root/trunk/docsrc/overview.dd

Ddoc

$(D_S Overview,

$(SECTION2 What is D?,

    $(P D is a general purpose systems and applications programming language.
    It is a high level language, but retains the ability
    to write high performance code and interface directly with the
    operating system
    <acronym title="Application Programming Interface">API</acronym>'s
    and with hardware.
    D is well suited to writing medium to large scale
    million line programs with teams of developers. D is easy
    to learn, provides many capabilities to aid the programmer,
    and is well suited to aggressive compiler optimization technology.

    <img src="../d3.png" border=0 align=right alt="D Man">
    )

    $(P D is not a scripting language, nor an interpreted language.
    It doesn't
    come with a <acronym title="Virtual Machine">VM</acronym>,
    a religion, or an overriding
    philosophy. It's a practical language for practical programmers
    who need to get the job done quickly, reliably, and leave behind
    maintainable, easy to understand code.
    )

    $(P D is the culmination of decades of experience implementing
    compilers for many diverse languages, and attempting to construct
    large projects using those languages. D draws inspiration from
    those other languages (most especially C++) and tempers it with
    experience and real world practicality.
    )
)


$(SECTION2 Why D?,

    $(P Why, indeed. Who needs another programming language?
    )

    $(P The software industry has come a long way since the C language was
    invented.
    Many new concepts were added to the language with C++, but backwards
    compatibility with C was maintained, including compatibility with
    nearly all the weaknesses of the original design.
    There have been many attempts to fix those weaknesses, but the
    compatibility issue frustrates it.
    Meanwhile, both C and C++ undergo a constant accretion of new
    features. These new features must be carefully fitted into the
    existing structure without requiring rewriting old code.
    The end result is very complicated - the C standard is nearly
    500 pages, and the C++ standard is about 750 pages!
    C++ is a difficult and costly language to implement,
    resulting in implementation variations that make it frustrating
    to write fully portable C++ code.
    )

    $(P C++ implements things like resizable arrays and string concatenation
    as part of the standard library, not as part of the core language.
    Not being part of the core language has several
    $(LINK2 cppstrings.html, suboptimal consequences).
    )

    $(P Can the power and capability of C++ be extracted, redesigned,
    and recast into a language that is simple, orthogonal,
    and practical?
    Can it all be put into a package
    that is easy for compiler
    writers to correctly implement, and
    which enables compilers to efficiently generate aggressively
    optimized code?
    )

    $(P Modern compiler technology has progressed to the point where language
    features for the purpose of compensating for primitive compiler
    technology can be omitted. (An 
    example of this would be the $(SINGLEQUOTE register) keyword in C, a more
    subtle example is the macro 
    preprocessor in C.)
    We can rely on modern compiler optimization technology to not
    need language features necessary to get acceptable code quality out of
    primitive compilers.
    )

$(SECTION3 Major Design Goals of D,

    $(P Everything in designing a language is a tradeoff. Keeping some
    principles in mind will help to make the right decisions.)

    $(OL
    $(LI Make it easier to write code that is portable from compiler
    to compiler, machine to machine, and operating system to operating
    system. Eliminate undefined and implementation defined behaviors
    as much as practical.)

    $(LI Provide syntactic and semantic constructs that eliminate or
    at least reduce common mistakes. Reduce or even eliminate the need
    for third party static code checkers.)

    $(LI Support memory safe programming.)

    $(LI Support multi-paradigm programming, i.e. at a minimum support
    imperative, structured, object oriented, generic and even
    functional programming paradigms.)

    $(LI Make doing things the right way easier than the wrong way.)

    $(LI Have a short learning curve for programmers comfortable with
    programming in C, C++ or Java.)

    $(LI Provide low level bare metal access as required. Provide a means
    for the advanced programmer to escape checking as necessary.)

    $(LI Make D reasonably easy to implement a compiler for.)

    $(LI Be compatible with the local C application binary interface.)

