2.12. c/c++ ref

  • The C memory model: global, local, and heap variables. Where they are stored, their properties, etc.

  • Variable scope

  • Using pointers, pointer arithmetic, etc.

  • Managing the heap: malloc/free, new/delete

2.12.1. C basic

  • #include <stdio.h>

  • printf(“the variable ‘i’ is: %d”, i);

  • can only use /* */ for comments

  • for constants: #define MAX_LEN 1024

2.12.2. malloc

  • malloc ex.

  • There is no bool keyword in C

  /* We're going to allocate enough space for an integer 
     and assign 5 to that memory location */
  int *foo;
  /* malloc call: note the cast of the return value
     from a void * to the appropriate pointer type */
  foo = (int *) malloc(sizeof(int));  
  *foo = 5;
  • char *some_memory = "Hello World";

  • this creates a pointer to a read-only part of memory

  • it’s disastrous to free a piece of memory that’s already been freed

  • variables must be declared at the beginning of a function and must be declared before any other code

2.12.3. memory

  • heap variables stay - they are allocated with malloc

  • local variables are stored on the stack

  • global variables are stored in an initialized data segment

2.12.4. structs

  struct point {
    int x;
    int y;
 struct point *p;
 p = (struct point *) malloc(sizeof(struct point));
 p->x = 0;
 p->y = 0;

2.12.5. strings

  • array with extra null character at end ‘\0’

  • strLen doesn’t include null character

2.12.6. pointers

int fake = NULL;
int val = 20;
int * x; // declare a pointer
x = &val; //take address of a variable
- can use pointer ++ and pointer -- to get next values

//Hello World #include using namespace std; //always comes after the includes, like a weaker version of packages //everything needs to be in a namespace otherwise you have to writed std::cout to look in iostream - you would use this for very long programs int main(){ //main function, not part of a class, must return an int cout << “Hello World” << endl; return 0; //always return this, means it didn’t crash }

Preprocessor #include //System file - angle brackets #include “ListNode.h” //user file - inserts the contents of the file in this place #ifndef - “if not defined” #define - defines a macro (direct text replacement) #define TRUE 0 //like a final int, we usually put it in all caps if(TRUE ==0) #define MY_OBJECT_H //doesn’t give it a value - all it does is make #ifdef true and #ifndef false #if/#ifdef needs to be closed with #endif if 2 files include each other, we get into an include loop we can solve this with the header of the .h files - everything is only defined once odd.h: #ifndef ODD_H #define ODD_H #include “even.h” bool odd (int x); #endif even.h: #ifndef EVEN_H #define EVEN_H #include “odd.h” bool even (int x); #endif

I/O #include using namespace std; int main(){ int x; cout << “Enter a value for x: “; //the arrows show you which way the data is flowing cin >> x; return 0; }

C++ Primitive Types int can be 16,32,64 bits depending on the platform double better than char

If statement can take an int, if (0) then false. Otherwise true. //don’t do single equals instead of double equals, will return false

Compiler: clang++

Functions - you can only call methods that are above you in the file function prototype - to compile mutually recursive functions, you need to declare the function with a semicolon instead of brackets and no body. bool even(int x); //called forward declaration / function prototype bool odd(int x){ if(x==0) return false; return even(x-1); } bool even(int x){ if(x==0) return true; return odd(x-1); } Classes Need 3 Separate files: 1. Header file that contains class definition - like an interface - IntCell.h #ifndef INTCELL_H //all .h files start w/ these #define INTCELL_H class IntCell{ public: //visibility blocks, everything in this block is public IntCell(int initialValue=0); //if you don’t provide a parameter, it assumes it is 0. You can call it with 1 or no parameters. ~IntCell(); //destructor, takes no parameters
int getValue() const; //the const keyword when placed here means the method doesn’t modify the object void setValue(int val); private: int storedvalue; int max(int m); }; #endif //all .h files end w/ these 2. C++ file that contains class implementation -IntCell.cpp #include “IntCell.h” using namespace std; // (not really necessary, but…) IntCell::IntCell( int initialValue ) : //default value only listed in .h file storedValue( initialValue ) { //put in all the fieldname(value), this is shorthand } int IntCell::getValue( ) const { return storedValue; } void IntCell::setValue( int val ) { //this is how you define the body of a method storedValue = val; } int IntCell::max(int m){ return 1; } 3. C++ file that contains a main() - TestIntCell.cpp #include #include “IntCell.h” using namespace std; int main(){ IntCell m1; //calls default constructor - we don’t use parentheses! IntCell m2(37); cout << m1.getValue() << ” ” << m2.getValue() << endl; m1 = m2; //there are no references - copies the bits in m2 into m1 m2.setValue(40); cout << m1.getValue() << ” ” << m2.getValue() << endl; return 0; }

