Aliasing

Aliasing is a phenomenon that occurs when two names refer to the same resource.

Aliasing happens in both languages with references and languages with pointers.

Aliasing can be controlled with:

• alias types,
• ownership,
• lifetimes.

Examples

In languages with references

Consider the following Java program:

void transpose(int[][] X, int[][] Y) {
  for (int i = 0; i < X.length; i++) {
 		for (int j = 0; j < X[0].length; j++) {
    		X[i][j] = Y[j][i];
    }
  }
}

The program represents matrices of integers as nested arrays. The aim of the code above is to transpose the matrix Y and put its result into the matrix X. Now suppose we initialise a matrix, and we make the following call:

int[][] Z = ...; // initialise matrix

... 

transpose(Z, Z);

As arrays in Java are passed as references, the formal parameters X and Y will refer to the same nested array, so the call will malfunction.

In languages with pointers

A similar example applies to a language with pointers.

For example, in C we could have

int **p = ... // initialise matrix

...

transpose(p, p);

References

• The definition and example for the case of a language with references comes from a classic POPL 1978 paper by John Reynolds:

@inproceedings{reynolds_1978,
	address = {New York, NY, USA},
	title = {Syntactic {Control} of {Interference}},
	doi = {10.1145/512760.512766},
	booktitle = {Proceedings of the 5th {ACM} {SIGACT}-{SIGPLAN} {Symposium} on {Principles} of {Programming} {Languages}},
	publisher = {Association for Computing Machinery},
	author = {Reynolds, John C.},
	year = {1978},
	pages = {39--46}
}