    $(LI Where D code looks the same as C code, have it either behave the
    same or issue an error.)

    $(LI Have a context-free grammar. Successful parsing must not require
    semantic analysis.)

    $(LI Easily support writing internationalized applications.)

    $(LI Incorporate Contract Programming and unit testing methodology.)

    $(LI Be able to build lightweight, standalone programs.)

    $(LI Reduce the costs of creating documentation.)

    $(LI Provide sufficient semantics to enable advances in compiler optimization
    technology.)

    $(LI Cater to the needs of numerical analysis programmers.)

    $(LI Obviously, sometimes these goals will conflict. Resolution will be
    in favor of usability.)
    )
)

$(SECTION3 Features To Keep,

    $(P The general look of D is like C and C++. This makes it easier to learn
    and port code to D. Transitioning from C/C++ to D should feel natural.
    The 
    programmer will not have to learn an entirely new way of doing things.
    )

    $(P Using D will not mean that the programmer will become restricted to a
    specialized runtime vm (virtual machine) like the Java vm or the
    Smalltalk vm.
    There is no D vm, it's a straightforward compiler that generates
    linkable object files.
    D connects to the operating system just like C does.
    The usual familiar tools like $(B make) will fit right in with
    D development.
    )

    $(UL
    $(LI The general $(B look and feel) of C/C++ is maintained.
    It uses the same algebraic syntax, most of the same expression
    and statement forms, and the general layout.
    )

    $(LI D programs can be written either in
    C style $(B function-and-data),
    C++ style $(B object-oriented),
    C++ style $(B template metaprogramming),
    or any mix of the three.
    )

    $(LI The $(B compile/link/debug) development model is
    carried forward,
    although nothing precludes D from being compiled into bytecode
    and interpreted. 
    )

    $(LI $(B Exception handling).
    More and more experience with exception handling shows it to be a 
    superior way to handle errors than the C traditional method of using
    error codes and errno globals.
    )

    $(LI $(B Runtime Type Identification).
    This is partially implemented in C++;
    in D it is taken to its 
    next logical step. Fully supporting it enables better garbage
    collection, better debugger support, more automated persistence, etc.
    )

    $(LI D maintains function link compatibility with the $(B C calling
    conventions). This makes 
    it possible for D programs to access operating system API's directly.
    Programmers' knowledge and experience with existing programming API's
    and paradigms can be carried forward to D with minimal effort.
    )

    $(LI $(B Operator overloading).
    D programs can overload operators enabling
    extension of the basic types with user defined types.
    )

    $(LI $(B Template Metaprogramming).
    Templates are a way to implement generic programming.
    Other ways include using macros or having a variant data type.
    Using macros is out. Variants are straightforward, but 
    inefficient and lack type checking.
    The difficulties with C++ templates are their
    complexity, they don't fit well into the syntax of the language,
    all the various rules for conversions and overloading fitted on top of
    it, etc. D offers a much simpler way of doing templates.
    )

    $(LI <acronym title="Resource Acquisition Is Initialization">$(B RAII)</acronym>
    (Resource Acquisition Is Initialization).
    RAII techniques are an essential component of writing reliable
    software.
    )

    $(LI $(B Down and dirty programming). D retains the ability to
    do down-and-dirty programming without resorting to referring to
    external modules compiled in a different language. Sometimes,
    it's just necessary to coerce a pointer or dip into assembly
    when doing systems work. D's goal is not to $(I prevent) down
    and dirty programming, but to minimize the need for it in
    solving routine coding tasks.
    )
    )
)

$(SECTION3 Features To Drop,

$(UL
    $(LI C source code compatibility. Extensions to C that maintain
    source compatibility 
    have already been done (C++ and ObjectiveC). Further work in this
    area is hampered by so much legacy code it is unlikely that significant
    improvements can be made.
    )

    $(LI Link compatibility with C++. The C++ runtime object model is just
    too complicated - properly supporting it would essentially imply
    making D a full C++ compiler too.
    )

