D is being designed with lessons learned from practical C++ usage rather than from a theoretical perspective. Even though it uses many C/C++ concepts it also discards some, and as such is not strictly backward compatible with C/C++ source code. It adds to the functionality of C++ by also implementing design by contract, unit testing, true modules, automatic memory management (garbage collection), first class arrays, associative arrays, dynamic arrays, array slicing, nested functions, inner classes, limited form of closures, anonymous functions, compile time function execution, lazy evaluation and has a reengineered template syntax. D retains C++'s ability to do low-level coding, and adds to it with support for an integrated inline assembler. C++ multiple inheritance is replaced by Java style single inheritance with interfaces and mixins. 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 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.
D supports three main programming paradigms—imperative, object-oriented, and metaprogramming.
Imperative programming is almost identical to C. Functions, data, statements, declarations and expressions work just as C, and the C runtime library can be accessed directly.
OO programming in D is based on a single inheritance hierarchy, with all classes derived from class Object. Multiple inheritance is possible from interfaces (interfaces are a lot like C++ abstract classes).
Metaprogramming is supported by a combination of templates, compile time function execution, tuples, and string mixins.
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 operators new and delete, and by simply calling C's malloc and free directly. Garbage collection can be disabled for individual objects, or even for a full program, if more control over memory management is desired. The manual gives many examples of how to implement different highly optimized memory management schemes for when garbage collection is inadequate in a program.
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 include support for enforcing const-correctness
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