D Programming Language 2.0

Last update Mon Dec 31 02:58:01 2012


What is D?

D is a general purpose systems and applications programming language. It is a high level language, but retains the ability to write high performance code and interface directly with the operating system API's and with hardware. D is well suited to writing medium to large scale million line programs with teams of developers. D is easy to learn, provides many capabilities to aid the programmer, and is well suited to aggressive compiler optimization technology. D Man

D is not a scripting language, nor an interpreted language. It doesn't come with a VM, a religion, or an overriding philosophy. It's a practical language for practical programmers who need to get the job done quickly, reliably, and leave behind maintainable, easy to understand code.

D is the culmination of decades of experience implementing compilers for many diverse languages, and attempting to construct large projects using those languages. D draws inspiration from those other languages (most especially C++) and tempers it with experience and real world practicality.

Why D?

Why, indeed. Who needs another programming language?

The software industry has come a long way since the C language was invented. Many new concepts were added to the language with C++, but backwards compatibility with C was maintained, including compatibility with nearly all the weaknesses of the original design. There have been many attempts to fix those weaknesses, but the compatibility issue frustrates it. Meanwhile, both C and C++ undergo a constant accretion of new features. These new features must be carefully fitted into the existing structure without requiring rewriting old code. The end result is very complicated - the C standard is nearly 500 pages, and the C++ standard is about 750 pages! C++ is a difficult and costly language to implement, resulting in implementation variations that make it frustrating to write fully portable C++ code.

C++ implements things like resizable arrays and string concatenation as part of the standard library, not as part of the core language.

Can the power and capability of C++ be extracted, redesigned, and recast into a language that is simple, orthogonal, and practical? Can it all be put into a package that is easy for compiler writers to correctly implement, and which enables compilers to efficiently generate aggressively optimized code?

Modern compiler technology has progressed to the point where language features for the purpose of compensating for primitive compiler technology can be omitted. (An example of this would be the ‘register’ keyword in C, a more subtle example is the macro preprocessor in C.) We can rely on modern compiler optimization technology to not need language features necessary to get acceptable code quality out of primitive compilers.

Major Design Goals of D

Everything in designing a language is a tradeoff. Keeping some principles in mind will help to make the right decisions.

  1. Make it easier to write code that is portable from compiler to compiler, machine to machine, and operating system to operating system. Eliminate undefined and implementation defined behaviors as much as practical.
  2. Provide syntactic and semantic constructs that eliminate or at least reduce common mistakes. Reduce or even eliminate the need for third party static code checkers.
  3. Support memory safe programming.
  4. Support multi-paradigm programming, i.e. at a minimum support imperative, structured, object oriented, generic and even functional programming paradigms.
  5. Make doing things the right way easier than the wrong way.
  6. Have a short learning curve for programmers comfortable with programming in C, C++ or Java.
  7. Provide low level bare metal access as required. Provide a means for the advanced programmer to escape checking as necessary.
  8. Be compatible with the local C application binary interface.
  9. Where D code looks the same as C code, have it either behave the same or issue an error.
  10. Have a context-free grammar. Successful parsing must not require semantic analysis.
  11. Easily support writing internationalized applications.
  12. Incorporate Contract Programming and unit testing methodology.
  13. Be able to build lightweight, standalone programs.
  14. Reduce the costs of creating documentation.
  15. Provide sufficient semantics to enable advances in compiler optimization technology.
  16. Cater to the needs of numerical analysis programmers.
  17. Obviously, sometimes these goals will conflict. Resolution will be in favor of usability.

Features To Keep

The general look of D is like C and C++. This makes it easier to learn and port code to D. Transitioning from C/C++ to D should feel natural. The programmer will not have to learn an entirely new way of doing things.

Using D will not mean that the programmer will become restricted to a specialized runtime vm (virtual machine) like the Java vm or the Smalltalk vm. There is no D vm, it's a straightforward compiler that generates linkable object files. D connects to the operating system just like C does. The usual familiar tools like make will fit right in with D development.

Features To Drop

Who D is For

Who D is Not For

Major Features of D

This section lists some of the more interesting features of D in various categories.

Object Oriented Programming


D's object oriented nature comes from classes. The inheritance model is single inheritance enhanced with interfaces. The class Object sits at the root of the inheritance hierarchy, so all classes implement a common set of functionality. Classes are instantiated by reference, and so complex code to clean up after exceptions is not required.