    $(LI The C preprocessor. Macro processing is an easy way to extend
    a language, adding in faux features that aren't really there (invisible
    to the symbolic debugger). Conditional compilation, layered with
    #include text, macros, token concatenation, etc., essentially forms
    not one language but two merged together with no obvious distinction
    between them. Even worse (or perhaps for the best) the C preprocessor
    is a very primitive macro language. It's time to step back, look at
    what the preprocessor is used for, and design support for those
    capabilities directly into the language.
    )

    $(LI Multiple inheritance. It's a complex
    feature of debatable value. It's very difficult to implement in an
    efficient manner, and compilers are prone to many bugs in implementing
    it. Nearly all the value of
    <acronym title="multiple inheritance">MI</acronym> can be handled with
    single inheritance
    coupled with interfaces and aggregation. What's left does not
    justify the weight of MI implementation.
    )

    $(LI Namespaces. An attempt to deal with the problems resulting from
    linking together independently developed pieces of code that
    have conflicting names. The idea of modules is simpler and works
    much better.
    )

    $(LI Tag name space. This misfeature of C is where the tag names
    of structs are in a separate but parallel symbol table. C++
    attempted to merge the tag name space with the regular name space,
    while retaining backward compatibility with legacy C code. The
    result is needlessly confusing.
    )

    $(LI Forward declarations. C compilers semantically only know
    about what has lexically preceded the current state. C++ extends this
    a little, in that class members can rely on forward referenced class
    members. D takes this to its logical conclusion, forward declarations
    are no longer necessary at the module level.
    Functions can be defined in a natural
    order rather than the typical inside-out order commonly used in C
    programs to avoid writing forward declarations.
    )

    $(LI Include files. A major cause of slow compiles as each
    compilation unit
    must reparse enormous quantities of header files. Include files
    should be done as importing a symbol table.
    )

    $(LI Trigraphs and digraphs. Unicode is the future.
    )

    $(LI Non-virtual member functions. In C++, a class designer decides
    in advance if a function is to be virtual or not. Forgetting to retrofit
    the base class member function to be virtual when the function gets
    overridden is a common (and very hard to find) coding error.
    Making all member functions virtual, and letting the compiler decide
    if there are no overrides and hence can be converted to non-virtual,
    is much more reliable.
    )

    $(LI Bit fields of arbitrary size.
    Bit fields are a complex, inefficient feature rarely used.
    )

    $(LI Support for 16 bit computers.
    No consideration is given in D for mixed near/far pointers and all the
    machinations necessary to generate good 16 bit code. The D language
    design assumes at least a 32 bit flat memory space. D will fit smoothly
    into 64 bit architectures.
    )

    $(LI Mutual dependence of compiler passes. In C++, successfully parsing
    the source text relies on having a symbol table, and on the various
    preprocessor commands. This makes it
    impossible to preparse C++ source, and makes writing code analyzers
    and syntax directed editors painfully difficult to do correctly.
    )

    $(LI Compiler complexity. Reducing the complexity of an implementation
    makes it more likely that multiple, $(I correct) implementations
    are available.
    )

    $(LI Dumbed down floating point. If one is using hardware that
    implements modern floating point, it should be available to the
    programmer rather than having floating point support dumbed down
    to the lowest common denominator among machines. In particular,
    a D implementation must support IEEE 754 arithmetic and if
    extended precision is available it must be supported.)

