Pointers

1. Pointers And Dynamic Memory CS-1030 Dr. Mark L. Hornick

2. Dynamic Memory • C++ has a new() operator like Java • new() returns an address of a newly-created object • You have to assign the address to a pointer Employee* pe = new Employee; CS-1030 Dr. Mark L. Hornick

3. Object access via pointers int id; Employee anEmp; // as an object id = anEmp.getID(); // object access Employee* pe = new Employee; // ptr id = pe->getID(); // ptr access //id = (*pe).getID() // never done CS-1030 Dr. Mark L. Hornick

4. Dynamic Memory C++ does not have a Garbage Collector • Who cleans up after you? CS-1030 Dr. Mark L. Hornick

5. Dynamic Memory C++ does not have a Garbage Collector • Who cleans up after you? • You do! CS-1030 Dr. Mark L. Hornick

6. The delete() operator • Syntax: delete p; // p is a pointer • Rules • Make sure p is pointing to a valid object • Don’t delete the same object more than once • The object must have been created with the new() operator • i.e. the object must be on the heap, not the stack • Never delete an object created on the stack:Employee e(“Tom”); // e created in stack memoryEmployee* pe = &e // points at object on stackdelete pe; // crash and burn!!! CS-1030 Dr. Mark L. Hornick

7. delete() and the destructor • The destructor is called when you delete an object via the pointerEmployee* p = new Employee(“Tom”);delete p; // calls ~Employee() • Never dereference the pointer after deleting the object! • The pointer still contains the object’s previous address, but the object is no longer valid, even if it’s “hanging around” in memory.Employee* p = new Employee(“Tom”);delete p; // object is invalidatedp->giveBigRaise(); // crash and burn !!!! CS-1030 Dr. Mark L. Hornick

8. Dynamic Memory Operators • new • Allocates memory from the heap • Invokes the constructor (by context) • Returns a pointer to the new object • delete • Invokes the destructor • Releases the memory back to the heap CS-1030 Dr. Mark L. Hornick

9. Object Types • static • Memory is fixed at compile time • Stored in a special area • AutomaticDog someDog; • Memory allocated as the program runs • Stored on the stack (CS280, CS285) • Automatically destroyed when it goes out of scope • DynamicDog* pDog; pDog = new Dog; • Drawn from a memory pool (heap) • Can be released and the memory reused CS-1030 Dr. Mark L. Hornick

10. Memory Leaks • Common and dangerous problem • Sequence of events • Dynamic memory is allocated • Pointer is lost • By reassignmentpDog = 0; • End of lifetime • pDog goes out of scope • Dynamic object is inaccessible • Cannot be deleted! Why is this bad? CS-1030 Dr. Mark L. Hornick

11. NULL Pointer Revisited • Purpose • This pointer points to no object • Value is 0 • Defined in <cstdlib> • Common convention • NULL is NEVER dereferenced • if new fails, it returns NULL • delete “ignores” NULL CS-1030 Dr. Mark L. Hornick

12. Usage of NULL #include <cstdlib> … Dog* pDog = NULL; pDog = new Dog; … delete pDog; pDog = NULL; // For safety CS-1030 Dr. Mark L. Hornick