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 in with standard D code—a technique often 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, but so far only the compiler supplied by Digital Mars implements a documentation generator.
Imperative programming in D is almost identical to C. Functions, data, statements, declarations and expressions work just as in C, and the C runtime library can be accessed directly. 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 of another and may access the enclosing function's local variables.
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++ pure abstract classes.
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 function-like style than those in C++. Here the use of static if, D's compile-time conditional construct, is demonstrated to construct a factorial template.
This is a regular function that performs the same calculation. The template version's code is similar to that of this function.
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
The std.metastrings.Format template performs printf-like data formatting, and the "msg" pragma displays the result at compile time.
Memory is usually managed with garbage collection, but specific objects can be finalized immediately when they go out of scope. Explicit memory management is possible using the overloaded operatorsnew and delete, and by simply calling C's malloc and free directly. Garbage collection can be controlled: programmers can add and exclude memory ranges from being observed by the collector, can pause and resume the collector and force a generational or a 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.
Interaction with other systems
C's application binary interface (ABI) is supported as well as all of C's fundamental and derived types, enabling direct access
to existing C code and libraries. C's standard library is part of standard D. Unless you use very explicit namespaces it can be somewhat messy to access, as it is spread throughout the D modules that use it -- but the pure D standard library is usually sufficient unless interfacing with C code.
C++'s ABI is not fully supported, although D can access C++ code that is written to the C ABI, and can access C++ COM (Component Object Model) code. The D parser understands an extern (C++) calling convention for linking to C++ objects, but it is only implemented in the currently experimental D 2.0.
D 2.0, a branch version of D that includes experimental features, was released on June 17, 2007. Some of these features are:
D differentiates between mutable references to immutable data, const references to mutable data, and combinations thereof
const and invariant keywords are transitive
Limited support for linking with code written in C++
Iteration with foreach over defined range only
Support for "real" closures. Previously closures couldn't be safely returned from functions, because stack-allocated variables would become inaccessible
Future support for "pure" functions which can only access immutable data and call other pure functions. This ensures a pure function has no side effects (the same stack inputs always result in the same outputs and outputs exist only through return values). Together with real closure support this allows Functional Programming in D and also opens theoretical paths for safe automatic threading.
Even though D is still under development, changes to the language are no longer made regularly since version 1.0 of January 2, 2007. The design is currently virtually frozen, and newer releases focus on resolving existing bugs. Version 1.0 is not completely compatible with older versions of the language and compiler. The official compiler by Walter Bright defines the language itself.
DMD: the Digital Mars D compiler, the official D compiler by Walter Bright. The compiler front end is licensed under both the Artistic License and the GNUGPL; sources for the front end are distributed along with the compiler binaries. The compiler back end is proprietary.
GDC: A front end for the GCC back end, built using the open DMD compiler sources. Development snapshots also support D version 2.0.
LLVMDC: A new compiler, also based on open DMD sources that uses LLVM as its compiler back end. It's in early development.
D is still lacking support in many IDEs, which is a potential stumbling block for some users. Editors used include Entice Designer, emacs, vim, SciTE, Smultron and Zeus among others. Vim supports both syntax highlighting and code completion (through patched ctags). A bundle is available for TextMate, and the 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).
There are at least two actively developed Eclipse plug-ins for D, Descent and Mmrnmhrm.
D applications can be debugged using any C/C++ debugger, like GDB or WinDbg, although support for various fundamental language features is extremely limited. A debugger with explicit support for D is Ddbg for Windows. The commercial 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 GUI.
Problems and controversies
D operator overloads are sometimes less powerful than the C++ counterparts. An example is the opIndex, which suffers because D does not allow returning references. This makes operations like obj[i]++; impossible. D's partial solution is the opIndexAssign operator, which only fixes cases where the indexed expression is only an L-value like obj[i] = 5 but not the original cases. In addition, the C++ way of returning a reference allows for the usage of the returned type's overloaded assignment operator. This is currently not possible in D. D 2.0 may fix this by introducing an opIndexLvalue - like operator overload, and deprecating opIndexAssign.
Division concerning the standard library
The standard library in D is called Phobos. Some members of the D community think Phobos is too simplistic and that it has numerous quirks and other issues, and a replacement of the library called Tango was written. However, Tango and Phobos are at the moment incompatible due to different run-time libraries (the garbage collector, threading support, etc). The existence of two libraries, both widely in use, could lead to significant problems where some packages use Phobos and others use Tango.
Unfinished support for shared/dynamic libraries
Unix's ELF shared libraries are supported to an extent using the GDC compiler. On Windows systems, DLLs are supported and allow D's garbage collector-allocated objects to be safely passed to C functions, since the garbage collector scans the stack for pointers. However, there are still limitations with DLLs in D including the fact that run-time type information of classes defined in the DLL is incompatible with those defined in the executable, and that any object created from within the DLL must be finalized before the DLL is unloaded.
D has no built-in support for weak references, although there are some libraries that implement them. Operations on Unicode strings are unintuitive (compiler accepts Unicode source code, standard library and foreach constructs operate on UTF-8, but string slicing and length property operate on bytes rather than characters).
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 in D 1.0, or invariant(char) in D 2.0 alpha. Newer versions of the language define string as an alias for char or invariant(char) , however, an explicit alias definition is necessary for compatibility with older versions.
The foreach statement can iterate over any collection, in this case it
is producing a sequence of indexes (i) and values (a) from the array
args. The index i and the value a have their types inferred from the type of the array args.
The following shows several capabilities of D in a very short program. It iterates the lines of a text file named 'words.txt' that contains a different word on each line, and prints all the words that are anagrams of other words.
The type of 'signature2words' is a built-in associative array that maps string keys to arrays of strings. It equals about to defaultdict(list) in Python.
BufferedFile yields lines lazily, without their newline, for performance the 'line' it yields is just a view on a string, so it has to be copied with 'dup' to have an actual string copy that can be used later.
The '~=' appends a new string to the values of the associate array.
'tolower' and 'join' are string functions that D allows to use with a method syntax, their names are often similar to Python string methods. The 'tolower' converts an ASCII string to lowercase and join(" ") joins an array of strings into a single string using a single space as separator.
The 'sort' sorts the array in place, creating an unique signature for words that are anagrams of each other.
The second foreach iterates on the values of the associative array, it's able to infer the type of 'words'.