    $(LI Template overloading of &lt; and &gt; symbols.
    This choice has caused years of bugs, grief, and confusion
    for programmers, C++ implementors, and C++ source parsing tool
    vendors. It makes it
    impossible to parse C++ code correctly without doing a nearly complete
    C++ compiler. D uses !( and ) which fit neatly and
    unambiguously into the grammar.
    )
)
)

$(SECTION3 Who D is For,

$(UL
    $(LI Programmers who routinely use lint or similar code analysis tools
    to eliminate bugs before the code is even compiled.
    )

    $(LI People who compile with maximum warning levels turned on and who 
    instruct the compiler to treat warnings as errors.
    )

    $(LI Programming managers who are forced to rely on programming style 
    guidelines to avoid common C bugs.
    )

    $(LI Those who decide the promise of C++ object oriented
    programming is not fulfilled due to the complexity of it.
    )

    $(LI Programmers who enjoy the expressive power of C++ but are
    frustrated by
    the need to expend much effort explicitly managing memory and finding
    pointer bugs.
    )

    $(LI Projects that need built-in testing and verification.
    )

    $(LI Teams who write apps with a million lines of code in it.
    )

    $(LI Programmers who think the language should provide enough
    features to obviate 
    the continual necessity to manipulate pointers directly.
    )

    $(LI Numerical programmers. D has many features to directly
    support features needed by numerics programmers, like
    extended floating point precision,
    core support for complex and imaginary floating types
    and defined behavior for
    <acronym title="Not A Number">NaN</acronym>'s and infinities.
    (These are added in the new
    C99 standard, but not in C++.)
    )

    $(LI Programmers who write half their application in scripting
    langauges like Ruby and Python, and the other half in C++ to
    speed up the bottlenecks. D has many of the productivity features
    of Ruby and Python, making it possible to write the entire app
    in one language.)

    $(LI D's lexical analyzer and parser are totally independent of each other and of the 
    semantic analyzer. This means it is easy to write simple tools to manipulate D source 
    perfectly without having to build a full compiler. It also means that source code can be 
    transmitted in tokenized form for specialized applications.
    )
)
)

$(SECTION3 Who D is Not For,

    $(UL
    $(LI Realistically, nobody is going to convert million line C or C++
    programs into D.
    Since D does not compile unmodified C/C++
    source code, D is not for 
    legacy apps.
    (However, D supports legacy C API's very well. D can connect
    directly to any code that exposes a C interface.)
    )

    $(LI As a first programming language - Basic or Java is more suitable
    for beginners. D makes an excellent second language for intermediate
    to advanced programmers.
    )

    $(LI Language purists. D is a practical language, and each feature
    of it is evaluated in that light, rather than by an ideal.
    For example, D has constructs and semantics that virtually eliminate
    the need for pointers for ordinary tasks. But pointers are still
    there, because sometimes the rules need to be broken.
    Similarly, casts are still there for those times when the typing
    system needs to be overridden.
    )
    )
)

)



$(SECTION2 Major Features of D,

    $(P This section lists some of the more interesting features of D
    in various categories.
    )

$(SECTION3 Object Oriented Programming,

    $(SECTION4 Classes,

    $(P D's object oriented nature comes from classes.
    The inheritance model is single inheritance enhanced
    with interfaces. The class Object sits at the root
    of the inheritance hierarchy, so all classes implement
    a common set of functionality.
    Classes are instantiated
    by reference, and so complex code to clean up after exceptions
    is not required.
    )
    )

    $(SECTION4 Operator Overloading,

    $(P Classes can be crafted that work with existing operators to extend
    the type system to support new types. An example would be creating
    a bignumber class and then overloading the +, -, * and / operators
    to enable using ordinary algebraic syntax with them.
    )
    )
)

$(SECTION3 Functional Programming,

    $(P Functional programming has a lot to offer in terms of encapsulation,
    concurrent programming, memory safety, and composition. D's support for
    functional style programming include:
    )

    $(UL
    $(LI Pure functions)
    $(LI Immutable types and data structures)
    $(LI Lambda functions and closures)
    )

)

$(SECTION3 Productivity,

    $(SECTION4 Modules,

    $(P Source files have a one-to-one correspondence with modules.
    Instead of #include'ing the text of a file of declarations,
    just import the module. There is no need to worry about
    multiple imports of the same module, no need to wrapper header
    files with $(TT #ifndef/#endif) or $(TT #pragma once) kludges,
    etc.
    )
    )