Operator Overloading

Classes can be crafted that work with existing operators to extend the type system to support new types. An example would be creating a bignumber class and then overloading the +, -, * and / operators to enable using ordinary algebraic syntax with them.

Functional Programming

Functional programming has a lot to offer in terms of encapsulation, concurrent programming, memory safety, and composition. D's support for functional style programming include:



Source files have a one-to-one correspondence with modules. Instead of #include'ing the text of a file of declarations, just import the module. There is no need to worry about multiple imports of the same module, no need to wrapper header files with #ifndef/#endif or #pragma once kludges, etc.

Declaration vs Definition

C++ usually requires that functions and classes be declared twice - the declaration that goes in the .h header file, and the definition that goes in the .c source file. This is an error prone and tedious process. Obviously, the programmer should only need to write it once, and the compiler should then extract the declaration information and make it available for symbolic importing. This is exactly how D works.


class ABC
    int func() { return 7; }
    static int z = 7;
int q;

There is no longer a need for a separate definition of member functions, static members, externs, nor for clumsy syntaxes like:

int ABC::func() { return 7; }
int ABC::z = 7;
extern int q;

Note: Of course, in C++, trivial functions like { return 7; } are written inline too, but complex ones are not. In addition, if there are any forward references, the functions need to be prototyped. The following will not work in C++:

class Foo
    int foo(Bar *c) { return c->bar(); }

class Bar
    int bar() { return 3; }

But the equivalent D code will work:

class Foo
    int foo(Bar c) { return c.bar; }

class Bar
    int bar() { return 3; }

Whether a D function is inlined or not is determined by the optimizer settings.


D templates offer a clean way to support generic programming while offering the power of partial specialization. Template classes and template functions are available, along with variadic template arguments and tuples.

Associative Arrays

Associative arrays are arrays with an arbitrary data type as the index rather than being limited to an integer index. In essence, associated arrays are hash tables. Associative arrays make it easy to build fast, efficient, bug-free symbol tables.


Documentation has traditionally been done twice - first there are comments documenting what a function does, and then this gets rewritten into a separate html or man page. And naturally, over time, they'll tend to diverge as the code gets updated and the separate documentation doesn't. Being able to generate the requisite polished documentation directly from the comments embedded in the source will not only cut the time in half needed to prepare documentation, it will make it much easier to keep the documentation in sync with the code. Ddoc is the specification for the D documentation generator. This page was generated by Ddoc, too.

Although third party tools exist to do this for C++, they have some serious shortcomings:


D has the expected support for ordinary functions including global functions, overloaded functions, inlining of functions, member functions, virtual functions, function pointers, etc. In addition:

Nested Functions

Functions can be nested within other functions. This is highly useful for code factoring, locality, and function closure techniques.

Function Literals

Anonymous functions can be embedded directly into an expression.

Dynamic Closures

Nested functions and class member functions can be referenced with closures (also called delegates), making generic programming much easier and type safe.

In, Out, and Ref Parameters

Not only does specifying this help make functions more self-documenting, it eliminates much of the necessity for pointers without sacrificing anything, and it opens up possibilities for more compiler help in finding coding problems.

Such makes it possible for D to directly interface to a wider variety of foreign API's. There would be no need for workarounds like "Interface Definition Languages".


C arrays have several faults that can be corrected:

D arrays come in several varieties: pointers, static arrays, dynamic arrays, and associative arrays.

See Arrays.


String manipulation is so common, and so clumsy in C and C++, that it needs direct support in the language. Modern languages handle string concatenation, copying, etc., and so does D. Strings are a direct consequence of improved array handling.

Resource Management

Automatic Memory Management

D memory allocation is fully garbage collected. Empirical experience suggests that a lot of the complicated features of C++ are necessary in order to manage memory deallocation. With garbage collection, the language gets much simpler.

Garbage collection eliminates the tedious, error prone memory allocation tracking code necessary in C and C++. This not only means much faster development time and lower maintenance costs, but the resulting program frequently runs faster.

For a fuller discussion of this, see garbage collection.

Explicit Memory Management

Despite D being a garbage collected language, the new and delete operations can be overridden for particular classes so that a custom allocator can be used.


RAII is a modern software development technique to manage resource allocation and deallocation. D supports RAII in a controlled, predictable manner that is independent of the garbage collection cycle.


Lightweight Aggregates

D supports simple C style structs, both for compatibility with C data structures and because they're useful when the full power of classes is overkill.

