Encyclopedia of Computer Science and Technology

376 pointers and indirectionA pointer is a variable whose value is an address location. HereMyPtr holds the address 101.second line above, therefore, MyPtr is a pointer to an integervariable. This means that the address of any integer variablecan be stored in MyPtr. The last line uses the & (ampersand)to represent the address of the variable MyVar. Therefore, itstores that address in MyPtr.Examining the lines above, one sees that the variableMyVar has the value 10. The pointer variable MyPtr has thevalue of whatever machine address contains the contentsof the variable MyVar. In an expression, putting an asteriskin front of a pointer name “dereferences” the pointer. Thismeans that it returns not the address stored in the pointer,but the value stored at the address stored in the pointer (seethe diagram). Therefore if one writes:AnotherVar = * MyPtr;What is the value of AnotherVar? The answer is thecurrent value of MyVar (whose address had been stored inMyPtr)—that value, as assigned earlier, is 10.The general concept of storing the address of anothervariable in a variable is called indirection, or indirectaddressing. It was first used in assembly language to workwith index registers—special memory locations in a processorthat store memory addresses.Uses for PointersAlthough the concept may seem esoteric, pointers have anumber of uses. For example, suppose one has a buffer(perhaps storing video graphics data) and one wants tocopy it from one area to another. One could declare the bufferto be an array (see array) and then reference each element,or memory location and copy it. However, this wouldbe rather awkward. Instead, one can declare a pointer, set itto the starting address of the buffer, and then simply use aloop to increment the pointer, pointing in turn to each locationin the buffer.A similar approach applies to strings in C and relatedlanguages. A string of characters in C is declared as anarray of char. In an array, the name of the array is actuallya pointer to the first data location. It is therefore easy tomanipulate strings by getting their starting address by referencingthe name and then using one or more pointers tostep through the data locations. For example, the followingfunction copies the contents of one string into another:strcpy(char *s1,char *s2){while (*s2)*s1++ = *s2++;}The function takes two strings, s1 and s2, declared aspointers to char. It then steps (increments) them (usingthe ++ operator) so that the value in each location in s2 iscopied into the corresponding location in s1. The loop exitswhen the value at s2 is 0 (null), indicating that the end ofstring marker has been reached.Another common use for pointers is in memory allocation.Typically, a program requests memory by giving thememory allocation function a pointer and the amount ofmemory requested. The function allocates the memory andthen returns the starting address of the new memory in thepointer, so the program knows how to access that memory.Pointers are also useful for passing a “bulky” variablesuch as a data record to a procedure or function. Suppose,for example, a program needs to pass a 65,000 byte recordto a procedure for printing a report. If it passes the actualrecord, the system has to make a copy of the whole record,tying up memory. If, instead, a pointer to the record is used,only the address is passed. The procedure can then accessthe record at that address without having to make a copy.In C and some other languages it is even possible tohave a pointer that points to another pointer. A commoncase is an array of strings, such asChar Form [80] [20];representing a form that has 20 lines of 80 characters. Eachline is an array of characters and the form as a whole is thus“an array of arrays of characters.” Therefore, to dereference(get the value of) a character one would first dereferencethe line, and then the column.Problems with PointersPointers may be useful, but they are also prone to causingprogramming problems. The simplest one is failing todistinguish between a pointer and its value. For example,suppose one writes:Total = Total + MyPtr;intending to add the value of the variable pointed to byMyPtr to Total. Unfortunately, the asterisk (dereferencingoperator) has been inadvertently omitted, so what getsadded to Total is the machine address stored in MyPtr!Another problem comes when a pointer is used to allocatememory, the memory is later deallocated, but thepointer is left pointing to it.Because pointers can potentially access any location inmemory (or at least attempt to), some computer scientistsview them as more dangerous than useful. It’s true thatmost things one might want to do with a pointer can beaccomplished by alternative means. One attempt to tamepointers is found in C++, which offers the “reference” datatype. A reference is essentially a constant pointer that onceassigned to a variable always dereferences that variable and