ⓘ D (programming language)


ⓘ D (programming language)

D, also known as Dlang, is a multi-paradigm system programming language created by Walter Bright at Digital Mars and released in 2001. Andrei Alexandrescu joined the design and development effort in 2007. Though it originated as a re-engineering of C++, D is a distinct language. It has redesigned some core C++ features, while also sharing characteristics of other languages, notably Java, Python, Ruby, C#, and Eiffel.

The design goals of the language attempted to combine the performance and safety of compiled languages with the expressive power of modern dynamic languages. Idiomatic D code was commonly as fast as equivalent C++ code, while also being shorter. The language as a whole was not memory-safe but does include optional attributes designed to check memory safety.

Type inference, automatic memory management and syntactic sugar for common types allow faster development, while bounds checking, design by contract features and a concurrency-aware type system help reduce the occurrence of bugs.


1. Features

D was designed with lessons learned from practical C++ usage, rather than from a purely theoretical perspective. Although the language uses many C and C++ concepts, it also discards some and was not compatible with C and C++ source code. D has, however, been constrained in its design by the rule that any code that was legal in both C and D should behave in the same way. D gained some features before C++, such as closures, anonymous functions, and compile time function execution. D adds to the functionality of C++ by also implementing design by contract, unit testing, true modules, garbage collection, first class arrays, associative arrays, dynamic arrays, array slicing, nested functions, lazy evaluation, and a re-engineered template syntax. D retains C++s ability to perform low-level programming and to add inline assembler. C++ multiple inheritance was replaced by Java-style single inheritance with interfaces and mixins. On the other hand, Ds declaration, statement and expression syntax closely matches that of C++.

The inline assembler typifies the differences between D and application languages like Java and C#. An inline assembler lets programmers enter machine-specific assembly code within standard D code, a method used by system programmers to access the low-level features of the processor needed to run programs that interface directly with the underlying hardware, such as operating systems and device drivers.

D has built-in support for documentation comments, allowing automatic documentation generation.


1.1. Features Imperative

Imperative programming in D is almost identical to that in C. Functions, data, statements, declarations and expressions work just as they do in C, and the C runtime library may be accessed directly. On the other hand, some notable differences between D and C in the area of imperative programming include Ds foreach loop construct, which allows looping over a collection, and nested functions, which are functions that are declared inside another and may access the enclosing functions local variables.

D also includes dynamic arrays and associative arrays by default in the language.


1.2. Features Object-oriented

Object-oriented programming in D is based on a single inheritance hierarchy, with all classes derived from class Object. D does not support multiple inheritance; instead, it uses Java-style interfaces, which are comparable to C++s pure abstract classes, and mixins, which separates common functionality from the inheritance hierarchy. D also allows the defining of static and final non-virtual methods in interfaces.


1.3. Features Metaprogramming

Metaprogramming is supported by a combination of templates, compile time function execution, tuples, and string mixins. The following examples demonstrate some of Ds compile-time features.

Templates in D can be written in a more imperative style compared to the C++ functional style for templates. This is a regular function that calculates the factorial of a number:

Here, the use of static if, Ds compile-time conditional construct, is demonstrated to construct a template that performs the same calculation using code that is similar to that of the function above:

In the following two examples, the template and function defined above are used to compute factorials. The types of constants need not be specified explicitly as the compiler infers their types from the right-hand sides of assignments:

This is an example of compile time function execution. Ordinary functions may be used in constant, compile-time expressions provided they meet certain criteria:

The std.string.format function performs printf -like data formatting also at compile-time, through CTFE, and the "msg" pragma displays the result at compile time:

String mixins, combined with compile-time function execution, allow generating D code using string operations at compile time. This can be used to parse domain-specific languages to D code, which will be compiled as part of the program:


1.4. Features Functional

D supports functional programming features such as function literals, closures, recursively-immutable objects and the use of higher-order functions. There are two syntaxes for anonymous functions, including a multiple-statement form and a "shorthand" single-expression notation:

There are two built-in types for function literals, function, which is simply a pointer to a stack-allocated function, and delegate, which also includes a pointer to the surrounding environment. Type inference may be used with an anonymous function, in which case the compiler creates a delegate unless it can prove that an environment pointer is not necessary. Likewise, to implement a closure, the compiler places enclosed local variables on the heap only if necessary. When using type inference, the compiler will also add attributes such as pure and nothrow to a functions type, if it can prove that they apply.

Other functional features such as currying and common higher-order functions such as map, filter, and reduce are available through the standard library modules std.functional and std.algorithm.

Alternatively, the above function compositions can be expressed using Uniform Function Call Syntax UFCS for more natural left-to-right reading:


1.5. Features Concurrent

Concurrent programming is fully implemented in the library, and does not require any special support from the compiler. Alternative implementations and methodologies of writing concurrent code are possible. The use of D typing system does help ensure memory safety.


