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 is commonly as fast as equivalent C++ code, while also being shorter. The language as a whole is 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.

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, or uses different approaches to achieve some goals. As such it is not source compatible with C and C++ source code in general. 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, compile time function execution, ranges, built-in container iteration concepts and type inference. 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, scoped code execution, 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, D's 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, as well as writing high performance code that is hard to generate by compiler automatically.
D has built-in support for documentation comments, allowing automatic documentation generation.

Programming paradigms

D supports five main programming paradigms: imperative, object-oriented, metaprogramming, functional and concurrent.

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 D's 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 function's local variables.

import std.stdio;
void main

D also includes dynamic arrays and associative arrays by default in the language.
Symbols can be declared in any order - forward declarations are not required. Similarly imports can be done almost in any order, and even be scoped.
D supports function overloading.

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 methods in interfaces.
Interfaces and inheritance in D support covariant types for return types of overridden methods.
D supports operator overloading, type forwarding, as well optional custom dynamic dispatch.
Classes in D can contain invariants which are automatically checked before and after entry to public methods. It is part of the design by contract methodology.
Many aspects of classes can be introspected automatically at compile time and at run time, to facilitate generic code or automatic code generation.

Metaprogramming

Metaprogramming is supported by a combination of templates, compile time function execution, tuples, and string mixins. The following examples demonstrate some of D's 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:

ulong factorial

Here, the use of static if, D's 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:

template Factorial

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:

enum fact_7 = Factorial!;

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

enum fact_9 = factorial;

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

import std.string : format;
pragma;
pragma;

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:

import FooToD; // hypothetical module which contains a function that parses Foo source code
// and returns equivalent D code
void main

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:

int function g;
g = ; // longhand
g = => x * x; // shorthand

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 function's 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.

import std.stdio, std.algorithm, std.range;
void main

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

auto result = a1.chain.reduce!mySum;
writeln;
result = a1.chain.reduce! => ;
writeln;

Parallel

Parallel programming concepts are implemented in the library, and doesn't require extra support from the compiler. However the D type system and compiler ensure that data sharing can be detected and managed transparently.

import std.stdio : writeln;
import std.range : iota;
import std.parallelism : parallel;
void main

iota.parallel is equivalent to std.parallelism.parallel by using UFCS.
The same module also supports taskPool that can be used to dynamic creation of parallel tasks, as well map-filter-reduce and fold style operations on ranges, which is useful when combined with functional operations:

import std.stdio : writeln;
import std.algorithm : map;
import std.range : iota;
import std.parallelism : taskPool;
void main

This code uses fact that the std.algorithm.map doesn't actually return an array, but a lazily evaluate range, this way the actual elements of the map are computed by each worker task in parallel automatically.

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.
import std.stdio, std.concurrency, std.variant;
void foo
void main

Memory management

Memory is usually managed with garbage collection, but specific objects may be finalized immediately when they go out of scope. This is what majority of programs and libraries written in D use.
In case more control about memory layout and better performance is needed, explicit memory management is possible using the overloaded operators new and delete, by calling C's malloc and free directly, or implementing custom allocator schemes. 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.
In functions, structs are by default allocated on the stack, while classes by default allocated on the heap. However this can be changed for classes, for example using standard library template std.typecons.scoped, or by using new for structs and assigning to pointer instead to value-based variable.
In function, static arrays are allocated on stack. For dynamic arrays one can use core.stdc.stdlib.alloca function into a dynamic array, by means of a slice.
A scope keyword can be used both to annotate parts of code, but also variables and classes/structs, to indicate they should be destroyed immediately on scope exit. Whatever the memory is deallocated also depends on implementation and class-vs-struct differences.
std.experimental.allocator contains a modular and compossible allocator templates, to create custom high performance allocators for special use cases.

SafeD

SafeD
is the name given to the subset of D that can be guaranteed to be memory safe. 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.
Scope Lifetime Safety
Initially under the banners of DIP1000 and DIP25, 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..
Lifetime Safety of Assignments
Within @safe code, the lifetime of an assignment involving a reference type is checked to ensure that the lifetime of the assignee is longer than that of the assigned.
For example:

@safe void test
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:
Storage ClassBehaviour of a Parameter with the storage class
scopereferences in the parameter cannot be escaped. Ignored for parameters with no references
returnParameter may be returned or copied to the first parameter, but otherwise does not escape from the function. Such copies are required not to outlive the argument they were derived from. Ignored for parameters with no references

An Annotated Example is given below.
@safe:
int* gp;
void thorin;
void gloin;
int* balin

