Dr. Dobb's is part of the Informa Tech Division of Informa PLC

This site is operated by a business or businesses owned by Informa PLC and all copyright resides with them. Informa PLC's registered office is 5 Howick Place, London SW1P 1WG. Registered in England and Wales. Number 8860726.

Channels ▼


The Case for D

D Fundamentals

D could be best described as a high-level systems programming language. It encompasses features that are normally found in higher-level and even scripting languages -- such as a rapid edit-run cycle, garbage collection, built-in hashtables, or a permission to omit many type declarations -- but also low-level features such as pointers, storage management in a manual (' la C's malloc/free) or semi-automatic (using constructors, destructors, and a unique scope statement) manner, and generally the same direct relationship with memory that C and C++ programmers know and love. In fact, D can link and call C functions directly with no intervening translation layer. The entire C standard library is directly available to D programs. However, you'd very rarely feel compelled to go that low because D's own facilities are often more powerful, safer, and just as efficient. By and large, D makes a strong statement that convenience and efficiency are not necessarily at odds. Aside from the higher-level topics that we'll discuss soon, no description of D would be complete without mentioning its attention to detail: all variables are initialized, unless you initialize them with void; arrays and associative arrays are intuitive and easy on the eyes; iteration is clean; NaN is actually used; overloading rules can be understood; support for documentation and unit testing is built-in. D is multi-paradigm, meaning that it fosters writing code in object-oriented, generic, functional, and procedural style within a seamless and remarkably small package. The following bite-sized sections give away some generalities about D.

Hello, Cheapshot

Let's get that pesky syntax out of the way. So, without further ado:

<b>import</b> std.stdio;
<b>void</b> main()
    writeln("Hello, world!");

Syntax is like people's outfits -- rationally, we understand it shouldn't make much of a difference and that it's shallow to care about it too much, but on the other hand we can't stop noticing it. (I remember the girl in red from The Matrix to this day.) For many of us, D has much of the "next door" familiar looks in that it adopted the C-style syntax also present in C++, Java, and C#. (I assume you are familiar with one of these, so I don't need to explain that D has par for the course features such as integers, floating-point numbers, arrays, iteration, and recursion.)

Speaking of other languages, please allow a cheapshot at the C and C++ versions of "Hello, world." The classic C version, as lifted straight from the second edition of K&R, looks like this:

#include <stdio.h> main() { printf("hello, world\n"); }

and the equally classic C++ version is (note the added enthusiasm):

<b>#include</b> <iostream>
<b>int</b> main()
     std::cout << "Hello, world!\n";

Many comparisons of the popular first program written in various languages revolve around code length and amount of information needed to understand the sample. Let's take a different route by discussing correctness, namely: what happens if, for whatever reason, writing the greeting to the standard output fails? Well, the C program ignores the error because it doesn't check the value returned by printf. To tell the truth, it's actually a bit worse; although on my system it compiles flag-free and runs, C's "hello world" returns an unpredictable number to the operating system because it falls through the end of main. (On my machine, it exits with 13, which got me a little scared. Then I realized why: "hello, world\n" has 13 characters; printf returns the number of characters printed, so it deposits 13 in the EAX register; the exit code luckily doesn't touch that register; so that's ultimately what the OS sees.) It turns out that the program as written is not even correct under the C89 or C99 standards. After a bit of searching, The Internet seems to agree that the right way to open the hailing frequencies in C is:

<b>#include</b> < stdio.h>
<b>int</b> main()
    printf("hello, world\n");
    <b>return</b> 0;

which does little in the way of correctness because it replaces an unpredictable return with one that always claims success, whether or not printing succeeded.

The C++ program is guaranteed to return 0 from main if you forgot to return, but also ignores the error because, um, at program start std::cout.exceptions() is zero and nobody checks for std::cout.bad() after the output. So both programs will claim success even if they failed to print the message for whatever reason. The corrected C and C++ versions of the global greet lose a little of their lip gloss:

<b>#include</b> <stdio.h>
<b>int</b> main()
   <b>return</b> printf("hello, world\n") < 0;


<b>#include</b> <iostream>
<b>int</b> main()
   std::cout << "Hello, world!\n";
   <b>return</b> std::cout.bad();

Further investigation reveals that the classic "hello, world" for other languages such as Java (code omitted due to space limitations), J# (a language completely -- I mean completely -- unrelated to Java), or Perl, also claim success in all cases. You'd almost think it's a conspiracy, but fortunately the likes of Python and C# come to the rescue by throwing an exception.

