POINTERS

1. POINTERS

2. Pointers A pointer type is one in which the variable has a range of values corresponding to memory addresses and a special value, nil, which means an invalid address.

3. Pointers: design issues • What is the scope and lifetime of a pointer variable? • What is the lifetime of a dynamic variable? • Are pointers restricted in the types of objects they can point to? • Are pointers used for dynamic storage management, indirect addressing or both?

4. Pointers: problems • Type checking - • the type a pointer can point to is called its domain type • Dangling pointers - • the pointer contains the address of a variable • which has been deallocated • Lost objects - • the pointer is destroyed, but the data to • which it points, may still be needed

5. Pointers: problems • in Pascal: • dangling pointers can be created with specific deallocation

6. Pointers: problems In Pascal, dangling pointers can be created with specific deallocation Pascal implementors can: - ignore the dispose - not include dispose in the language - deallocate the dynamic variable and set its pointer to nil - implement dispose completely and correctly disallowing dangling pointers

7. Pointers: problems In C, dangling pointers and lost objects can be created as below: Dangling pointer: int *p1, *p2; p1 = (int *) malloc (sizeof(int)); p2 = p1; free (p1); p2 is now a dangling pointer

8. Pointers: problems In C++, dangling pointers and lost objects can be created as below: Dangling pointer: int *p1, *p2; p1 = new int; p2 = p1; delete p1; p2 is now a dangling pointer

9. Pointers: problems In C, dangling pointers and lost objects can be created Lost object: int *p1; p1 = (int *) malloc (5*sizeof(int)); ... p1 = (int *) malloc (10*sizeof(int)); The first block of allocated memory is now a lost object.

10. Pointers: problems In C++, dangling pointers and lost objects can be created Lost object: int *p1; p1 = new int[10]; ... p1 = new int[15]; The first block of allocated memory is now a lost object.

11. Pointers: problems In C/C++, dangling pointers and lost objects can be created, but additional capabilities are available - dereferencing - pointer arithmetic - parameter passing - pointers to functions