Call The Project Dynamic-Memory

Transcription

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3 Call The Project Dynamic-Memory

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5 Copy-Paste Main # include "Utilities.hpp" int main(int argc, char * argv[]) { short int *PtrNo; (*PtrNo) = 5; printf ("(*PtrNo) = %d\n", (*PtrNo)); } getchar(); return(0); Run The Program

6 # include "Utilities.hpp" Why Does It Not Work? int main(int argc, char * argv[]) { short int *PtrNo; // (*PtrNo) = 5; // printf ("(*PtrNo) = %d\n", (*PtrNo)); } getchar(); return(0); What Is The Value In &1000? 4 bytes * short int?? *PtrNo &1000

7 short int No = 12, *PtrNo; Pointer Review - 1 Our & Addresses Will Not Be The Same 2 bytes short int Sketch The Memory For No Sketch The Memory For *PtrNo 12 No & bytes * short int?? *PtrNo &1002 Write The Line Of C/C++ Code To Display The Contents Of No Write The Line Of C/C++ Code To Display The Address Of No (10) Write The Line Of C/C++ Code To Display The Address Of No (16)

8 short int No = 12, *PtrNo; Pointer Review - 2 Our & Addresses Will Not Be The Same 2 bytes short int 4 bytes * short int 12 No?? *PtrNo & & Write The Line Of C/C++ Code To Display The Contents Of PtrNo Write The Line Of C/C++ Code To Display The Address Of PtrNo (10) Write The Line Of C/C++ Code To Display The Address Of PtrNo (16)

9 Pointer Review - 3 short int No = 12, *PtrNo; Our & Addresses Will Not Be The Same 2 bytes short int 4 bytes * short int 12 No & *PtrNo & & Write The Line Of C/C++ Code To Make PtrNo Point To No Write The Line Of C/C++ Code To Display The Address In PtrNo (10) Write The Line Of C/C++ Code To Display The Value Pointed To By PtrNo (16)

10 Pointer Review - 4 short int No = 12, *PtrNo; Our & Addresses Will Not Be The Same 2 bytes short int 4 bytes * short int No & *PtrNo & & Write The Line Of C/C++ Code To Increment Value Pointed To By PtrNo Write The Line Of C/C++ Code To Display The Contents Of No Write The Line Of C/C++ Code To Display The Value Pointed To By PtrNo (16)

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12 Add Student.hpp & Student.cpp To Project

13 Set The Diagnostic Level To 1 Run The Program

14 Allocate Memory Write The C/C++ Code To Allocate Memory For SPtr.

15 Display The Student - 1 Write The C/C++ Code To Display Our Student.

16 Display The Student - 2 Two Ways To Display Our Student.

17 Change The Student Info Write The C/C++ Code To Change The Student Info To ^

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19 malloc & new You may remember using malloc to allocate dynamic memory in C. We are going to use the C++ equivalent, called new. (Type Safe) 19

20 Pointer Review - 1 short int, *PtrNo; Our & Addresses Will Not Be The Same Sketch Memory 4 bytes * short int?? &1004 *PtrNo 2 bytes short int?? &1004 &1000 Type This Line Of C/C++ Code To Dynamically Allocate Memory For PtrNo Our Only Reference To This Block Of Memory Is Through The Pointer PtrNo!

21 Dynamic Memory - 2 *PtrNo 4 bytes * short int?? &1004 & bytes short int 12 &1004 When The Programmer Fails To Return Dynamic Memory To The Memory Manager, That Memory Is Lost Until The Computer, or Virtual Computer, Is Restarted! This Is Referred To As A Memory Leak!

22 Dynamic Memory - 3 *PtrNo 4 bytes * short int?? &1004 & bytes short int 12 &1004 Function delete Is Used To Return The Dynamic Memory To The Memory Manager. Just as free is used with malloc, delete is used with new.

23 Dynamic Memory - 4 *PtrNo 4 bytes * short int?? &1004 & bytes short int 12 &1004 If The Return Of The Dynamic Memory To The Memory Manager Is Not At The End Of The Program/Function, Good Programmers Will Always Set The Pointer To NULL To Avoid Dangling Pointers!

24 *PtrNo 4 bytes * short int?? &1004 &1000 Dynamic Memory bytes short int?????? Write The Line Of C/C++ Code To Allocate An Array Of 5 Short Integers Pointed to By PtrNos Don't Execute Yet!???? &1004 Write The Block Of C/C++ Code To Fill The Array With 5, 10, 15, Did This Work? How Do You Know!

25 Dynamic Memory - 6 Write The Block Of C/C++ Code To Display The Contents Of The Array Pointed To By PtrNos Discuss The Line Of C/C++ Code To Return The Dynamic Memory delete PtrNos[ ]; vs. delete PtrNos;

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27 class Name { public: Name(void); ~Name(void); void Set (char NewFirst[] = "", char NewLast[] = ""); private: int NoCharsInFirstName, NoCharsInLastName; char *First, *Last; }; Name::Name(void) { } Not A Good Constructor! Nothing Initialized! Construct A Memory Map [Sketch Of Memory] For Name N1; Initial N1 Map Address Symbolic Name Contents &1000 *FirstName? &1004 *LastName? &1008 NoCharsInFirst? &1012 NoCharsInLast? Dangling Pointers FirstName & LastName - Pointers Reference Memory Incorrect Memory! 27 27