1.6. Features Memory management

Memory is usually managed with garbage collection, but specific objects may be finalized immediately when they go out of scope. Explicit memory management is possible using the overloaded operators new and delete, and by simply calling Cs malloc and free directly. Garbage collection can be controlled: programmers may add and exclude memory ranges from being observed by the collector, can disable and enable the collector and force either a generational or full collection cycle. The manual gives many examples of how to implement different highly optimized memory management schemes for when garbage collection is inadequate in a program.


1.7. Features SafeD

SafeD is the name given to the subset of D that can be guaranteed to be memory safe no writes to memory that were not allocated or that have already been recycled. Functions marked safe are checked at compile time to ensure that they do not use any features that could result in corruption of memory, such as pointer arithmetic and unchecked casts, and any other functions called must also be marked as safe or trusted. Functions can be marked trusted for the cases where the compiler cannot distinguish between safe use of a feature that is disabled in SafeD and a potential case of memory corruption.


1.8. Features Scope Lifetime Safety

Initially under the banners of DIP1000 and DIP25 now part of the language specification, D provides protections against certain ill-formed constructions involving the lifetimes of data.

The current mechanisms in place primarily deal with function parameters and stack memory however it is a stated ambition of the leadership of the programming language to provide a more thorough treatment of lifetimes within the D programming language.


1.9. Features Lifetime Safety of Assignments

Within safe code, the lifetime of an assignment involving a reference type is checked to ensure to the lifetime of the assignee is longer than that of the assigned.

For example:


1.10. Features Function Parameter Lifetime Annotations within safe code

When applied to function parameter which are either of pointer type or references, the keywords return and scope constrain the lifetime and use of that parameter.

The Standard Dictates the following behaviour:

An Annotated Example is given below.


1.11. Features Interaction with other systems

Cs application binary interface ABI is supported, as well as all of Cs fundamental and derived types, enabling direct access to existing C code and libraries. D bindings are available for many popular C libraries. Additionally, Cs standard library is a part of standard D.

On Microsoft Windows, D can access Component Object Model COM code.


1.12. Features Interaction with C++ code

D takes a permissive but realistic approach to interoperation with C++ code.

For D code marked as externC++, the following features are specified:

  • The vtable shall be matched up to single inheritance The only level supported by the D language specification.
  • The name mangling conventions shall match those of C++ on the target.
  • For Function Calls, the ABI shall be equivalent.

C++ namespaces are used via the syntax externC++, namespace where namespace is the name of the C++ namespace.


1.13. Features Better C

The D programming language has an official subset known as Better C ". This subset forbids access to D features requiring use of runtime libraries other than that of C

Accessed via- on all current implementations – the "-betterC" flag during compilation, Better C may only call into D code compiled under the same flag and linked code other than D but code compiled without the Better C option may call into code compiled with it: This will, however, lead to slightly different behaviours due to differences in how C and D handle asserts.


1.14. Features Features available in the Better C subset

  • Array slicing, and array bounds checking
  • Memory safety protections
  • Full metaprogramming facilities
  • COM classes and C++ classes
  • RAII
  • scopeexit
  • final switch
  • Nested functions, nested structs, delegates and lambdas
  • Unrestricted use of compile-time features for example, Ds dynamic allocation features can be used at compile time to pre-allocate D data
  • Member functions, constructors, destructors, operating overloading, etc.
  • unittest blocks
  • switch with strings
  • Interfacing with C++
  • assert failures are directed to the C runtime library
  • The full module system

1.15. Features Features unavailable in the Better C subset

  • Static module constructors or destructors
  • Garbage Collection
  • TypeInfo and ModuleInfo
  • Exceptions
  • Built-in threading e.g. core.thread
  • Dynamic arrays though slices of static arrays work and associative arrays
  • synchronized and core.sync

2. History

Walter Bright started working on a new language in 1999. D was first released in December 2001 and reached version 1.0 in January 2007. The first version of the language D1 concentrated on the imperative, object oriented and metaprogramming paradigms, similar to C++.

Dissatisfied with Phobos, Ds official runtime and standard library, members of the D community created an alternative runtime and standard library named Tango. The first public Tango announcement came within days of D 1.0s release. Tango adopted a different programming style, embracing OOP and high modularity. Being a community-led project, Tango was more open to contributions, which allowed it to progress faster than the official standard library. At that time, Tango and Phobos were incompatible due to different runtime support APIs. This made it impossible to use both libraries in the same project. The existence of two libraries, both widely in use, has led to significant dispute due to some packages using Phobos and others using Tango.

In June 2007, the first version of D2 was released. The beginning of D2s development signaled D1s stabilization. The first version of the language has been placed in maintenance, only receiving corrections and implementation bugfixes. D2 introduced breaking changes to the language, beginning with its first experimental const system. D2 later added numerous other language features, such as closures, purity, and support for the functional and concurrent programming paradigms. D2 also solved standard library problems by separating the runtime from the standard library. The completion of a D2 Tango port was announced in February 2012.

