Basic Types, Variables, Literals, Constants

Basic Types, Variables, Literals, Constants

What is in a Word? • A byte is the basic addressable unit of memory in RAM • Typically it is 8 bits (octet) • But some machines had 7, or 9, or ... • A word is the basic unit of operation by the CPU • Most registers are this size • Largest unit of transfer between RAM and CPU in single instruction

What is a Type? • A type is a qualifier that is used by the compiler • Machine languages do not have types • The type of a variable or constant tells the compiler: • How much space the object occupies • What operations on the object mean

What is a Type? • Given an address in RAM, what does it mean? • Could be anything! • Types tell the compiler • Which instruction to apply • Integer addition, floating pt addition • How to increment pointers • Array references, fields

some Arithmetic Types in C++

Which Type to Use? • General Usage • int – most integer arithmetic • double – for floating point computation • char – only for chars • bool – only for Booleans • Unsigned • Used to save space – who cares? • Do not mix signed and unsigned! • Can cause headaches easily - avoid

Type Conversion • Casting, or conversion • Needed when different type expected • Compiler handles automatically • Bool • LHS = {false if 0, true if non-0} • RHS = {0 if false, 1 if true}

Type Conversion • Integer ↔ Floating point number • Truncate FPN when int on LHS • Fractional part 0 when int on RHS • Can lose precision • Out-of-Range • LHS unsigned, then residue mod size • LHS signed, then undefined (bad)

What is a Literal? • A literal is a fixed, explicit value that is known at compile time • Can be used to initialize variables • Can be used to initialize constants • Can be used in expressions • Generally bad programming style • It may be int, char, bool, etc. • 5, 5.0, -3, 'a', “a”, “Go Gators!”, '\n'

Special Characters • Some characters are not printable • Some characters have special meaning to the language • For these, we need escape sequences • All start with backslash \ • Some predefined: \n newline • Any by \x where x is a number

Special Characters Can use as single character: std::cout << '\n'; std::cout << “\tHello!\n”; Generalized escape sequence: \12 = \014 = x0c = newline in decimal, octal, hex Note: only the first 3 octal digits are accepted Note: hex uses all the following digits (!)

Special Literals • Boolean • true • false • Pointer • nullptr ← preferred literal • 0 • NULL (must #include cstdlib) • Never any other

Variables • A variable is a logically named, typed, structured piece of storage • Name allows us to refer to the stored structure • Type allows us to know structure • Variables can be assigned new values • Program can manipulate them!!

Variables • Definition: allocates space for storage • Declaration: specifies name and type • So variable can be referenced here • … and defined elsewhere • Type var_name, var_name, …; • All vars have type given at start • Good practice: one per line of code!

Variable Definition int sum = 0, value, // all type int total = 0; /* sum and total initialized to 0 */ Sales_item item; /* type Sales_item initialized to default value */ std::string name(“Dr. Newman”); /* string is a type from std library variable length character sequence */

Initialization • Good idea: ALWAYS INITIALIZE!!!! • Initialization – object gets value at time of definition (when created) • May be any expression that can be evaluated at time of creation • Name becomes visible immediately • Hence can be used in subsequent initializations in same line!

Variable Initialization int i = 0, j = 2*i; /* j init uses value of i immediately */ int k = sizeof(double); /* value is a function that can be evaluated when k is defined */

“List” Initialization int i = 0; /* i initialized with literal value */ int i(0); /* here also */ int i = {0}; /* i initialized with literal value, but restricted */ int i{0}; /* same here */ double pi = 3.14; int a{pi}, b = {pi}; int c(pi), d = pi; /* floating pt */ /* fail - requires narrowing */ /* OK, but value … … truncated */

Declaration vs. Definition • Definition – allocate space • Declaration – state type and name • Name can be used in current file • “Makes promise” it will be defined later • Only define in ONE file extern int i; /* declares but doesn't define i*/ int j; // declares and defines j

Identifiers • Identifier = name – for variable, function, constant, class, type, etc. • Cannot be a keyword in C++ • Identifiers may be composed of letters, digits, and underscore char • Must begin with _ or letter • Identifiers are case-sensitive • Main is not main is not MAIN!

C++ Keywords alignas alignof asm auto bool break case catch char char16_t char32_t class const constexpr const_cast continue decltype default delete do double dynamic_cast else enum explicit export extern false float for friend goto if inline int long mutable namespace new

C++ Keywords noexcept nullptr operator private protected public register reinterpret-cast return short signed sizeof static static_assert static_cast struct switch template this thread_local throw true try typedef typeid typename union unsigned using virtual void volatile wchar_t while

C++ Alternative Operator Names or_eq xor xor_eq and and_eq bitand bitor compl not not_eq or

Identifier Conventions • Identifier should hint toward purpose • Constants are all upper case • Variables are lower case • Classes start with upper case • Multi-word identifiers should distinguish each word using a capital or underscore • Sales_item, booksSold, totalRbis

Scoping • When is a name visible/usable? • Most scopes delimited by {} blocks • Names can be reused across scopes • Global scope – defined outside a fcn • Block scope – accessible from point defined onward within block • Nested scopes – name visible in outer scope can be redefined in inner scope

Basic Types, Variables, Literals, Constants