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29 class Person { public: Person (char NewFirst[] = "", char NewLast[] = ""); ~Person (void); void Set (char NewFirst[] = "", char NewLast[] = ""); private: int NoCharsInFirstName, NoCharsInLastName; char *First, *Last; }; In order to avoid dangling pointers, the constructor should either allocate the needed dynamic memory or set the pointer to NULL. Create A Good Constructor For Person P1; Person::Person (char NewFirst[] = "", char NewLast[] = "") { NoCharsInFirstName = strlen(newfirst)+1; First = new char[nocharsinfirstname]; strcpy(first, NewFirst); You Do! NoCharsInLastName = strlen(newlast)+1; Last = new char[nocharsinlastname]; strcpy(first, NewFirst); }; 29 29

30 class Person { public: Person (char NewFirst[] = "", char NewLast[] = ""); ~Person (void); void Set (char NewFirst[] = "", char NewLast[] = ""); private: int NoCharsInFirstName, NoCharsInLastName; char *First, *Last; }; Person::~Person (void) { Create A Good Destructor For Person P1; delete [] First; delete [] Last; You Do! }; 30 30

31 class Person { public: Person (char NewFirst[] = "", char NewLast[] = ""); ~Person (void); void Set (char NewFirst[] = "", char NewLast[] = ""); private: int NoCharsInFirstName, NoCharsInLastName; char *First, *Last; }; Create A Good Memory Map For Person P1 ( Mickey, Mouse ); Initial P1 Map Address Symbolic Name Contents &1000 *FirstName &2000 &2000-&2006 Mickey &1004 *LastName & You &1008 Do! NoCharsInFirst &1012 NoCharsInLast 6 &3174-&3180 Mouse

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33 Person Mickey ( Mickey, Mouse ), Donald ( Donald, Duck ); Initial Mickey Map Address Symbolic Name Contents &1000 *FirstName &2000 &2000-&2006 Mickey &1004 *LastName & &1008 NoCharsInFirst &1012 NoCharsInLast 6 &3174-&3180 Mouse Initial Donald Map Address Symbolic Name Contents &1016 *FirstName &2400 &2400-&2406 Donald &1020 *LastName & &1024 NoCharsInFirst &1028 NoCharsInLast 5 &2822-&2826 Duck Donald = Mickey; Shallow Copy! No Operator Overload For = What Happens? Alter The Memory Maps Accordingly!

34 Donald = Mickey; Shallow Copy! Initial Mickey Map Address Symbolic Name Contents &1000 *FirstName &2000 &2000-&2006 Mickey &1004 *LastName & &1008 NoCharsInFirst &1012 NoCharsInLast 6 &3174-&3180 Mouse Initial Donald Map Address Symbolic Name Contents &1016 *FirstName &2400 &2400-&2406 Donald &1020 *LastName & Memory Leak &1024 NoCharsInFirst &1028 NoCharsInLast 5 &2822-&2826 Duck Shallow copy of Dynamic Variables Creates Memory Leak When Either Mickey Or Donald Go Out Of Scope, The Destructor Would Cause Other To Have Dangling Pointers

35 Problems With A Shallow Copy 1. Shallow Copy only applies to objects which contain pointers 2. Shallow copy transfers pointer addresses, but fails to properly manage dynamic memory 3. Immediately creates a Memory Leak If processing occurs in actual memory [like in Windows, MacOS, etc.] re-booting the computer is generally the easiest way to return the lost memory to the Operating System If processing occurs in a shell [like in UNIX, Linux] logging out and then re-logging in is generally the easiest way to return the lost memory to the Environment. 4. When one of the assignment variables goes out of scope, the destructor for that variable will cause the other assigned variable to have one or more Dangling Pointers. 35

36 I Hope You Are Asking Yourself? How Do I Avoid The Problems Generated By Shallow Copies? Potential Problem In Many Languages Not Unique To C & C++ Must Be Some Solution? 36

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38 class Person { public: Name (char NewFirst[] = "", char NewLast[] = ""); ~Name(void); void Set (char NewFirst[] = "", char NewLast[] = ""); void operator = (Person P); private: int NoCharsInFirstName, NoCharsInLastName; char *First, *Last; }; Create A Good Operator Overload For Person Mickey ( Mickey, Mouse ), Donald ( Donald, Duck ); Mickey = Donald; 38

39 Part 1/2 Person Mickey ( Mickey, Mouse ), Donald ( Donald, Duck ); Mickey = Donald; void Person::operator = (Person P) { // Fill the Indigenous data NoCharsInFirstName = P.NoCharsInFirstName; NoCharsInLastName = P.NoCharsInLastName; // Free the memory associated with First and Last if (First!= NULL) delete [] First; if (Last!= NULL) delete [] Last; 39

40 // Allocate memory for First and fill it with P.First if appropriate if (N.First == NULL) else { } First = NULL; First = new char [NoCharsInFirstName]; strcpy (First, P.First); // Allocate memory for Last and fill it with P.Last if appropriate } if (N.Last == NULL) else { } Last = NULL; Last = new char [NoCharsInLastName]; strcpy (Last, P.Last); Part 2/2 There Are Other Solutions! Deep Copy - makes an actual copy of the data to which the 40 pointers are pointing.

41 Deep Copy Solves Problems With A Shallow Copy 1. Object has one or more pointers 2. Shallow copy transfers pointer addresses and properly manage dynamic memory 3. Creates no Memory Leak 4.Creates No Dangling Pointers. 41

42 So Why Do We Really Care About This Shallow Copy & Deep Copy Stuff? Are We Ever Going To Use It? 42

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