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The D Programming Language

C has a long and venerable history. But Can a language written in the 1970s work thirty years on? Dermot Hogan considers whether D is a worthy successor to C...?

See also our interview with Walter Bright - creator of the D language

There have been several evolutions of the C language – Objective C, C++, Java, C# to name but the major ones. Each has added on something to the basic C structure, with more or less success; and each has been defined by its time (and what was capable of running efficiently on a machine of that time).

It seems to me that C++ hasn’t been so much designed as ‘grown’ – and not very successfully at that. But no one, as far as I am aware, has sat down and carefully looked at the core of C and tried to figure out the good and bad parts and, more importantly, which features from other languages might be successfully grafted onto the core language. Until a few years ago that is: enter Walter Bright with the D language.

Just so you know where I stand on the matter: I like C. The original, that is. I regularly program in C on Microchip dsPIC devices and for that for that sort of thing it’s the ideal language. But I have to admit that straight C on a 3GHz processor with 1.5GB of memory and a 200GB disk on a PC running Windows XP just isn’t efficient. Don’t get me wrong – in one sense C is wonderfully efficient – as it can be used to write fast-running code. But for programming a modern GUI and the rest, it’s terrible – it’s far too low level. In that sense, it’s inefficient because it wastes so much of my time!

But the main alternative to pure C, C++, sucks: it has no ‘conceptual integrity’, an idea I’ll come back to later. On the other hand, C# and Java aren’t really C. I treat C# as being a mechanism for implementing Microsoft’s .NET framework; Java I have no use for – I don’t write in Cobol, either of the old or new variety. And since Microsoft, in its wisdom, has recast Visual Basic as an inferior C#, that’s no longer in the frame, either.

D, though, looks interesting. It is C, or at least, a close relative – but not a compatible superset like Objective C or a series of ad-hoc ‘patches’ as in C++. Instead, it’s a rewrite of C but aimed at systems programming rather than application level work. I’ll pick out some of the interesting features here. For a full list, you’ll need to download the (very readable) manual from www.digitalmars.com.

My Picks...

First, D is object oriented. Not like Smalltalk where everything is an object or even C# where most things are objects. It’s object oriented in a sensible, practical sort of way. Object orientation is treated as a tool, not as the Word Of God. For example, classes are classes, as in other languages, but with single inheritance. But structs are what they should be: record layouts. If you’ve ever tried to use C# or Visual Basic .NET to talk to a micro-controller – and I have - you’ll appreciate the problem. A struct should be an unmanaged memory tool, otherwise what use is it? You might as well use a class. The C# distinction between stack allocation and heap allocation is worthless.

Secondly, D uses managed memory and garbage collection. This means that, for most of the time, you can forget about freeing stuff up. I’ve just waded through a Microsoft example C++ program (dealing with Visual Studio debugging extensions, if you really want to know) where every bit of memory was tediously allocated and then freed. I’d forgotten just how time consuming it is just writing C++ code – let alone debugging it.

And now a subject close to my heart. Engraved on it almost: COM. Currently, from Microsoft’s perspective, COM is sort of ‘undead’. Few in Microsoft seem to acknowledge its existence, but it’s there all the same. All pervasive, in fact. Now, there are good bits to COM and there are bad bits. The good bits are the way the interface definitions work – ‘austere’ is a good description. The bad bits are the threading and apartment models – ‘baroque’, I think, is an appropriate term. COM was supposed to have gone away with .NET, but it most certainly hasn’t. I’ve been recently programming some Visual Studio extensions (using the good bits of COM) and I can tell you that COM is alive and well and living in Visual Studio. And in many other Microsoft products too.

The problem is that programming COM from a managed tool like C# isn’t always that easy. Sure, it’s simple enough to define the interface, and the Visual Studio 2005 IDE makes it easy to knock out a boilerplate implementation. The trouble comes from the fact that C# also defines its own interfaces – which aren’t necessarily COM ones. I also don’t like the rather kludgey way that you implicitly use QueryInterface by casting. It all works, but it’s not elegant and you do have to keep littering your C# code with Guid and COM attributes. In contrast, a D interface is pretty much the same as a COM type interface – a set of pointers into an external DLL. Further, all D objects with a COM interface (derived from IUnknown) are automatically COM objects. In other words, you can use COM cleanly and explicitly from D.