Pointers Stores a memory address of another object //we will assume everyhing is 32 bit Can be a primitive type or a class type int * x; pointer to int char *y; pointer to char Rational * rPointer; pointer to Rational all pointers are 32 bits in size because they are just addresses in a definition, * defines pointer type: int * x; in an expression, * dereferences: *x=2; (this sets a value for what the pointer points to) in a definition, & defines a reference type &x means get the address of x int x = 1; //Address 1000, value 1 - don’t forget to make the pointee int y = 5; //Address 1004, value 5 int * x_pointer = &x; //Address 1008, value 1000 cout << x_pointer; //prints the address 1000 cout << *x_pointer; //prints the value at the address *x_pointer = 2; //this changes the value of x to 2 x_pointer = &y; //this means x_pointer now stores the address of y *x_pointer = 3; //this changes the value of y to 3

int n = 30;
int * p;                //variables are not initialized to any value
*p = n;                 //this throws an error because you have not requested enough memory, unless it happens to be pointing to memory that you have allocated
int *p = NULL;          //this will still crash, but it is a better way to initialize
void swap(int * x, int * y) {
    int temp = *x;      //temp takes the value x is pointing to
    *x = *y;            //x points to the value that y was pointing to
    *y = temp;          //y points to the value 3
}                       //at the end, x and y still are the same addresses
int main() {
    int a=0;
    int b=3;
    cout << "Before swap(): a: " << a << "b: " 
         << b << endl;
    cout << "After swap(): a: " << a << "b: " 
         << b << endl;
    return 0;

Dynamic Memory Allocation //not very efficient Static Memory Allocation - the compiler knows at compile time how much memory is needed int someArray[10]; //declare array of 10 elements int *value1_address = &someArray[3]; // declare a pointer to int new keyword returns a pointer to newly created “thing” int main() { int n; cout << “Please enter an integer value: ” ; // read in a value from the user cin >> n; int * ages = new int [n];// use the user’s input to create an array of int using new for (int i=0; i < n; i++) { // use a loop to prompt the user to initialize array cout << “Enter a value for ages[ ” << i << ” ]: “; cin >> ages[i]; } for(int i=0; i<n; i++) { // print out the contents of the array cout << “ages[ ” << i << ” ]: ” << ages[i]; delete [] ages; //finished with the array - clean up the memory used by calling delete return 0; //everything you allocate with new needs to be deleted, this is faster than java } Generally, SomeTypePtr = new SomeType; int * intPointer = new int; delete intPointer; //for array, delete [] ages; -this only deals with the pointee, not the pointer Accessing parts of an object regular object: Rational r; r.num = 3; for a pointer, dereference it: Rational *r = new Rational(); (r).num=4; //or r->num = 4; (shorthand) char x,y; //y is not a pointer! Write like char *x,y; Linked Lists List object keeps track of size, pointers to head, tail head and tail are dummy nodes ListNode holds a value, previous, and next ListItr has pointer to current ListNode Friend class ListNode { public: ListNode(); //Constructor private: //only this class can modify these fields int value; ListNode next, previous; //for doubly linked lists friend class List; //these classes can bypass private visibility friend class ListItr; }; Constructor - just has to initialize fields Foo() { ListNode head = new ListNode(); //because we put the class type ListNode, then we are creating a new local variable and not modifying the field //head = new Listnode() - this works } Foo() { ListNode temp; head = &temp; //this ListNode is deallocated after the constructor ends, doesn’t work } Assume int x has been declared And int y is from user input Consider these separate C++ lines of code: x = new int[10]; // 40 bytes x = new int; // 4 bytes x = new int[y]; // y4 bytes sizeof(int) -> tells you how big an integer is (4 bytes) References - like a pointer holds an address, with 3 main differences 1. Its address cannot change (its address is constant) 2. It MUST be initialized upon declaration Cannot (easily) be initialized to NULL 3. Has implicit dereferencing If you try to change the value of the reference, it automatically assumes you mean the value that the reference is pointing to //can’t use it when you need to change ex. ListItr has current pointer that changes a lot Declaration List sampleList List & theList = sampleList;//references has to be initialized to the object, not the address void swap(int & x, int & y) { //this passes in references int temp = x; x = y; y = temp; } int main() { //easier to call, references are nice when dealing with parameters int a=0; int b=3; cout << “Before swap(): a: ” << a << “b: “ << b << endl; swap(b,a); cout << “After swap(): a: ” << a << “b: “ << b << endl; return 0; } You can access its value with just a period Location * & Definition “pointer to” “reference to” Statement “dereference” “address of” subroutines methods are in a class functions are outside a class Parameter passing Call by value - actual parameter is copied into formal parameter This is what Java always does - can be slow if it has to copy a lot -actual object can’t be modified Call by reference - pass references as parameters Use when formal parameter should be able to change the value of the actual argument void swap (int &x, int &y); Call by constant reference - parameters are constant and are passed by reference Both efficient and safe bool compare(const Rational & left, const Rational & right); Can also return by different ways C++ default class 1. Destructor //this will do nothing Frees up any resources allocated during the use of an object 2. Copy Constructor //copies something over Creates a new object based on an old object IntCell copy = original; //or Intcell copy(original) automatically called when object is passed by value into a subroutine automatically called when object is returned by value from a subroutine 3. operator=() also known as the copy assignment operator intended to copy the state of original into copy called when = is applied to two objects after both have been previously constructed IntCell original; //constructor called IntCell copy; copy = original; //operator called overrides the = operator //operator overrides only work on objects, not pointers (and a default constructor, if you don’t supply one) //this will do nothing