    $(SECTION4 Declaration vs Definition,

    $(P C++ usually requires that functions and classes be declared twice - the declaration
    that goes in the .h header file, and the definition that goes in the .c source
    file. This is an error prone and tedious process. Obviously, the programmer 
    should only need to write it once, and the compiler should then extract the 
    declaration information and make it available for symbolic importing. This is 
    exactly how D works.
    )

    $(P Example:
    )

-----------------------
class ABC
{
    int func() { return 7; }
    static int z = 7;
}
int q;
-----------------------

    $(P There is no longer a need for a separate definition of member functions, static
    members, externs, nor for clumsy syntaxes like:
    )

$(CCODE
int ABC::func() { return 7; }
int ABC::z = 7;
extern int q;
)

    $(P Note: Of course, in C++, trivial functions like $(TT { return 7; })
    are written inline too, but complex ones are not. In addition, if
    there are any forward references, the functions need to be prototyped.
    The following will not work in C++:
    )

$(CCODE
class Foo
{
    int foo(Bar *c) { return c->bar(); }
};

class Bar
{
  public:
    int bar() { return 3; }
};
)

    $(P But the equivalent D code will work:
    )

-----------------------
class Foo
{
    int foo(Bar c) { return c.bar; }
}

class Bar
{
    int bar() { return 3; }
}
-----------------------

    $(P Whether a D function is inlined or not is determined by the
    optimizer settings.
    )
    )

    $(SECTION4 Templates,

    $(P D templates offer a clean way to support generic programming while
    offering the power of partial specialization.
    Template classes and template functions are available, along
    with variadic template arguments and tuples.
    )
    )

    $(SECTION4 Associative Arrays,

    $(P Associative arrays are arrays with an arbitrary data type as
    the index rather than being limited to an integer index.
    In essence, associated arrays are hash tables. Associative
    arrays make it easy to build fast, efficient, bug-free symbol
    tables.
    )
    )

$(V1
    $(SECTION4 Real Typedefs,

    $(P C and C++ typedefs are really type $(I aliases), as no new
    type is really introduced. D implements real typedefs, where:
    )

-----------------------
typedef int handle;
-----------------------

    $(P really does create a new type $(B handle). Type checking is
    enforced, and typedefs participate in function overloading.
    For example:
    )

-----------------------
int foo(int i);
int foo(handle h);
-----------------------
    )
)

    $(SECTION4 Documentation,

    $(P Documentation has traditionally been done twice - first there
    are comments documenting what a function does, and then this gets
    rewritten into a separate html or man page.
    And naturally, over time, they'll tend to diverge as the code
    gets updated and the separate documentation doesn't.
    Being able to generate the requisite polished documentation directly
    from the comments embedded in the source will not only cut the time
    in half needed to prepare documentation, it will make it much easier
    to keep the documentation in sync with the code.
    $(LINK2 ddoc.html, Ddoc) is the specification for the D
    documentation generator. This page was generated by Ddoc, too.
    )

    $(P Although third party tools exist to do this for C++, they have some
    serious shortcomings:

    $(UL

    $(LI It is spectacularly difficult to parse C++ 100% correctly. To
    do so really requires a full C++ compiler. Third party tools tend to
    parse only a subset of C++ correctly, so their use will constrain
    the source code to that subset.)

    $(LI Different compilers support different versions of C++ and have
    different extensions to C++. Third party tools have a problem matching
    all these variations.)

    $(LI Third party tools may not be available for all the desired
    platforms, and they're necessarily on a different upgrade cycle
    from the compilers.)

    $(LI Having it builtin to the compiler means it is standardized across
    all D implementations. Having a default one ready to go at all times
    means it is far more likely to be used.)

