1. ENERGY 211 / CME 211
Lecture 10
October 13, 2008

2. Pointers
A pointer to an object is a variable whose value is the object's address
To declare a pointer: type *var-name;
To access the underlying object's value, use the dereferencing operator *
Use the address operator & to obtain an object's address, whose type is a pointer to the object's type
To output address, can use <<:
int x; int *p = &x; cout << p;

3. Pointers as Arguments C++ inherited pointers from C
In C, values of variables passed to functions cannot be changed
How can functions change values of arguments?
Solution: pass address of the actual parameter; function accepts pointer
Inside function, dereference pointer using * to access value (and change it!)

4. Arrays and Pointers
A pointer can point to an element of an array: int x[4]; int *p = &x[0];
Can add to a pointer to access another element: value of *(p+1) is x[1]
Pointer arithmetic accounts for size of each element, so use the right types!
If a function accepts a pointer as an argument, can pass an array of the same type

5. Dynamic Instances
Variables declared inside a function go away when the function exits
Arrays must be declared to have a fixed size, known at compile time
Can allocate memory dynamically
Form: type *name = new type;
Array: type *name = new type[expr]; where expr is a positive integer
Can use any constructor for type

6. Dynamic Instances, cont'd
A pointer/array created using new can be used like any other pointer/array
If you allocate memory, you must de-allocate when you're done
Form: delete ptr;, where ptr was previously allocated using new
Failure to do so causes a memory leak
Form for array: delete [] name;
Can't delete pointer more than once!

7. NULL Pointers
A NULL pointer is a pointer that does not actually point to any object
Its value, in a sense, is zero
Can always delete a NULL pointer
Common uses:
Initializing pointers
Testing whether a pointer is usable
Before you dereference a pointer, make sure it's not NULL or your program will crash!

8. C++ Smart Pointers C pointers are prone to misuse
C++ adds a smart pointer, a "wrapper" variable that contains a C pointer
When wrapper is de-allocated automatically, like any other variable, pointer inside is deleted
Form of declaration (T is a type):
std::auto_ptr<T> name(new T);
Can dereference like a C pointer

9. Iterators as Pointers
Recall that iterators allow access to elements of a sequence such as a vector or string
You can dereference an iterator to access the underlying element
Example: *(s.begin()) is first character of string s
Pointer arithmetic is valid with random-access iterators: *(s.begin()+1) is second character of string s

10. Next Time More on pointers! Improving efficiency using pointers
Matrix allocation strategies