Kĩ thuật lập trình - Chapter 3: Objects, types, and values

Chapter 3Objects, types, and valuesBjarne Stroustrup www.stroustrup.com/ProgrammingAbstractMost programming tasks involve manipulating data. Today, we will:describe how to input and output datapresent the notion of a variable for holding dataintroduce the central notions of “Type” and “Type Safety” *Stroustrup/Programming/2015OverviewStrings and string I/OIntegers and integer I/OTypes and objectsType safety*Stroustrup/Programming/2015Input and output// read first name:#include "std_lib_facilities.h" // our course headerint main(){ cout > first_name; cout >first_name; reads characters until a whitespace character is seen (“a word”)White space: space, tab, newline, *Stroustrup/Programming/2015String input// read first and second name:int main(){ cout > first >> second; // read two strings string name = first + ' ' + second; // concatenate strings // separated by a space cout > first_name >> age; // read cout > reads a wordcout > reads a numbercout > n; cout > length; cout Boolean literalstrue falseCharacter literals'a', 'x', '4', '\n', '$'Integer literals0, 1, 123, -6, 034, 0xa3Floating point literals1.2, 13.345, .3, -0.54, 1.2e3, .3FString literals "asdf", "Howdy, all y'all!"Complex literalscomplex(12.3,99)complex(1.3F)*If (and only if) you need more details, see the book!Stroustrup/Programming/2015Types C++ provides a set of typesE.g. bool, char, int, doubleCalled “built-in types”C++ programmers can define new typesCalled “user-defined types”We'll get to that eventuallyThe C++ standard library provides a set of typesE.g. string, vector, complexTechnically, these are user-defined types they are built using only facilities available to every user*Stroustrup/Programming/2015Declaration and initializationint a = 7;int b = 9;char c = 'a';double x = 1.2;string s1 = "Hello, world";string s2 = "1.2";*79'a'1.212 | "Hello, world"3 | "1.2"a:b:c:x:s1:s2:Stroustrup/Programming/2015ObjectsAn object is some memory that can hold a value of a given typeA variable is a named objectA declaration names an objectint a = 7;char c = 'x';complex z(1.0,2.0);string s = "qwerty";*7'x'1.0 "qwerty"2.06 a:s:c:z:Stroustrup/Programming/2015Type safetyLanguage rule: type safetyEvery object will be used only according to its typeA variable will be used only after it has been initializedOnly operations defined for the variable's declared type will be appliedEvery operation defined for a variable leaves the variable with a valid valueIdeal: static type safetyA program that violates type safety will not compileThe compiler reports every violation (in an ideal system)Ideal: dynamic type safetyIf you write a program that violates type safety it will be detected at run timeSome code (typically "the run-time system") detects every violation not found by the compiler (in an ideal system)*Stroustrup/Programming/2015Type safetyType safety is a very big dealTry very hard not to violate it“when you program, the compiler is your best friend”But it won’t feel like that when it rejects code you’re sure is correctC++ is not (completely) statically type safeNo widely-used language is (completely) statically type safeBeing completely statically type safe may interfere with your ability to express ideasC++ is not (completely) dynamically type safeMany languages are dynamically type safeBeing completely dynamically type safe may interfere with the ability to express ideas and often makes generated code bigger and/or slowerAlmost all of what you’ll be taught here is type safeWe’ll specifically mention anything that is not*Stroustrup/Programming/2015Assignment and increment// changing the value of a variableint a = 7; // a variable of type int called a // initialized to the integer value 7a = 9; // assignment: now change a's value to 9a = a+a; // assignment: now double a's valuea += 2; // increment a's value by 2++a; // increment a's value (by 1)*79182021a:Stroustrup/Programming/2015A type-safety violation(“implicit narrowing”)// Beware: C++ does not prevent you from trying to put a large value// into a small variable (though a compiler may warn)int main(){ int a = 20000; char c = a; int b = c; if (a != b) // != means “not equal” cout > val >> unit) { // keep reading if (unit == 'i') // 'i' for inch cout << val << "in == " << val*cm_per_inch << "cm\n"; else if (unit == 'c') // 'c' for cm cout << val << "cm == " << val/cm_per_inch << "in\n"; else return 0; // terminate on a “bad unit”, e.g. 'q' }}*Stroustrup/Programming/2015C++11 hintAll language standards are updated occasionallyOften every 5 or 10 yearsThe latest standard has the most and the nicest featuresCurrently C++14The latest standard is not 100% supported by all compilersGCC (Linux) and Clang (Mac) are fineMicrosoft C++ is OKOther implementations (many) varyStroustrup/Programming/2015*C++14 HintYou can use the type of an initializer as the type of a variable// “auto” means “the type of the initializer”auto x = 1; // 1 is an int, so x is an intauto y = ′c′; // ′c′ is a char, so y is a charauto d = 1.2; // 1.2 is a double, so d is a doubleauto s = ″Howdy″; // ″Howdy″ is a string literal of type const char[] // so don’t do that until you know what it means!auto sq = sqrt(2); // sq is the right type for the result of sqrt(2) // and you don’t have to remember what that isauto duh; // error: no initializer for autoStroustrup/Programming/2015*The next lectureWill talk about expressions, statements, debugging, simple error handling, and simple rules for program construction*Stroustrup/Programming/2015