The release of Andrei Alexandrescus book The D Programming Language on June 12, 2010, marked the stabilization of D2, which today is commonly referred to as just "D".

In January 2011, D development moved from a bugtracker / patch-submission basis to GitHub. This has led to a significant increase in contributions to the compiler, runtime and standard library.

In December 2011, Andrei Alexandrescu announced that D1, the first version of the language, would be discontinued on December 31, 2012. The final D1 release, D v1.076, was on December 31, 2012.

Code for the official D compiler, the Digital Mars D compiler by Walter Bright, was originally released under a custom license, qualifying as source available but not conforming to the open source definition. In 2014 the compiler front-end was re-licensed as open source under the Boost Software License. This re-licensed code excluded the back-end, which had been partially developed at Symantec. On April 7, 2017, the entire compiler was made available under the Boost license after Symantec gave permission to re-license the back-end, too. On June 21, 2017, the D Language was accepted for inclusion in GCC.

As of GCC 9, the D language frontend was merged into GCC.


3. Implementations

Most current D implementations compile directly into machine code for efficient execution.

  • GCC – The GNU Compiler Collection, merged GDC into GCC 9
  • D Compiler for.NET – A back-end for the D programming language 2.0 compiler. It compiles the code to Common Intermediate Language CIL bytecode rather than to machine code. The CIL can then be run via a Common Language Infrastructure CLI virtual machine. The project has not been updated in years and the author indicated the project is not active anymore.
  • SDC – The Stupid D Compiler uses a custom front-end and LLVM as its compiler back-end. It is written in D and uses a scheduler to handle symbol resolution in order to elegantly handle the compile-time features of D. This compiler currently supports a limited subset of the language.
  • DMD – The Digital Mars D compiler by Walter Bright is the official D compiler; open sourced under the Boost Software License. The DMD frontend is shared by GDC now in GCC and LDC, to improve compatibility between compilers. Initially fronted was written in C++, but now most of it is written in D language itself self-hosting. The backend and machine code optimizers are based on Symantec compiler. Initially it supported 32-bit x86, with support added for 64-bit amd64 and PowerPC by Walter Bright. Later the backend and almost entire compiler was ported from C++ to D for full self-hosting.
  • LDC – A compiler based on the DMD front-end that uses LLVM as its compiler back-end. The first release-quality version was published on 9 January 2009. It supports version 2.0.

Using above compilers and toolchains, it is possible to compile D programs to target many different architectures, including x86, amd64, AArch64, PowerPC, MIPS64, DEC Alpha, Motorola m68k, Sparc, s390, WebAssembly. The primary supported operating system are Windows and Linux, but various compiler supports also Mac OS X, FreeBSD, NetBSD, AIX, Solaris/OpenSolaris and Android, either as a host or target, or both. WebAssembly target supported via LDC and LLVM can operate in any WebAssembly environment, like modern web browser, or dedicated Wasm virtual machines.


4. Development tools

Editors and integrated development environments IDEs supporting D include Eclipse, Microsoft Visual Studio, SlickEdit, Emacs, vim, SciTE, Smultron, TextMate, MonoDevelop, Zeus, and Geany among others.

  • Eclipse plug-ins for D include: DDT and Descent dead project.
  • A plugin for Xcode 3 is available, D for Xcode, to enable D-based projects and development.
  • An AddIn for MonoDevelop is available, named Mono-D.
  • Visual Studio Code integration with extensions as Dlang-Vscode or Code-D.
  • KDevelop as well as its text editor backend, Kate autocompletion plugin is available.
  • Vim supports both syntax highlighting and code completion
  • Coedit, an open source IDE dedicated to D.
  • A bundle is available for TextMate, and the Code Blocks IDE includes partial support for the language. However, standard IDE features such as code completion or refactoring are not yet available, though they do work partially in Code Blocks due to Ds similarity to C.
  • Visual Studio integration is provided by VisualD.

Open source D IDEs for Windows exist, some written in D, such as Poseidon, D-IDE, and Entice Designer.

D applications can be debugged using any C/C++ debugger, like GDB or WinDbg, although support for various D-specific language features is extremely limited. On Windows, D programs can be debugged using Ddbg, or Microsoft debugging tools WinDBG and Visual Studio, after having converted the debug information using cv2pdb. The ZeroBUGS debugger for Linux has experimental support for the D language. Ddbg can be used with various IDEs or from the command line; ZeroBUGS has its own graphical user interface GUI.


5. Examples

Example 1

This example program prints its command line arguments. The main function is the entry point of a D program, and args is an array of strings representing the command line arguments. A string in D is an array of characters, represented by char otherwise sort refuses to sort it. There are more efficient ways to write this program using just UTF-8.


6. Uses

Notable organisations that use the D programming language for projects include Facebook, eBay, and Netflix.

D has been successfully used for AAA games, a JavaScript virtual machine, an operating system kernel, GPU programming, web development, numerical analysis, GUI applications, and a passenger information system.

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