Additionally, casting is far more intelligently used. In C#, a cast is typically either a call to QueryInterface or it’s used to up-cast or down-cast as required. Or even as in the original C to tell the compiler to stop moaning and just do it. D introduces a new cast operator which makes it very clear that you are requesting the compiler to cast; cast has the bonus that you can search for all casts using a text editor. Neat.


Lastly, let's talk about the rather peculiar installation procedure. There isn’t one. You just unzip the download into a directory of your choice and path it (if you want to). That’s it. No registry entries, no installation wizard – just unzip and go. And it works too - much to my surprise. It harks back to a simpler, earlier era of programming. After weeks of doing battle with Visual Studio and the registry, I don’t think that’s a bad thing at all.

Ah! The good old days! A DOS box, a text editor and a C compiler (here, in fact, a D compiler) …it takes me back. I got quite misty eyed over this example.

Other goodies

There’s an awful lot in D, so it’s difficult in a short article to cover everything, so here’s a list of some of the other features that caught my eye:

  • Nested functions. Yes – you can have a function enclosed within a function. Originally, C didn’t include these for efficiency reasons. However, there’s no a priori reason why you shouldn’t have these or use them.
  • Delegates. A delegate is a function pointer with an object reference. I’ve never been able to figure out why Java doesn’t have them. And yes, I’ve read Sun’s ‘justification’ of why delegates are A Bad Thing. It strikes me as total garbage: delegates just make life easier.
  • Operator overloading. The only examples I’ve seen of operator overloading in C++ produced unreadable code. Elegant, but unreadable: try figuring out celestial mechanics when you have to keep working out what the [] operator is doing. It’s not easy, I can tell you. Still, should you wish to use operator overloading, you can.
  • Templates. I’m a recent convert to templates and I think that any C type language must have them. D has them.

Actually, these aren’t downsides so much as the natural consequences of having flexibility and of D being aimed at ‘systems programming’. The first issue is pointers. D is not a ‘safe’ language like Java or C#. You can futz about with pointers far more easily than you can in C# (in Java you can’t – end of story). The problem comes with the garbage collector collecting things that you’ve messed with. If you have so messed, disaster will pretty quickly follow.

However, the upside to this is that you can allocate memory by using malloc and if you are careful, you can avoid the use of the garbage collector completely. This means that you can implement a real-time – defined as responding to an event within a predetermined time – system in D. You simply cannot do this in C# or Java. This may not be important to most people, but to me it is. I’ve recently written a real-time telescope controller and in this type of application, you simply cannot – cannot – wait for a few milliseconds while a garbage collection occurs. The heavens don’t stop moving because you’ve run out of memory.

But there are no regular expressions in D! Admittedly, this is a personal beef. And I’d have to allow that those who think that regular expressions have no place whatsoever in a systems programming language have a point. But once you’ve used Perl or Ruby, where regular expressions are ‘first class objects’, than you really miss them in other languages.

Lastly, I also have my doubts about D’s ‘contracts’. A D contract is an assertion that something is or isn’t the case. For example, that a number is greater than zero when a square root operation occurs. D builds these assertions into the language. Contracts (in general – they also occur in Chrome and Eiffel) seem to me to be just one more ‘magic bullet’ that will lead to the promised land of bug free programming. I remain sceptical.


There’s something I like about Walter Bright (the man behind D) – he’s straightforward in his views. For example, on C tags: “This misfeature of C is where the tag names of structs are in a separate but parallel symbol table. C++ attempted to merge the tag name space with the regular name space, while retaining backward compatibility with legacy C code. The result is not printable.” You know where you stand with this guy.

D is the evolution of Mr. Bright’s considerable expertise in the practical implementation of C and C++ compilers. He knows what works and what doesn’t. With D, he seems to have distilled many years of experience into a practical re-write of the original C language. One way of viewing D is as ‘C done right’.

Earlier I used the term ‘conceptual integrity’. This comes from ‘The Mythical Man Month’ by Fred Brookes. He uses it to distinguish the excellent from the indifferent. Good systems and designs have it; bad ones don’t. Naturally, C++ doesn’t have conceptual integrity.

On the other hand, D isn’t merely good; D has conceptual integrity.

See also our interview with Walter Bright - creator of the D language


May 2006


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