    )
    )
    )
)

$(SECTION3 Functions,

    $(P D has the expected support for ordinary functions including
    global functions, overloaded functions, inlining of functions,
    member functions, virtual functions, function pointers, etc.
    In addition:
    )

    $(SECTION4 Nested Functions,

    $(P Functions can be nested within other functions.
    This is highly useful for code factoring, locality, and
    function closure techniques.
    )
    )

    $(SECTION4 Function Literals,

    $(P Anonymous functions can be embedded directly into an expression.
    )
    )

    $(SECTION4 Dynamic Closures,

    $(P Nested functions and class member functions can be referenced
    with closures (also called delegates), making generic programming
    much easier and type safe.
    )
    )

    $(SECTION4 In Out and Inout Parameters,

    $(P Not only does specifying this help make functions more
    self-documenting, it eliminates much of the necessity for pointers
    without sacrificing anything, and it opens up possibilities
    for more compiler help in finding coding problems.
    )

    $(P Such makes it possible for D to directly interface to a
    wider variety of foreign API's. There would be no need for
    workarounds like "Interface Definition Languages".
    )
    )
)

$(SECTION3 Arrays,

    $(P C arrays have several faults that can be corrected:
    )

    $(UL

    $(LI Dimension information is not carried around with
    the array, and so has to be stored and passed separately.
    The classic example of this are the argc and argv
    parameters to $(TT main(int $(D_PARAM argc), char *$(D_PARAM argv)[])).
    (In D, main is declared as $(TT main(char[][] $(D_PARAM args))).)
    )

    $(LI Arrays are not first class objects. When an array  is passed to a function, it is
    converted to a pointer, even though the prototype confusingly says it's an 
    array. When this conversion happens, all array type information
    gets lost.
    )

    $(LI C arrays cannot be resized. This means that even simple aggregates like a stack 
    need to be constructed as a complex class.)

    $(LI C arrays cannot be bounds checked, because they don't know
    what the array bounds are.)

    $(LI Arrays are declared with the [] after the identifier. This leads to
    very clumsy 
    syntax to declare things like a pointer to an array:

$(CCODE
int (*array)[3];
)

    $(P In D, the [] for the array go on the left:
    )

-----------------------
int[3]* array;      // declares a pointer to an array of 3 ints
long[] func(int x); // declares a function returning an array of longs
-----------------------

    $(P which is much simpler to understand.
    )
    )
    )

    $(P D arrays come in several varieties: pointers, static arrays, dynamic
    arrays, and associative arrays.
    )

    $(P See $(LINK2 arrays.html, Arrays).
    )

    $(SECTION4 Strings,

    $(P String manipulation is so common, and so clumsy in C and C++, that
    it needs direct support in the language. Modern languages handle
    string concatenation, copying, etc., and so does D. Strings are
    a direct consequence of improved array handling.
    )
    )
)

$(SECTION3 Resource Management,

    $(SECTION4 Automatic Memory Management,

    $(P D memory allocation is fully garbage collected. Empirical experience
    suggests that a lot of the complicated features of C++ are necessary
    in order to manage memory deallocation. With garbage collection, the
    language gets much simpler.
    )

    $(P There's a perception that garbage collection is for lazy, junior
    programmers. I remember when that was said about C++, after all,
    there's nothing in C++ that cannot be done in C, or in assembler
    for that matter.
    )

    $(P Garbage collection eliminates the tedious, error prone memory
    allocation
    tracking code necessary in C and C++. This not only means much
    faster development time and lower maintenance costs,
    but the resulting program frequently runs
    faster!
    )

    $(P Sure, garbage collectors can be used with C++, and I've used them
    in my own C++ projects. The language isn't friendly to collectors,
    however, impeding the effectiveness of it. Much of the runtime
    library code can't be used with
    collectors.
    )

    $(P For a fuller discussion of this, see
    $(LINK2 garbage.html, garbage collection).
    )
    )

    $(SECTION4 Explicit Memory Management,

    $(P Despite D being a garbage collected language, the new and delete
    operations can be overridden for particular classes so that
    a custom allocator can be used.
    )
    )

