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
C basic
- #include
- printf(“the variable ‘i’ is: %d”, i);
- can only use /* */ for comments
- for constants:
#define MAX_LEN 1024
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;
free(foo);
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
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
structs
struct point {
int x;
int y;
};
struct point *p;
p = (struct point *) malloc(sizeof(struct point));
p->x = 0;
p->y = 0;
strings
- array with extra null character at end ‘\0’
- strLen doesn’t include null character
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
Preprocessor
#include
I/O
#include
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
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;
swap(&b,&a);
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]; // y*4 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);