C++ has visibility on the inheritance

class Name { public: Name(void) : myName(“”) { } ~Name(void) { } void SetName(string theName) { myName = theName; } void print(void) { cout << myName << endl; }

private: string myName; };

class Contact: public Name { //this is like contact extends name public: Contact(void) { myAddress = “”; } ~Contact(void) { } void SetAddress(string theAddress) { myAddress = theAddress; } void print(void) { Name::print(); //this can’t access private fields in Name, needs to call print from super class cout << myAddress << endl; } private: string myAddress; };

C++ has multiple inheritance - you can have as many parent classes as you want class Sphere : public Shape, public Comparable, public Serializable { };

Dispatch Static - Decision on which member function to invoke made using compile-time type of an object when you have a pointer Person *p; p = new Student(); p.print(); //will alway call the Person print method - uses type of the pointer Dynamic - Decision on which member function to invoke made using run-time type of an object Incurs runtime overhead Program must maintain extra information Compiler must generate code to determine which member function to invoke Syntax in C++: virtual keyword (Java does this by default, i.e. everything is virtual) Example class A virtual void foo()
class B : public A virtual void foo() void main () int which = rand() % 2; A *bar; if ( which ) bar = new A(); else bar = new B(); bar->foo(); return 0;

    Virtual method tables - stores the virtual methods in an array
        Each object contains a pointer to the virtual method table
            In addition to any other fields
        That table has the addresses of the methods
            Any virtual method must follow the pointer to the object... (one pointer dereference)
            Then follow the virtual method table pointer... (second pointer dereference)
            Then lookup the method pointer
                In Java default is Dynamic
                In C++, default is Static - this is faster
        When creating a subclass object, the constructor of each subclass overwrites the appropriate pointers in the virtual method table with the overridden method pointers

Abstract Classes class foo { public: virtual void bar() = 0; };

Types of multiple inheritance 1. Shared What Person is in the diagram on the previous slide 2. Replicated (or repeated) What gp_list_node is in the diagram on the previous slide 3. Non-replicated (or non-repeated) A language that does not allow shared or replicated (i.e. no common ancestors allowed) 4. Mix-in What Java (and others) use to fake multiple inheritance through the use of interfaces

In C++, replicated is the default
    Shared can be done by specifying that a base class is virtual:
        class student: public virtual person, public gp_list_node {
        class professor: public virtual person, public gp_list_node {
Java has ArrayStoreException - makes sure the thing you are adding to the array is of the correct type
    String[] a = new String[1];
    Object[] b = a;
    b[0] = new Integer (1);