Interaction with other systems

's application binary interface is supported, as well as all of C's fundamental and derived types, enabling direct access to existing C code and libraries. D bindings are available for many popular C libraries. Additionally, C's standard library is part of standard D.
On Microsoft Windows, D can access Component Object Model code.
As long as memory management is properly taken care of, many other languages can be mixed with D in a single binary. For example GDC compiler allow to link C, C++, and other supported language codes to be intermixed. D code can also be marked as using C, C++, Pascal ABIs, and thus be passed to the libraries written in these languages as callbacks. Similarly data can be interchanged between the codes written in these languages in both ways. This usually restricts use to primitive types, pointers, some forms of arrays, unions, structs, and only some types of function pointers.
Because many other programming languages often provide the C API for writing extensions or running the interpreter of the languages, D can interface directly with these languages as well, using standard C bindings. For example, there are bi-directional bindings for languages like Python, Lua and other languages, often using compile-time code generation and compile-time type reflection methods.

Interaction with C++ code

D takes a permissive but realistic approach to interoperation with C++ code.
For D code marked as extern, the following features are specified:
C++ namespaces are used via the syntax extern where namespace is the name of the C++ namespace.
An Example of C++ interoperation
The C++ side

  1. include
using namespace std;
class Base
class Derived : public Base
int Derived::mul
Derived *createInstance
void deleteInstance
The D side

extern
void main

Better C

The D programming language has an official subset known as "". This subset forbids access to D features requiring use of runtime libraries other than that of C.
Enabled via the compiler flags "-betterC" on DMD and LDC, and "-fno-druntime" on GDC, may only call into D code compiled under the same flag but code compiled without the 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.

Features available in the Better C subset

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 concentrated on the imperative, object oriented and metaprogramming paradigms, similar to C++.
Some members of the D community dissatisfied with Phobos, D's official runtime and standard library, created an alternative runtime and standard library named Tango. The first public Tango announcement came within days of D 1.0's 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 D2's development signaled D1's 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 Alexandrescu's 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, GDC, a D language frontend based on DMD open source frontend was merged into GCC.

Implementations

Most current D implementations compile directly into machine code for efficient execution.
Production ready compilers:
Toy and proof-of-concept compilers:
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 can operate in any WebAssembly environment, like modern web browser, or dedicated Wasm virtual machines.

Development tools

Editors and integrated development environments supporting D include Eclipse, Microsoft Visual Studio, SlickEdit, Emacs, vim, SciTE, Smultron, TextMate, MonoDevelop, Zeus, and Geany among others.
Additionally many other editors and IDE support syntax highlighting and partial code / identifier completion for D.
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 , or Microsoft debugging tools, after having converted the debug information using . The 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.
A DustMite is a powerful tool for minimize D source code, useful when finding compiler or tests issues.
dub is a popular package and build manager for D applications and libraries, and is often integrated into IDE support.

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 immutable.

import std.stdio: writefln;
void main

The foreach statement can iterate over any collection. In this case, it is producing a sequence of indexes and values from the array args. The index i and the value arg have their types inferred from the type of the array args.

Example 2

The following shows several D capabilities and D design trade-offs in a short program. It iterates over the lines of a text file named words.txt, which contains a different word on each line, and prints all the words that are anagrams of other words.

import std.stdio, std.algorithm, std.range, std.string;
void main
  1. signature2words is a built-in associative array that maps dstring keys to arrays of dstrings. It is similar to defaultdict in Python.
  2. lines) yields lines lazily, with the newline. It has to then be copied with idup to obtain a string to be used for the associative array values. Built-in associative arrays require immutable keys.
  3. The ~= operator appends a new dstring to the values of the associate dynamic array.
  4. toLower, join and chomp are string functions that D allows the use of with a method syntax. The name of such functions are often similar to Python string methods. The toLower converts a string to lower case, join joins an array of strings into a single string using a single space as separator, and chomp removes a newline from the end of the string if one is present. The w.dup.sort.release.idup is more readable, but equivalent to release.idup for example. This feature is called UFCS, and allows extending any built-in or third party package types with method-like functionality. The style of writing code like this is often referenced as pipeline or Fluent interface.
  5. The sort is an std.algorithm function that sorts the array in place, creating a unique signature for words that are anagrams of each other. The release method on the return value of sort is handy to keep the code as a single expression.
  6. The second foreach iterates on the values of the associative array, it's able to infer the type of words.
  7. signature is assigned to an immutable variable, its type is inferred.
  8. UTF-32 dchar is used instead of normal UTF-8 char otherwise sort refuses to sort it. There are more efficient ways to write this program using just UTF-8.

    Uses

Notable organisations that use the D programming language for projects include Facebook, eBay, and Netflix.
D has been successfully used for AAA games,, language interpreters, virtual machines, an operating system kernel, GPU programming, web development,, numerical analysis, GUI applications,, a passenger information system, machine learning, text processing, web and application servers and research.