Concepts of Programming Languages

6.11 Pointer and Reference Types 297

All variables in the object-oriented languages Smalltalk, Python, Ruby, and

Lua are references. They are always implicitly dereferenced. Furthermore, the

direct values of these variables cannot be accessed.

6.11.7 Evaluation

The problems of dangling pointers and garbage have already been discussed at

length. The problems of heap management are discussed in Section 6.11.8.3.

Pointers have been compared with the goto. The goto statement widens the

range of statements that can be executed next. Pointer variables widen the range

of memory cells that can be referenced by a variable. Perhaps the most damning

statement about pointers was made by Hoare (1973): “Their introduction into

high-level languages has been a step backward from which we may never recover.”

On the other hand, pointers are essential in some kinds of programming

applications. For example, pointers are necessary to write device drivers, in

which specific absolute addresses must be accessed.

The references of Java and C# provide some of the flexibility and the

capabilities of pointers, without the hazards. It remains to be seen whether

programmers will be willing to trade the full power of C and C++ pointers for

the greater safety of references. The extent to which C# programs use pointers

will be one measure of this.

6.11.8 Implementation of Pointer and Reference Types

In most languages, pointers are used in heap management. The same is true

for Java and C# references, as well as the variables in Smalltalk and Ruby, so

we cannot treat pointers and references separately. First, we briefly describe

how pointers and references are represented internally. We then discuss two

possible solutions to the dangling pointer problem. Finally, we describe the

major problems with heap-management techniques.

6.11.8.1 Representations of Pointers and References

In most larger computers, pointers and references are single values stored in

memory cells. However, in early microcomputers based on Intel micropro-

cessors, addresses have two parts: a segment and an offset. So, pointers and

references are implemented in these systems as pairs of 16-bit cells, one for

each of the two parts of an address.

6.11.8.2 Solutions to the Dangling-Pointer Problem

There have been several proposed solutions to the dangling-pointer problem.

Among these are tombstones (Lomet, 1975), in which every heap-dynamic

variable includes a special cell, called a tombstone, that is itself a pointer to the

heap-dynamic variable. The actual pointer variable points only at tombstones