Pointers in programming languages

It is likely that few features cause as much problems as pointers and references in statement-oriented languages, such as C, C++ and Java. They are powerful, yes, and they allow us to control quite precisely how a program is to represent something. We can use them to conveniently compose objects and data without the redundancy of replicating information massively. In languages like C they are even more powerful than in Java, since just about any part of memory can be viewed as if it were just about anything through the use of pointer arithmetic, which is indeed frightening.

But they also complicate reasoning about programs enormously. Both human reasoning and automated reasoning. Pointers allow any part of the program to have side effects in any other part of the program (if we have a reference to an object that originated there), and they make it very hard to reason about the properties that an object might have at a given point in time (since we generally have no idea who might hold a reference to it – it is amazing that programmers are forced to track this in their heads, more or less). In my effort to design my own language, multiple pointers to the same objects – aliases – have come back from time to time to bite me and block elegant, attractive designs. I believe that this is a very hard problem to design around. Aliased pointers set up communication channels between arbitrary parts of a program.

Nevertheless attempts have been made, in academia and in research labs, to solve this problem. Fraction-based permissions track how many aliases exist and endow each alias with specific permissions to access the object that is referred to. Ownership analysis forces access to certain objects to go through special, “owning” objects. Unique or “unshared” pointers in some language extensions restrict whether aliases may be created or not. But so far no solution has been extremely attractive and convenient, and none has made it into mainstream languages. (I know that someone Philipp Haller made a uniqueness plugin for the Scala compiler, but it is not in wide use, I believe.)

If we are to attempt further incremental evolution of the C-family languages, aliased pointers are one of the most important issues we can attack in my opinion.

Category: Computer science, Software development | Tags: , , , , , , , 2 comments »

2 Responses to “Pointers in programming languages”

  1. saurabh

    What are the languages that support pointers???

  2. Johan Nystrom

    Saurabh, actually, it’s hard to name a language that does not support pointers or references in some form. C/C++, Java, Python, Haskell, Lisp and their derivatives, PHP, etc.

    C/C++ are the only languages in this list to support pointer arithmetic, which is a different problem from the one discussed above.

    I think the goal is not to get rid of pointers, but to contain the effects they have on language design.

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