D (programming language)
For other programming languages named D, see D (disambiguation) § Computing.
For other uses, see D (disambiguation).
|Paradigm||Multi-paradigm: functional, imperative, object-oriented|
|Designed by||Walter Bright, Andrei Alexandrescu (since 2007)|
|Developer||D Language Foundation|
|First appeared||December 8, 2001; 19 years ago (2001-12-08)|
/ November 20, 2020; 24 days ago (2020-11-20)
|Typing discipline||Inferred, static, strong|
|OS||FreeBSD, Linux, macOS, Windows|
Andrei Alexandrescu joined the design and development effort in 2007.
Though it originated as a re-engineering of C++, D is a distinct language.
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.
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 (and syntax) to achieve some goals.
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 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 (deferred) code execution, and a re-engineered template syntax.
On the other hand, D's declaration, statement and expression syntax closely matches that of 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 (i.e. using vector extensions, SIMD) that is hard to generate by the compiler automatically.
D has built-in support for documentation comments, allowing automatic documentation generation.
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.
D also includes dynamic arrays and associative arrays by default in the language.
Symbols (functions, variables, classes) can be declared in any order - forward declarations are not required.
Similarly imports can be done almost in any order, and even be scoped (i.e. import some module or part of it inside a function, class or unittest only).
D supports function overloading.
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.
Interfaces and inheritance in D support covariant types for return types of overridden methods.
Classes (and interfaces) 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 (and structs) can be introspected automatically at compile time (a form of reflection using type traits) and at run time (RTII / TypeInfo), to facilitate generic code or automatic code generation (usually using compile-time techniques).
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:
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:
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:
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:
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 (for example, if a closure is returned by another function, and exits that function's scope).
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.
Alternatively, the above function compositions can be expressed using Uniform Function Call Syntax (UFCS) for more natural left-to-right reading:
Parallel programming concepts are implemented in the library, and don't require extra support from the compiler.
However the D type system and compiler ensure that data sharing can be detected and managed transparently.
iota(11).parallel is equivalent to std.parallelism.parallel(iota(11)) by using UFCS.
The same module also supports taskPool that can be used for dynamic creation of parallel tasks, as well map-filter-reduce and fold style operations on ranges (and arrays), which is useful when combined with functional operations:
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 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.
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 (i.e. on stack with fallback, RAII style allocation, reference counting, shared reference counting).
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 (with only reference to the class instance being on the stack).
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 (of known size) are allocated on stack.
For dynamic arrays one can use core.stdc.stdlib.alloca function (similar to C function alloca, to allocate memory on stack.
The returned pointer can be used (recast) into a (typed) dynamic array, by means of a slice (however resizing array, including appending must be avoided; and for obvious reasons they must not be returned from the function).
A scope keyword can be used both to annotate parts of code, but also variables and classes/structs, to indicate they should be destroyed (destructor called) 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 composable allocator templates, to create custom high performance allocators for special use cases.
SafeD is the name given to the subset of D that can be guaranteed to be memory safe (no writes to memory that has not been allocated or that has 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.
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.
(Influenced by ideas from Rust 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.
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 Class||Behaviour (and constraints to) of a Parameter with the storage class|
|scope||references in the parameter cannot be escaped. Ignored for parameters with no references|
|return||Parameter 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(s) they were derived from. Ignored for parameters with no references|
An Annotated Example is given below.
Interaction with other systems
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 (COM) 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 (functions) 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 (with a thin D interface file).
Interaction with C++ code
D takes a permissive but realistic approach to interoperation with C++ code.
For D code marked as extern(C++), the following features are specified:
- The name mangling conventions shall match those of C++ on the target.
- For Function Calls, the ABI shall be equivalent.
- The vtable shall be matched up to single inheritance (The only level supported by the D language specification).
C++ namespaces are used via the syntax extern(C++, namespace) where namespace is the name of the C++ namespace.
An Example of C++ interoperation
The C++ side
The D side
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.
Enabled via the compiler flags "-betterC" on DMD and LDC, and "-fno-druntime" on GDC, 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.
Features available in the Better C subset
- Unrestricted use of compile-time features (for example, D's dynamic allocation features can be used at compile time to pre-allocate D data)
- Full metaprogramming facilities
- Nested functions, nested structs, delegates and lambdas
- Member functions, constructors, destructors, operating overloading, etc.
- The full module system
- Array slicing, and array bounds checking
- Memory safety protections
- Interfacing with C++
- COM classes and C++ classes
- assert failures are directed to the C runtime library
- switch with strings
- final switch
- unittest blocks
- Garbage Collection
- TypeInfo and ModuleInfo
- Built-in threading (e.g. core.thread)
- Dynamic arrays (though slices of static arrays work) and associative arrays
- synchronized and core.sync
- Static module constructors or destructors
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++.
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 (the garbage collector, threading support, etc.).
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 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.
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 (short for GNU D Compiler, or GCC D Compiler), a D language frontend based on DMD open source frontend was merged into GCC.
Production ready compilers:
- 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 the frontend was written in C++, but now most of it is now written in D itself (self-hosting). The backend and machine code optimizers are based on the Symantec compiler. At first it supported only 32-bit x86, with support added for 64-bit amd64 and PowerPC by Walter Bright. Later the backend and almost the entire compiler was ported from C++ to D for full self-hosting.
- GCC – The GNU Compiler Collection, merged GDC into GCC 9 on 2018-10-29. The first working versions of GDC with GCC, based on GCC 3.3 and GCC 3.4 on 32-bit x86 on Linux and MacOS X was released on 2004-03-22. Since then GDC was gaining support for more platforms, and improving performance and fixing bugs, while tracking upstream DMD code for the frontend and language specification.
- 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.
Toy and proof-of-concept compilers:
- 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.
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.
WebAssembly target (supported via LDC and LLVM) can operate in any WebAssembly environment, like modern web browser (Google Chrome, Mozilla Firefox, Microsoft Edge, Apple Safari), or dedicated Wasm virtual machines.
- Dexed (formely Coedit) a D focused graphical IDE written in Object Pascal
- Mono-D is a feature rich cross-platform D focused graphical IDE based on MonoDevelop / Xamarin Studio, primarily written in C#.
- Eclipse plug-ins for D include: DDT and Descent (dead project).
- Visual Studio integration is provided by VisualD.
- Visual Studio Code integration with extensions as Dlang-Vscode or Code-D.
- Vim supports both syntax highlighting and code completion
- 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 D's similarity to C).
- 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.
- KDevelop (as well as its text editor backend, Kate) autocompletion plugin is available.
Additionally many other editors and IDE support syntax highlighting and partial code / identifier completion for D.
On Windows, D programs can be debugged using , or Microsoft debugging tools (WinDBG and Visual Studio), 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 (GUI).
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.
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.
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/D (programming language).