    $(SECTION4 RAII,

    $(P RAII is a modern software development technique to manage resource
    allocation and deallocation. D supports RAII in a controlled,
    predictable manner that is independent of the garbage collection
    cycle.
    )
    )
)


$(SECTION3 Performance,

    $(SECTION4 Lightweight Aggregates,

    $(P D supports simple C style structs, both for compatibility with
    C data structures and because they're useful when the full power
    of classes is overkill.
    )
    )

    $(SECTION4 Inline Assembler,

    $(P Device drivers, high performance system applications, embedded systems,
    and specialized code sometimes need to dip into assembly language
    to get the job done. While D implementations are not required
    to implement the inline assembler, it is defined and part of the
    language. Most assembly code needs can be handled with it,
    obviating the need for separate assemblers or DLL's.
    )

    $(P Many D implementations will also support intrinsic functions
    analogously to C's support of intrinsics for I/O port manipulation,
    direct access to special floating point operations, etc.
    )
    )
)


$(SECTION3 Reliability,

    $(P A modern language should do all it can to help the programmer flush
    out bugs in the code. Help can come in many forms;
    from making it easy to use more robust techniques, 
    to compiler flagging of obviously incorrect code, to runtime checking.
    )

    $(SECTION4 Contracts,

    $(P Contract Programming (invented by B. Meyer) is a revolutionary
    technique
    to aid in ensuring the correctness of programs. D's version of
    DBC includes function preconditions, function postconditions, class
    invariants, and assert contracts.
    See $(LINK2 dbc.html, Contracts) for D's implementation.
    )
    )

    $(SECTION4 Unit Tests,

    $(P Unit tests can be added to a class, such that they are automatically
    run upon program startup. This aids in verifying, in every build,
    that class implementations weren't inadvertently broken. The unit
    tests form part of the source code for a class. Creating them
    becomes a natural part of the class development process, as opposed
    to throwing the finished code over the wall to the testing group.
    )

    $(P Unit tests can be done in other languages, but the result is kludgy
    and the languages just aren't accommodating of the concept.
    Unit testing is a main feature of D. For library functions it works
    out great, serving both to guarantee that the functions
    actually work and to illustrate how to use the functions.
    )

    $(P Consider the many C++ library and application code bases out there for
    download on the web. How much of it comes with *any* verification
    tests at all, let alone unit testing? Less than 1%? The usual practice
    is if it compiles, we assume it works. And we wonder if the warnings
    the compiler spits out in the process are real bugs or just nattering
    about nits.
    )

    $(P Along with Contract Programming, unit testing makes D far and away
    the best language for writing reliable, robust systems applications.
    Unit testing also gives us a quick-and-dirty estimate of the quality
    of some unknown piece of D code dropped in our laps - if it has no
    unit tests and no contracts, it's unacceptable.
    )
    )


    $(SECTION4 Debug Attributes and Statements,

    $(P Now debug is part of the syntax of the language.
    The code can be enabled or disabled at compile time, without the
    use of macros or preprocessing commands. The debug syntax enables
    a consistent, portable, and understandable recognition that real
    source code needs to be able to generate both debug compilations and
    release compilations.
    )
    )

    $(SECTION4 Exception Handling,

    $(P The superior $(I try-catch-finally) model is used rather than just
    try-catch. There's no need to create dummy objects just to have
    the destructor implement the $(I finally) semantics.
    )
    )

    $(SECTION4 Synchronization,

    $(P Multithreaded programming is becoming more and more mainstream,
    and D provides primitives to build multithreaded programs with.
    Synchronization can be done at either the method or the object level.
    )

-----------------------
synchronized int func() { ... }
-----------------------

    $(P Synchronized functions allow only one thread at a time to be
    executing that function.
    )

    $(P The synchronize statement puts a mutex around a block of statements,
    controlling access either by object or globally.
    )
    )

