1. Dynamic Memory
A whole heap of fun…

2. What will this do? getPointerToTen Initialize a variable
Makes a pointer to it
Returns that pointer
Main prints twice

3. The Stack C++ allocates variables on a stack 1000 pTen ??
int* getBadPointer() {
int x = 10;
int* px = &x;
return px; }
void foo() {
int z = 5;
int main(){
int* pTen = getBadPointer();
cout << *pTen << endl;
foo();
return 0;
Address
Identifier
Value
1000
pTen ??

4. The Stack C++ allocates variables on a stack 1008 px ?? 1004 x 1000
int* getBadPointer() {
int x = 10;
int* px = &x;
return px; }
void foo() {
int z = 5;
int main(){
int* pTen = getBadPointer();
cout << *pTen << endl;
foo();
return 0;
Address
Identifier
Value
1008 px ??
1004 x
1000
pTen

5. The Stack C++ allocates variables on a stack 1008 px 1004 x 10 1000
int* getBadPointer() {
int x = 10;
int* px = &x;
return px; }
void foo() {
int z = 5;
int main(){
int* pTen = getBadPointer();
cout << *pTen << endl;
foo();
return 0;
Address
Identifier
Value
1008 px
1004 x 10
1000
pTen ??

6. The Stack C++ allocates variables on a stack 1008 1004 10 1000 pTen
int* getBadPointer() {
int x = 10;
int* px = &x;
return px; }
void foo() {
int z = 5;
int main(){
int* pTen = getBadPointer();
cout << *pTen << endl;
foo();
return 0;
Address
Identifier
Value
1008
1004 10
1000
pTen

7. The Stack C++ allocates variables on a stack 1008 1004 10 1000 pTen
int* getBadPointer() {
int x = 10;
int* px = &x;
return px; }
void foo() {
int z = 5;
int main(){
int* pTen = getBadPointer();
cout << *pTen << endl; //10
foo();
cout << *pTen << endl;
return 0;
Address
Identifier
Value
1008
1004 10
1000
pTen

8. The Stack C++ allocates variables on a stack 1008 1004 z 10 1000 pTen
int* getBadPointer() {
int x = 10;
int* px = &x;
return px; }
void foo() {
int z = 5;
int main(){
int* pTen = getBadPointer();
cout << *pTen << endl; //10
foo();
cout << *pTen << endl;
return 0;
Address
Identifier
Value
1008
1004 z 10
1000
pTen

9. The Stack C++ allocates variables on a stack 1008 1004 z 5 1000 pTen
int* getBadPointer() {
int x = 10;
int* px = &x;
return px; }
void foo() {
int z = 5;
int main(){
int* pTen = getBadPointer();
cout << *pTen << endl; //10
foo();
cout << *pTen << endl;
return 0;
Address
Identifier
Value
1008
1004 z 5
1000
pTen

10. The Stack C++ allocates variables on a stack 1008 1004 5 1000 pTen
int* getBadPointer() {
int x = 10;
int* px = &x;
return px; }
void foo() {
int z = 5;
int main(){
int* pTen = getBadPointer();
cout << *pTen << endl; //10
foo();
cout << *pTen << endl;
return 0;
Address
Identifier
Value
1008
1004 5
1000
pTen

11. The Stack C++ allocates variables on a stack 1008 1004 5 1000 pTen
int* getBadPointer() {
int x = 10;
int* px = &x;
return px; }
void foo() {
int z = 5;
int main(){
int* pTen = getBadPointer();
cout << *pTen << endl; //10
foo();
cout << *pTen << endl; //5
return 0;
Address
Identifier
Value
1008
1004 5
1000
pTen

12. The Stack Traditional model: Stack grows down in memory CODE GLOBALS

13. The Stack Traditional model: Stack grows down in memory
Each function adds a Stack Frame : new set of local variables
CODE
GLOBALS
STACK
STACK FRAME

14. The Stack Traditional model: Stack grows down in memory
Each function adds a Stack Frame : new set of local variables
Exiting a function removes a stack frame
CODE
GLOBALS
STACK
STACK FRAME

15. The Heap The Heap is the extra space Managed by the OS Aka Free Store
C++ functions request parts of heap from OS
CODE
GLOBALS
STACK
STACK FRAME
HEAP

16. The Heap Heap is unaffected by changes to stack CODE GLOBALS STACK

17. The Heap Heap is unaffected by changes to stack
CODE
GLOBALS
STACK
HEAP
Stays until explicitly freed

18. Dynamic Allocation Dynamic Allocation : Allocate space on heap
Done with new keyword
Address
Identifier
Value
2000
1996
1992
1988
1984
1980
1976 …
1000
999
998
997
996
995

19. Dynamic Allocation Dynamic Allocation : Allocate space on heap
New returns pointer, must store
Address
Identifier
Value
2000 p
1000
1996
1992
1988
1984
1980
1976 …
???
999
998
997
996
995
Values in heap do not have identifiers… must have pointer to them!

