Returning to the Heap in .NET framework

Drawer PDF417 in .NET framework Returning to the Heap

The problems that exist for simple types of pointers plague class object pointers as well In particular, you must make sure that the pointer you re using actually points to a valid object For example, don t return a reference to an object defined local to the function:

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MyClass* myFunc() { // the following does not work MyClass mc; MyClass* pMC = &mc; return pMC; }

Upon return from myFunc(), the mc object goes out of scope The pointer returned by myFunc() is not valid in the calling function The problem of returning memory that s about to go out of scope is dis cussed in 9 Allocating the object off the heap solves the problem:

MyClass* myFunc() { MyClass* pMC = new MyClass; return pMC; }

I hate to keep referencing pointers and pointing to references, but new pro grammers often wonder why both are needed Actually, you could argue that you don t need both C# and most other lan guages don t use pointers However, pointer variables are an ingrained part of good ol standard non-Visual StudioNET specific C++

The syntax for manipulating a reference is similar to that used with normal objects So why not just stick with references and never look back at pointers Objects and their addresses aren t the same thing Many times, the syntax for a reference actually becomes more complicated than that for pointers Consider the following examples:

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class Student { public: int semesterHours; float gpa; Student valFriend; Student& refFriend; Student* ptrFriend; }; int main(int nNumberofArgs, char* pszArgs[]) { // the following declares a reference off of the heap // (simple enough) Student& student = *new Student; studentgpa = 10; // ditto Student& studentFriend = *new Student; studentFriendgpa = 20; // the following copies the value of one Student // object into the second studentvalFriend = studentFriend; // this doesn t work at all Student& refFriend; refFriend = studentFriend; // this does work studentpFriend = &studentFriend; return 0; }

As you can see, I modified that Student class so that one Student can refer ence her best buddy I tried to use the reference variable type to do so I cre ated two students in main() in an attempt to link the one student object to its studentFriend The first assignment in the body of the program copies the contents of the friend into the data member Student object contains a body double The second assignment doesn t work at all C++ can t differentiate assigning an object to a reference variable from assignment to an object itself Only the third assignment works The student object points to the address of the studentFriend, which is exactly what you want

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The second most common structure after the array is called a list Lists come in different sizes and types; however, the most common one is the linked list In the linked list, each object points to the next member in a sort of chain that extends through memory The program can simply point the last element in the list to an object to add it to the list This means that the user doesn t have to declare the size of the linked list at the beginning of the program you can cause the linked list to grow (and shrink) as necessary by adding (and removing) objects The cost of such flexibility is speed of access You can t just reach in and grab the tenth element, for example, like you would in the case of an array Now, you have to start at the beginning of the list and link ten times from one object to the next A linked list has one other feature besides its run-time expandability (that s good) and its difficulty in accessing an object at random (that s bad) a linked list makes significant use of pointers This makes linked lists a great tool for giving you experience in manipulating pointer variables Not every class can be used to create a linked list You declare a linkable class as follows:

class LinkableClass { public: LinkableClass* pNext; // other members of the class };

The key is using the pNext pointer to an object of class LinkableClass At first blush, this seems odd indeed a class contains a pointer to itself Actually, this says that the class Linkable contains a pointer to another object also of class Linkable The pNext pointer is similar to the appendage used to form a chain of chil dren crossing the street The list of children consists of a number of objects, all of type child Each child holds onto another child The head pointer is simply a pointer of type LinkableClass*: To keep tor turing the child chain analogy, the teacher points to an object of class child (It s interesting to note that the teacher is not a child the head pointer is not of type LinkableClass*)

LinkableClass* pHead = (LinkableClass*)0;

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