What you should know about C++Lecture 1.5: CMSC 420C++•Almost every C program is a C++ program.•That was an explicit goal of the design of C++.•Why are we covering C++?-Important language, nearly the lingua franca of computer science.-Languages like Python, Ruby, C#, Java are used more and more, but there’s still a huge C++ code base.-Many features of C++ are designed to support abstract data types and general data structures.•If you want, think of C++ as C with some features to make your life easier.Javaclass IntStack { public IntStack(int max) { stack = new int[max]; top = -1; } protected void finalize() { // nothing to do here } public void push(int k) { stack[++top] = k; } // ... protected int [] stack; protected int top;};Finalize() may or may not be called when instance is garbage collectedConstructorMajor Addition to C: Classesclass IntStack { public: IntStack(int max=100); ~IntStack(); void push(int); int pop(); protected: int * stack; int top; public: int size();};Constructor has same name as class. Called when object is created.Destructor called ~ClassName;Called when object is deleted.Functions declared inside class arecalled member functions. They can access the data in the class.protected things can only be seen by subclasses; public things can be seen by everyone; private things can only be seen by this class.Comparison to Javaclass IntStack { public: IntStack(int max=100); ~IntStack(); void push(int); int pop(); protected: int * stack; int top; public: int size();};class IntStack { public IntStack(int max) { stack = new int[max]; top = -1; } protected void finalize() { // nothing to do here } public void push(int k) { stack[++top] = k; } // ... protected int [] stack; protected int top;};C++Javanew and delete operators•new operator similar to Java•new and delete in C++ return explicit pointers.•Java: int[] A = new int[3];•C++: int * A = new int[3];•C++: Node * n = new Node;•In Java, there’s garbage collection. In C++, have to delete explicitly:•C++: delete [] A; •C++: delete myStack;Classes –!function implementationsIntStack::InStack(int max=100) { stack = new int[max]; top = -1;}IntStack::~IntStack(){ delete [] stack;}void IntStack::push(int k) { top++; stack[top] = k}Syntax “new TYPE[SIZE]” creates a new array of length SIZE containing objects of type TYPE.Member functions can access class variables without any special syntax.“delete [] X” frees the memory for the array pointed to by X.To free a single object, omit the “[]”.•Stored as a local variable:•Stored on heap:Classes –!Example use{ IntStack S(10000); S.push(10); S.push(12);} // ~InStack automatically called{ IntStack * S = new IntStack(10000); S->push(10); S->push(12); delete S;}Structuresstruct A { int key; struct A * next;};struct A myrecord;myrecord.key = 10;struct A { int key; A * next; A(int k) {key = k;}};A myrecord(10);•In C++ structures are just classes where everything is public by default:•Syntax a little nicer for C++ structures (e.g. can include constructors):struct Foo { ...};class Foo { public: ...};C C++I/O•You can use all C functions for input or output.•OR you can use C++ streams (but don’t mix the two).•Standard streams:-stdin is called cin.-stdout is called cout.•Reading values from stdin (whitespace is ignored): •Writing same to stdout:int i;float f;string s;cin >> i >> f >> s;cout << i << “ “ << f << “ “ << s << endl;Strings#include <string>using namespace std;int main() { string s = “abcdefg”; string s2 = “cat”; cout << s[0] << s[2] << endl; // “ac” s.append(s2); cout << s << endl; // “abcdefgcat” s.insert(2, s2); cout << s << endl; // “abcatcdefgcat” cout << s.find(“tcd”); // 4}http://www.sgi.com/tech/stl/basic_string.htmlA “make it work” instruction.References•A way to give the same variable several names.•Value of a reference must be specified when it is created and can never be changed.•It’s like a pointer that always points to the same variable, and the dereferencing operation (*x) is automatic.int x = 10;int & y = x;cout << y; // prints 10x = 30;cout << y; // prints 30y = 72;cout << y; // prints 72cout << x; // prints 72References – Pass by Reference•References are most commonly used to pass variables to functions so that the function can change them:•Common case: want to pass a big object to a function, so don’t want to copy, but want to be sure object isn’t changed:int add1(int * x) { (*x) += 1; } /* C-style */int add1(int & x) { x += 1; } // C++-styleint foo(const Image & pict) {/*...*/}Operator Overloadingstruct Point { int x, y; Point(int xx, int yy) { x=xx; y=yy; }};bool operator==(const Point & A, const Point & B) { return A.x == B.x && A.y == B.y;}int main() { Point p1(10, 4); Point p2(-12, -100); Point p3(10, 4); if(p1 == p2) { /* FALSE */ } if(p1 == p3) { /* TRUE */ }}Variable Declarations•Can declare variables in the middle of blocks: e.g. put “int x;” any place you can have a statement:•Also, can declare variables inside initialization section of for loops:int i;for(i=0; i < len; i++) total += X[i]for(int i=0; i < len; i++) total += X[i]// i not visible after loop{ int i; // some code int j; // more code}C C++Minor Differences From C•Comments: // until the end of line (in addition to /* */)•bool is a built-in type, with values true and false.•namespaces: collect functions into groups. Probably you’ll only use to say:•Doesn’t support int foo(a,b) int a, int b { /* ... */ } syntax.•Function arguments can have default values: int foo(int a=0) { /* ... */ }.•“g++” instead of “gcc”, .cc extension instead of .cusing namespace std;Other differences•Templates: write code to work with any type of variables.-In practice, can be hard to get to work right.-Won’t need for this class (but can use if you want).•const modifier means variable cannot be changed. •Good idea in theory, except that const “infects” everything it touches •e.g. can’t pass a const variable to any function that hasn’t explicitly labeled the parameter const.•Just as well to avoid using it.Resources•SGI STL documentation: http://www.sgi.com/tech/stl/table_of_contents.html•C++