    $(SECTION4 Support for Robust Techniques,

    $(UL
    $(LI Dynamic arrays instead of pointers)

    $(LI Reference variables instead of pointers)

    $(LI Reference objects instead of pointers)

    $(LI Garbage collection instead of explicit memory management)

    $(LI Built-in primitives for thread synchronization)

    $(LI No macros to inadvertently slam code)

    $(LI Inline functions instead of macros)

    $(LI Vastly reduced need for pointers)

    $(LI Integral type sizes are explicit)

    $(LI No more uncertainty about the signed-ness of chars)

    $(LI No need to duplicate declarations in source and header files.)

    $(LI Explicit parsing support for adding in debug code.)
    )
    )

    $(SECTION4 Compile Time Checks,

    $(UL
    $(LI Stronger type checking)

    $(LI No empty ; for loop bodies)

    $(LI Assignments do not yield boolean results)

    $(LI Deprecating of obsolete API's)
    )
    )

    $(SECTION4 Runtime Checking,

    $(UL
    $(LI assert() expressions)

    $(LI array bounds checking)

    $(LI undefined case in switch exception)

    $(LI out of memory exception)

    $(LI In, out, and class invariant Contract Programming support)
    )
    )
)

$(SECTION3 Compatibility,

    $(SECTION4 Operator precedence and evaluation rules,

    $(P D retains C operators and their precedence rules, order of
    evaluation rules, and promotion rules. This avoids subtle
    bugs that might arise from being so used to the way C
    does things that one has a great deal of trouble finding
    bugs due to different semantics.
    )
    )

    $(SECTION4 Direct Access to C API's,

    $(P Not only does D have data types that correspond to C types,
    it provides direct access to C functions. There is no need
    to write wrapper functions, parameter swizzlers, nor code to copy
    aggregate members one by one.
    )
    )

    $(SECTION4 Support for all C data types,

    $(P Making it possible to interface to any C API or existing C
    library code. This support includes structs, unions, enums,
    pointers, and all C99 types.
    D includes the capability to
    set the alignment of struct members to ensure compatibility with
    externally imposed data formats.
    )
    )

    $(SECTION4 OS Exception Handling,

    $(P D's exception handling mechanism will connect to the way
    the underlying operating system handles exceptions in
    an application.
    )
    )

    $(SECTION4 Uses Existing Tools,

    $(P D produces code in standard object file format, enabling the use
    of standard assemblers, linkers, debuggers, profilers, exe compressors,
    and other analyzers, as well as linking to code written in other
    languages.
    )
    )
)

$(SECTION3 Project Management,

    $(SECTION4 Versioning,

    $(P D provides built-in support for generation of multiple versions
    of a program from the same text. It replaces the C preprocessor
    #if/#endif technique.
    )
    )

    $(SECTION4 Deprecation,

    $(P As code evolves over time, some old library code gets replaced
    with newer, better versions. The old versions must be available
    to support legacy code, but they can be marked as $(I deprecated).
    Code that uses deprecated versions will be normally flagged
    as illegal, but would be allowed by a compiler switch.
    This will make it easy for maintenance
    programmers to identify any dependence on deprecated features.
    )
    )
)
)


$(SECTION2 Sample D Program (sieve.d),

--------------------------
/* Sieve of Eratosthenes prime numbers */

import std.stdio;

bool[8191] flags;
 
int main()
{   int i, count, prime, k, iter;

    writefln("10 iterations");
    for (iter = 1; iter <= 10; iter++)
    {   count = 0;
    flags[] = 1;
    for (i = 0; i < flags.length; i++)
    {   if (flags[i])
        {   prime = i + i + 3;
        k = i + prime;
        while (k < flags.length)
        {
            flags[k] = 0;
            k += prime;
        }
        count += 1;
        }
    }
    }
    writefln("%d primes", count);
    return 0;
}
--------------------------
)

)

Macros:
    TITLE=Overview
    WIKI=Overview