Inline Assembler

Device drivers, high performance system applications, embedded systems, and specialized code sometimes need to dip into assembly language to get the job done. While D implementations are not required to implement the inline assembler, it is defined and part of the language. Most assembly code needs can be handled with it, obviating the need for separate assemblers or DLL's.

Many D implementations will also support intrinsic functions analogously to C's support of intrinsics for I/O port manipulation, direct access to special floating point operations, etc.


A modern language should do all it can to help the programmer flush out bugs in the code. Help can come in many forms; from making it easy to use more robust techniques, to compiler flagging of obviously incorrect code, to runtime checking.


Contract Programming (invented by B. Meyer) is a revolutionary technique to aid in ensuring the correctness of programs. D's version of DBC includes function preconditions, function postconditions, class invariants, and assert contracts. See Contracts for D's implementation.

Unit Tests

Unit tests can be added to a class, such that they are automatically run upon program startup. This aids in verifying, in every build, that class implementations weren't inadvertently broken. The unit tests form part of the source code for a class. Creating them becomes a natural part of the class development process, as opposed to throwing the finished code over the wall to the testing group.

Unit tests can be done in other languages, but the result is kludgy and the languages just aren't accommodating of the concept. Unit testing is a main feature of D. For library functions it works out great, serving both to guarantee that the functions actually work and to illustrate how to use the functions.

Consider the many C++ library and application code bases out there for download on the web. How much of it comes with *any* verification tests at all, let alone unit testing? Less than 1%? The usual practice is if it compiles, we assume it works. And we wonder if the warnings the compiler spits out in the process are real bugs or just nattering about nits.

Along with Contract Programming, unit testing makes D far and away the best language for writing reliable, robust systems applications. Unit testing also gives us a quick-and-dirty estimate of the quality of some unknown piece of D code dropped in our laps - if it has no unit tests and no contracts, it's unacceptable.

Debug Attributes and Statements

Now debug is part of the syntax of the language. The code can be enabled or disabled at compile time, without the use of macros or preprocessing commands. The debug syntax enables a consistent, portable, and understandable recognition that real source code needs to be able to generate both debug compilations and release compilations.

Exception Handling

The superior try-catch-finally model is used rather than just try-catch. There's no need to create dummy objects just to have the destructor implement the finally semantics.


Multithreaded programming is becoming more and more mainstream, and D provides primitives to build multithreaded programs with. Synchronization can be done at either the method or the object level.

synchronized int func() { ... }

Synchronized functions allow only one thread at a time to be executing that function.

The synchronize statement puts a mutex around a block of statements, controlling access either by object or globally.

Support for Robust Techniques

Compile Time Checks

Runtime Checking


Operator precedence and evaluation rules

D retains C operators and their precedence rules, order of evaluation rules, and promotion rules. This avoids subtle bugs that might arise from being so used to the way C does things that one has a great deal of trouble finding bugs due to different semantics.

Direct Access to C API's

Not only does D have data types that correspond to C types, it provides direct access to C functions. There is no need to write wrapper functions, parameter swizzlers, nor code to copy aggregate members one by one.

Support for all C data types

Making it possible to interface to any C API or existing C library code. This support includes structs, unions, enums, pointers, and all C99 types. D includes the capability to set the alignment of struct members to ensure compatibility with externally imposed data formats.

OS Exception Handling

D's exception handling mechanism will connect to the way the underlying operating system handles exceptions in an application.

Uses Existing Tools

D produces code in standard object file format, enabling the use of standard assemblers, linkers, debuggers, profilers, exe compressors, and other analyzers, as well as linking to code written in other languages.

Project Management


D provides built-in support for generation of multiple versions of a program from the same text. It replaces the C preprocessor #if/#endif technique.


As code evolves over time, some old library code gets replaced with newer, better versions. The old versions must be available to support legacy code, but they can be marked as deprecated. Code that uses deprecated versions will be normally flagged as illegal, but would be allowed by a compiler switch. This will make it easy for maintenance programmers to identify any dependence on deprecated features.

Sample D Program (sieve.d)

/* Sieve of Eratosthenes prime numbers */

import std.stdio;

bool[8191] flags;
int main()
    int i, count, prime, k, iter;

    writefln("10 iterations");
    for (iter = 1; iter <= 10; iter++)
        count = 0;
	    flags[] = 1;
	    for (i = 0; i < flags.length; i++)
            if (flags[i])
                prime = i + i + 3;
		        k = i + prime;
		        while (k < flags.length)
		            flags[k] = 0;
		            k += prime;
		        count += 1;
    writefln("%d primes", count);
    return 0;

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