duration_s; string artist_;

public: Song(...); ...

// this is not declaring this method // this method is not a member method of Song friend ostream& operator<<(ostream& os, const Song& s);};

Questions?

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