CSCE 121: Introduction to Program Design and Concepts, Honors Dr. J. Michael Moore Spring 2015 Set 3: Objects, Types, and Values 1 Based on slides created by Bjarne Stroustrup and Jennifer Welch

CSCE 121: Set 3: Objects, Types and Values  Object: region of memory with a type that specifies what kind of information can be stored there and what operations can be performed on it  Variable: object with a name  Declaration: statement that gives a name to an object  Definition: declaration that also sets aside memory for a variable  Value: data item put into a variable  Examples: int age = 42; string name = “Fred”; 2

CSCE 121: Set 3: Objects, Types and Values Common Built-in Types int num = 39; // integer double ratio = 4.1; // real number char qmark = ‘?’; // single character string name = “Fred”; // sequence of characters bool finished = false; // true or false 39, 4.1, ‘?’, “Fred”, and false are examples of literals. 3

CSCE 121: Set 3: Objects, Types and Values More Types C++ has a few more built-in types C++ programmers can define new types: “user-defined” types C++ standard library provides a set of types string, vector, complex,… (Technically, these are also user-defined types) C++ standard library provides a set of types string, vector, complex,… (Technically, these are also user-defined types) 4

CSCE 121: Set 3: Objects, Types and Values Types and Operations Strings cin >> reads a string cout <<writes a string + concatenates ++ error! - error! … Numbers (int, double,…) cin >> reads a number cout << writes a number + adds ++ increments by 1 - subtracts … Variable’s type determines which operations are valid and what they mean (“overloading”). Compiler makes sure you use each variable according to its type. 5

CSCE 121: Set 3: Objects, Types and Values Names Used for variables, functions, types,… Rules: Start with a letter Contain only letters, digits and underscores _ Cannot (re)use keywords (e.g., int, if, while, double) Good ideas: Don’t start with _ : reserved for low-level entities Choose meaningful names! Not too short, not too long, but just right. 6

CSCE 121: Set 3: Objects, Types and Values Initialization vs. Assignment Initialization gives a variable its initial value, when declared/defined int n = 10; Assignment gives an existing variable a new value n = n*2; 7

CSCE 121: Set 3: Objects, Types and Values Initialization Always initialize your variables! Beware that “debug mode” in your compiler may initialize for you Valid exception to this rule is an input variable 8

CSCE 121: Set 3: Objects, Types and Values Initialization with C++11 You can use the type of an initializer as the type of a variable with “auto”: auto x = 1; // 1 is an int, so x is an int auto y = ‘c’; // ‘c’ is a char, so y is a char auto d = 1.2; // 1.2 is a double, so d is a double auto s = “Howdy”; // s has same type as “Howdy” // (more later on that) auto sq = sqrt(2);// sq has right type for result of // sqrt(2), and you don’t have to // remember what it is 9

CSCE 121: Set 3: Objects, Types and Values Type Safety Definition: every object will be used only according to its type Variable is only used after it is initialized Only operations defined for the variables type will be applied Every operation defined for a variable results in a valid value Ideal: static type safety Compiler finds all type safety violations Ideal: dynamic type safety Run-time system finds all type safety violations not found by compiler 10

CSCE 121: Set 3: Objects, Types and Values Type Safety Type safety is very important! Try very hard not to violate it Compiler is your friend C++ is not completely statically type safe No widely-used language is Reduces ability to express ideas C++ is not completely dynamically type safe Many languages are, but… Being dynamically type safe can cause performance problems Almost everything in this class will be type safe 11

CSCE 121: Set 3: Objects, Types and Values Type Conversion 12 C++ allows for implicit conversions. One type converted to another type automatically Beware!!! Safe conversions No information lost char c = ‘x’; int i1 = c; int i2 = ‘x’; Unsafe conversions “Narrowing” conversions double x = 2.7; int y = x; int a = 1000; char b = a; How to ‘outlaw’ unsafe conversions???

CSCE 121: Set 3: Objects, Types and Values About Efficiency For now, don’t worry about “efficiency” Concentrate on correctness and simplicity of code C++ is derived from C, a systems programming language A char stored in a byte An int is stored in a word A double fits in a floating-point register C++ built-in operations map directly to machine instructions Integer + is implemented by integer add operation, etc. C++ helps programmers build safer, more elegant and efficient code using built-in types and operations 13

CSCE 121: Set 3: Objects, Types and Values A Bit of Philosophy Like other branches of engineering, programming involves tradeoffs Conflicting ideals for a program Type safety Run-time performance Portability across platforms Compatibility with other code Each of construction Ease of maintenance Don’t skimp on correctness or testing! Aim for type safety and portability 14

CSCE 121: Set 3: Objects, Types and Values Acknowledgments Photo on slide 1: “Broken Type” by Javier Garcia, licensed under CC BY-NC 2.0“Broken Type” Javier GarciaCC BY-NC 2.0 Slides are based on those for the textbook: 15