20. Dynamic Allocation Dynamic Allocation : Allocate space on heap
Deference to access
Address
Identifier
Value
2000 p
1000
1996
1992
1988
1984
1980
1976 …
100
999
998
997
996
995

21. Dynamic Allocation
Address
Identifier
Value
2000 p
1000
1996
1992
1988
1984
1980
1976 …
100
999
998
997
996
995
Initalize heap memory as it is allocated with type(value)

22. Power of Heap
How will this time be different?

23. The Stack
int* getGoodPointerToTen() { int* px = new int(10); return px; } void foo() { int z = 5; int main() { int* pTen = getGoodPointerToTen(); cout << *pTen << endl; foo();
Address
Identifier
Value
2000
pTen ??
1996
1992
1988
1984
1980
1976 …
1000
999
998
997
996
995

24. The Stack
int* getGoodPointerToTen() { int* px = new int(10); return px; } void foo() { int z = 5; int main() { int* pTen = getGoodPointerToTen(); cout << *pTen << endl; foo();
Address
Identifier
Value
2000
pTen ??
1996 px
1992
1988
1984
1980
1976 …
1000
999
998
997
996
995

25. The Stack
int* getGoodPointerToTen() { int* px = new int(10); return px; } void foo() { int z = 5; int main() { int* pTen = getGoodPointerToTen(); cout << *pTen << endl; foo();
Address
Identifier
Value
2000
pTen ??
1996 px
1992
1988
1984
1980
1976 …
1000 10
999
998
997
996
995

26. The Stack
int* getGoodPointerToTen() { int* px = new int(10); return px; } void foo() { int z = 5; int main() { int* pTen = getGoodPointerToTen(); cout << *pTen << endl; foo();
Address
Identifier
Value
2000
pTen ??
1996 px
1000
1992
1988
1984
1980
1976 … 10
999
998
997
996
995

27. The Stack
int* getGoodPointerToTen() { int* px = new int(10); return px; } void foo() { int z = 5; int main() { int* pTen = getGoodPointerToTen(); cout << *pTen << endl; foo();
Address
Identifier
Value
2000
pTen
1000
1996
1992
1988
1984
1980
1976 … 10
999
998
997
996
995

28. The Stack
int* getGoodPointerToTen() { int* px = new int(10); return px; } void foo() { int z = 5; int main() { int* pTen = getGoodPointerToTen(); cout << *pTen << endl; //10 foo(); cout << *pTen << endl;
Address
Identifier
Value
2000
pTen
1000
1996
1992
1988
1984
1980
1976 … 10
999
998
997
996
995

29. The Stack
int* getGoodPointerToTen() { int* px = new int(10); return px; } void foo() { int z = 5; int main() { int* pTen = getGoodPointerToTen(); cout << *pTen << endl; //10 foo(); cout << *pTen << endl;
Address
Identifier
Value
2000
pTen
1000
1996 z
1992
1988
1984
1980
1976 … 10
999
998
997
996
995

30. The Stack
int* getGoodPointerToTen() { int* px = new int(10); return px; } void foo() { int z = 5; int main() { int* pTen = getGoodPointerToTen(); cout << *pTen << endl; //10 foo(); cout << *pTen << endl;
Address
Identifier
Value
2000
pTen
1000
1996 z 5
1992
1988
1984
1980
1976 … 10
999
998
997
996
995

31. The Stack
int* getGoodPointerToTen() { int* px = new int(10); return px; } void foo() { int z = 5; int main() { int* pTen = getGoodPointerToTen(); cout << *pTen << endl; //10 foo(); cout << *pTen << endl;
Address
Identifier
Value
2000
pTen
1000
1996 5
1992
1988
1984
1980
1976 … 10
999
998
997
996
995

32. The Stack
int* getGoodPointerToTen() { int* px = new int(10); return px; } void foo() { int z = 5; int main() { int* pTen = getGoodPointerToTen(); cout << *pTen << endl; //10 foo();
Address
Identifier
Value
2000
pTen
1000
1996 5
1992
1988
1984
1980
1976 … 10
999
998
997
996
995

33. Leaks

34. Dangers Losing track of memory "memory leak" 2000 myData 1000 1996
Address
Identifier
Value
2000
myData
1000
1996
1992
1988
1984
1980
1976
1972
1968 … 5
996
992
988
984
980
Losing track of memory "memory leak"

35. Dangers Losing track of memory "memory leak"
Address
Identifier
Value
2000
myData
996
1996
1992
1988
1984
1980
1976
1972
1968 …
1000 5 8
992
988
984
980
Losing track of memory "memory leak"
Asked for two ints, only remember where one is!

36. Accessing Heap Values delete tells OS we are done with memory 2000
Address
Identifier
Value
2000
myData
1000
1996
1992
1988
1984
1980
1976
1972
1968 … 5
996
992
988
984
980
delete tells OS we are done with memory

37. Accessing Heap Values delete tells OS we are done with memory 2000
Address
Identifier
Value
2000
myData
1000
1996
1992
1988
1984
1980
1976
1972
1968 … 5
996
992
988
984
980
delete tells OS we are done with memory

38. Accessing Heap Values delete tells OS we are done with memory
Address
Identifier
Value
2000
myData
1996
1992
1988
1984
1980
1976
1972
1968 …
1000 5
996
992
988
984
980
delete tells OS we are done with memory
Nulling pointer prevents using that memory