How does the D version fare? Well, it doesn't need any change: writeln throws on failure, and an exception issued by main causes the exception's message to be printed to the standard error stream (if possible) and the program to exit with a failure exit code. In short, the right thing is done automatically. I wouldn't have taken this cheapshot if it weren't for two reasons. One, it's fun to imagine the street riots of millions of betrayed programmers crying how their "Hello, world" program has been a sham. (Picture the slogans: "Hello, world! Exit code 13. Coincidence?" or "Hello, sheeple! Wake up!" etc.) Second, the example is not isolated, but illustrative for a pervasive pattern -- D attempts not only to allow you to do the right thing, it systematically attempts to make the right thing synonymous to the path of least resistance whenever possible. And it turns out they can be synonymous more often than one might think. (And before you fix my code, "void main()" is legal D and does what you think it should. Language lawyers who destroy noobs writing "void main()" instead of "int main()" in C++ newsgroups would need to find another favorite pastime if they switch to D.)

Heck, I planned to discuss syntax and ended up with semantics. Getting back to syntax, there is one notable departure from C++, C#, and Java: D uses T!(X, Y, Z) instead of T<X, Y, Z>(and T!(X) or simply T!X for T<X>) to denote parameterized types, and for good reasons. The choice of angular brackets, when clashing with the use of '<', '>', and '>>' as arithmetic operands, has been a huge source of parsing problems for C++, leading to a hecatomb of special rules and arbitrary disambiguations, not to mention the world's least known syntax object.template fun<arg>(). If one of your C++ fellow coders has Superman-level confidence, ask them what that syntax does and you'll see Kryptonite at work. Java and C# also adopted the angular brackets but wisely chose to disallow arithmetic expressions as parameters, thus preemptively crippling the option to ever add them later. D extends the traditional unary operator '!' to binary uses and goes with the classic parentheses (which (I'm sure) you always pair properly).

Compilation Model

D's unit of compilation, protection, and modularity is the file. The unit of packaging is a directory. And that's about as sophisticated as it goes. There's no pretense that the program source code would really feel better in a super-duper database. This approach uses a "database" tuned by the best of us for a long time, integrating perfectly with version control, backup, OS-grade protection, journaling, what have you, and also makes for a low entry barrier for development as all you need is an editor and a compiler. Speaking of which, specialized tool support is at this time scarce, but you can find things like the emacs mode d-mode, vim support, the Eclipse plug-in Descent, the Linux debugger ZeroBugs, and the full IDE Poseidon.

Generating code is a classic two-stroke compile and link cycle, but that happens considerably faster than in most similar environments, for two reasons, no, three.

  • One, the language's grammar allows separate and highly optimized lexing, parsing, and analysis steps.
  • Two, you can easily instruct the compiler to not generate many object files like most compilers do, and instead construct everything in memory and make only one linear commit to disk.
  • Three, Walter Bright, the creator and original implementor of D, is an inveterate expert in optimization.

This low latency means you can use D as a heck of an interpreter (the shebang notation is supported, too).

D has a true module system that supports separate compilation and generates and uses module summaries (highbrowspeak for "header files") automatically from source, so you don't need to worry about maintaining redundant files separately, unless you really wish to, in which case you can. Yep, that stops that nag right in mid-sentence.

Related Reading

More Insights

Currently we allow the following HTML tags in comments:

Single tags

These tags can be used alone and don't need an ending tag.

<br> Defines a single line break

<hr> Defines a horizontal line

Matching tags

These require an ending tag - e.g. <i>italic text</i>

<a> Defines an anchor

<b> Defines bold text

<big> Defines big text

<blockquote> Defines a long quotation

<caption> Defines a table caption

<cite> Defines a citation

<code> Defines computer code text

<em> Defines emphasized text

<fieldset> Defines a border around elements in a form

<h1> This is heading 1

<h2> This is heading 2

<h3> This is heading 3

<h4> This is heading 4

<h5> This is heading 5

<h6> This is heading 6

<i> Defines italic text

<p> Defines a paragraph

<pre> Defines preformatted text

<q> Defines a short quotation

<samp> Defines sample computer code text

<small> Defines small text

<span> Defines a section in a document

<s> Defines strikethrough text

<strike> Defines strikethrough text

<strong> Defines strong text

<sub> Defines subscripted text

<sup> Defines superscripted text

<u> Defines underlined text

Dr. Dobb's encourages readers to engage in spirited, healthy debate, including taking us to task. However, Dr. Dobb's moderates all comments posted to our site, and reserves the right to modify or remove any content that it determines to be derogatory, offensive, inflammatory, vulgar, irrelevant/off-topic, racist or obvious marketing or spam. Dr. Dobb's further reserves the right to disable the profile of any commenter participating in said activities.

Disqus Tips To upload an avatar photo, first complete your Disqus profile. | View the list of supported HTML tags you can use to style comments. | Please read our commenting policy.