Have you heard of the adage "All problems in computer science can be solved by another level of indirection."? If you are a programmer, chances are you have read about it in a book or an article talking about how to best structure the software you write. It's been dubbed the fundamental theorem of software engineering (the FTSE) so you should know what it is about. If you don't then, quickly, go read up on the subject at ... no, I won't do the googling for you.
A common explanation you will find is that the FTSE is talking about abstraction layers: another level of indirection is achieved by raising the abstraction level. That's a good way to view it, but only some of it. Abstraction, when realized in software, often results in a layer so that the details of whatever is on the other side remain hidden, and a layer causes references to go in round about ways to get to the point, hence indirection. However, there are other forms of indirection where one just wants to reduce coupling between two parts. Here we are not really getting rid of any details, but maybe creating a common language about them. Think of the Adaptor pattern for example. Not convinced? Go read the insightful commentary on the subject by Zed Shaw at http://zedshaw.com/essays/indirection_is_not_abstraction.html.
So, just for the fun of it, how would you explain the FTSE to a neophyte? Wikipedia defines indirection as the ability to reference something using a name, reference, or container instead of the value itself. This is both misleading and revelatory. Misleading because this is a much more general definition than the intuition most programmers have about the FTSE. After all, everything relies on naming and references, so what could a theorem stating the obvious have to teach us? But it's also revelatory because it hints at the fact that much of software engineering, and therefore much of computing, is about answering the equally famous question what's in a name. Therefore, to apply the FTSE in practice we need to allow ourselves to answer that question the best we can at any point in time. That is, we need to be able to define the meaning of a name within any given context. The meaning of a symbol in a software system of even moderate complexity quickly starts exhibiting nuances and goes through changes much like words in natural language. This is because just like natural language is a complicated symbolic model of the messy world, a software system can be similarly characterized.
In a sense software, any software, is a model of the world. The symbols we use inside a software program acquire meaning (or semantics if you will) by virtue of what the entities they refer to actually do. A function's meaning is its implementation. The meaning of a piece of data is how it's used, how it interacts with other data. And programming types govern that usage to some extent, but mostly it is about what we do with the data inside the program. For instance, to discover the meaning of a field in a data structure about which you have no documentation, you would trace where and how it is being used. New requirements or changes and adjustments to old requirements are nothing more than a refined model of the world and/or system that needs to be reflected in software. This process of refining a model is enriching the meaning of the terms used to describe things and that translates into modifying the semantics of certain symbols in the software. Which is what we call "changing the implementation". Now, the practice of programming in not seen as creating and refining meaning of symbols, but I believe that is a very important perspective. It is a perspective where the resolution of symbolic references in context is at the foundation of programming, instead of the usual "giving instructions to a machine" view. I came to this conclusion about 15 years ago while designing a programming language and looking at various theoretical constructs in PL design, their limitations and trade-offs. Over the years, I have developed a reflex to see a lot of design problems in terms the important symbols in play, the context, both lexical and runtime (or static and dynamic if you prefer), and what is the resolution process of those symbolic references. I also see a lot of programming language constructs as being in essence a bag of reference resolution tools. That is, programming constructs are in large part tools a programmer has at their disposal to define the meaning of symbolic references. Let's take a look at some.
Note that we are not talking here about the distinction between "reference variables" vs. "primitive data variables" or whatever. Though that's an important distinction, what we are concerned about is merely the fact that variables are names of things. What is thought of as "reference variables" (or pointers) in various languages has to do with how they are copied during an assignment operation or as a function argument, whether the value is mutable or not etc.
Aliases and MacrosAliases are relatively uncommon as a separate construct in modern languages. When pointers are assigned to each other, this is called aliasing because we have two names for the same memory location, i.e. two references with the same referent. For example, the assignment of any variable of Object type in Java is considered aliasing. While we do have another level of indirection here since we could change one reference without disturbing the other, this type of aliasing is not particularly interesting. But consider another type of aliasing, through macros (think C #define) where a name is explicitly declared to be a replacement of another name. The indirection here involves an extra translation step and the meaning of the alias in this case is not that it has the same referent, but that its referent is the original name. As a consequence, mentioning the alias at a place in the program where the same symbol is used for an entirely different thing will make the alias refer that that thing. Another benefit of this kind of aliasing is that it can be used to refer to other things besides variables, for example types, especially when a type description is a complex expression that we don't want to write over and over again. Macros are also in the same spirit - languages that recognize the value of compile-time decision making will offer a macro facility. And it is a shame they are not more popular. They just have a bad aura because they are associated with purely textual manipulation, a completely separate language on its own. However, if one sees them as a way to do structured code generation, or compile-time evaluation, they are much more appealing. One has to realize that macros and aliases are about name & conquer just as much as variables and functions are, and that they are in fact a great level of indirection mechanism. A mechanism that occupies another spot in the compile-run time dimension. Speaking of which, the fact that there is nothing in between that strict compile vs. run-time separation is strong limitation in language expressiveness. Partial evaluation techniques could be what macros at run time look like, but those are still confined to academic research mainly.
To sum up so far: the key difference between variables and aliases is the timing of the reference resolution process. With variables, the referent is obtained when the program is running while with aliases it is obtained at compile time. In one case we have the context of a running program, in the other the context of a running compiler.
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