Download - Pointers and References
Pointers and References
Pointers & Memory
0x00 0x04 0x08 0x0B 0x10 0x14 0x18
0x1B
0x20
5
int x = 5;
x
0x00 0x04 0x08 0x0B 0x10 0x14 0x18
0x1B
0x20
Pointers & Memory
Pointers & Memory
5
int x = 5;int* y = &x
x y0x04
0x00 0x04 0x08 0x0B 0x10 0x14 0x18
0x1B
0x20
Pointers & Memory
5
int x = 5;int* y = &xint* z = y;
x y0x04 0x04
z
0x00 0x04 0x08 0x0B 0x10 0x14 0x18
0x1B
0x20
Pointers & Memory
0
int x = 5;int* y = &xint* z = y;*z = 0;
x y0x04 0x04
z
0x00 0x04 0x08 0x0B 0x10 0x14 0x18
0x1B
0x20
Allocating memory using new
Point *p = new Point(5, 5);
• new can be thought of a function with slightly strange syntax
• new allocates space to hold the object.• new calls the object’s constructor.• new returns a pointer to that object.
Deallocating memory using delete
// allocate memoryPoint *p = new Point(5, 5);
...// free the memorydelete p;
For every call to new, there must beexactly one call to delete.
Using new with arrays
int x = 10;int* nums1 = new int[10]; // okint* nums2 = new int[x]; // ok
• Initializes an array of 10 integers on the heap.• C++ equivalent of the following C codeint* nums = (int*)malloc(x * sizeof(int));
Using delete on arrays// allocate memoryint* nums1 = new int[10];int* nums3 = new int[x][4][5];
...// free the memorydelete[] nums1;delete[] nums3;
• Have to use delete[].
Destructors
• delete calls the object’s destructor.• delete frees space occupied by the object.
• A destructor cleans up after the object.• Releases resources such as memory.
Destructors – an Example
class Segment{public: Segment(); virtual ~Segment();private: Point *m_p0, *m_p1;};
Destructors – an ExampleSegment::Segment(){ m_p0 = new Point(0, 0); m_p1 = new Point(1, 1);}Segment::~Segment(){ if (m_p0) delete m_p0; if (m_p1) delete m_p1;}
Syntactic Sugar “->”
Point *p = new Point(5, 5);
// Access a member function:(*p).move(10, 10);
// Or more simply:p->move(10, 10);
Stack vs. HeapOn the Heap / Dynamic allocation
On the Stack / Automatic allocation
Point *p = new Point();
Point *ps = new Point[n];
Point p;
Point ps[10];
What happens when p goes out of scope?
Dynamic Memory
S 1 2 3 4 5 6 7
Dynamic Memory
S
T
1 2 3 4 5 6 7
0 0 0 0 0 0 0 0 0 0 0 0 0 0
int* T = new int[2*n];
Dynamic Memory
S
T
1 2 3 4 5 6 7
1 2 3 4 5 6 7 0 0 0 0 0 0 0
for (i = 0; i < n; i++) T[i] = S[i];
Dynamic Memory
S
T
1 2 3 4 5 6 7
1 2 3 4 5 6 7 0 0 0 0 0 0 0
delete[] S;
Dynamic Memory
S
T 1 2 3 4 5 6 7 0 0 0 0 0 0 0
S = T;
Passing by value
void Math::square(int i) { i = i*i;}
int main() { int i = 5; Math::square(i); cout << i << endl;}
Passing by reference
void Math::square(int &i) { i = i*i;}
int main() { int i = 5; Math::square(i); cout << i << endl;}
What is a reference?
• An alias – another name for an object. int x = 5; int &y = x; // y is a // reference to x y = 10;
Introducing: const
void Math::printSquare(const int &i){ i = i*i; cout << i << endl;}
int main() { int i = 5; Math::printSquare(i); Math::printCube(i);}
Won’t compile.
Can also pass pointers to const
void Math::printSquare(const int *pi) {
*pi = (*pi) * (*pi); cout << pi << endl;}
int main() { int i = 5; Math::printSquare(&i); Math::printCube(&i);}
Still won’t compile.
Declaring things const
const River nile;
const River* nilePc;
River* const nileCp;
const River* const nileCpc
Read pointer declarations right to left
// A const Riverconst River nile;
// A pointer to a const Riverconst River* nilePc;
// A const pointer to a RiverRiver* const nileCp;
// A const pointer to a const Riverconst River* const nileCpc
Exercises1. Create a class called Point with two private data members, a public
print function, and constructors as well as a destructor that prints a message when it is called.
2. Create an array of 10 Points on the stack, initialize them with non-zero data, and print them. Verify that that the destructor is called for all 10 points when your program exits.
3. Create an array of 10 Points on the heap, initialize them with non-zero data, and print them. Do not delete the array and verify the destructor is not called (you have created a memory leak). Now write your program to delete the array at the end. Verify that only one destructor is called and your program crashes (you have created a memory leak and a bug). Now write your program to delete[] the array at the end. Verify that all destructors are called.