root/trunk/docsrc/faq.dd

Ddoc

$(D_S $(TITLE),

    $(P The same questions keep cropping up, so the obvious thing to do is
    prepare a FAQ.)

    $(P $(LINK2 http://www.digitalmars.com/d/1.0/faq.html, D 1.0 FAQ))
    $(P $(LINK2 http://www.digitalmars.com/faq.html, C++ FAQ))

<h2>D 2.0 FAQ</h2>

    $(UL
    $(LI $(LINK2 features2.html, What's the difference between D 2.0 and D 1.0?))

    $(LI Where is my simple language?)

    $(LI $(LINK2 cpp0x.html, What's D 2.0's answer to C++0x's proposed new features?))

    $(LI What other cool features are in the plans for D 2.0?)

    $(LI I suggested a great feature. Why was the suggestion ignored?)

    $(LI Why const and immutable?)

    $(LI Why the $(I names) const and immutable?)

    $(LI How exactly is immutable related to multicores?)

    $(LI Ok, I'm fine with immutable for safe data sharing among threads. But why do we need the uninformative const?)

    $(LI Why are immutable strings favored in D 2.0?)

    $(LI I want to contribute to D 2.0. How can I effect that?)

    $(ITEMR case_range, Why doesn't the case range statement
     use the $(TT case X..Y:) syntax?)

    $(ITEMR shared_guarantees, What guarantees is shared supposed to provide?)

    $(ITEMR shared_synchronized, What does shared have to do with synchronization?)

    $(ITEMR shared_memory_barriers, What does shared have to do with memory barriers?)

    $(ITEMR casting_to_shared, What are the semantics of casting FROM unshared TO shared?)

    $(ITEMR casting_from_shared, What are the semantics of casting FROM shared TO unshared?)

    )

<h2>General D FAQ</h2>

    $(UL

    $(LI $(LINK2 http://www.wikiservice.at/wiki4d/wiki.cgi?FaqRoadmap, The D wiki FAQ page)
    with many more questions answered)
    $(LI $(LINK2 comparison.html, What does D have that C++ doesn't?))
    $(ITEMR q1, Why the name D?)
    $(LI $(LINK2 http://www.digitalmars.com/d/download.html, Where can I get a D compiler?))
    $(ITEMR q1_2, Is there linux port of D?)
    $(ITEMR gdc, Is there a GNU version of D?)
    $(ITEMR backend, How do I write my own D compiler for CPU X?)
    $(ITEMR gui, Where can I get a GUI library for D?)
    $(ITEMR ide, Where can I get an IDE for D?)
    $(ITEMR q2, What about templates?)
    $(ITEMR q3, Why emphasize implementation ease?)
    $(ITEMR q4, Why is [expletive deleted] printf left in?)
    $(ITEMR q5, Will D be open source?)
    $(ITEMR q6, Why fall through on switch statements?)
    $(ITEMR q7, Why should I use D instead of Java?)
    $(ITEMR q7_2, Doesn't C++ support strings, etc. with STL?)
    $(ITEMR q7_3, Can't garbage collection be done in C++ with an add-on library?)
    $(ITEMR q7_4, Can't unit testing be done in C++ with an add-on library?)
    $(ITEMR q8, Why have an asm statement in a portable language?)
    $(ITEMR real, What is the point of 80 bit reals?)
    $(ITEMR anonymous, How do I do anonymous struct/unions in D?)
    $(ITEMR printf, How do I get printf() to work with strings?)
    $(ITEMR nan, Why are floating point values default initialized to NaN rather than 0?)
    $(ITEMR assignmentoverloading, Why is overloading of the assignment operator not supported?)
    $(ITEMR keys, The $(SINGLEQUOTE ~) is not on my keyboard?)
    $(ITEMR omf, Can I link in C object files created with another compiler?)
    $(ITEMR regexp_literals, Why not support regular expression literals
        with the /foo/g syntax?)
    $(ITEMR dogfood, Why is the D front end written in C++ rather than D?)
    $(ITEMR cpp_to_D, Why aren't all Digital Mars programs translated to D?)
    $(ITEMR foreach, When should I use a foreach loop rather than a for?)
    $(ITEMR cpp_interface, Why doesn't D have an interface to C++ as well as C?)
    $(ITEMR reference-counting, Why doesn't D use reference counting for garbage collection?)
    $(ITEMR gc_1, Isn't garbage collection slow and non-deterministic?)
    $(ITEMR pure, Can't a sufficiently smart compiler figure out that a function is pure automatically?)
    )

$(ITEM case_range, Why doesn't the case range statement
     use the $(TT case X..Y:) syntax?)

    $(P See the $(LINK2 statement.html#CaseRangeStatement, case range statement).)

    $(P The usages of .. would then be:
    $(OL
    $(LI $(TT case X..Y:))
    $(LI $(TT foreach(e; X..Y)))
    $(LI $(TT array[X..Y]))
    )
    Case (1) has a VERY DIFFERENT meaning from (2) and (3).
    (1) is inclusive of Y, and (2) and (3) are exclusive of Y.
    Having a very different meaning means it should have a distinctly different syntax. 
    )

$(ITEM shared_guarantees, What guarantees is shared supposed to provide?)

    $(P Shared means that multiple threads can access the data. The guarantee is that if it is
    not shared, and not immutable, that only the current thread can see it.
    )

$(ITEM shared_synchronized, What does shared have to do with synchronization?)

    $(P Only shared data can be synchronized. It makes no sense to synchronize thread local
    data.
    )

$(ITEM shared_memory_barriers, What does shared have to do with memory barriers?)

    $(P Reading/writing shared data emits memory barriers to ensure sequential consistency (not
    implemented).
    )

$(ITEM casting_to_shared, What are the semantics of casting FROM unshared TO shared?)

    $(P Make sure there are no other unshared references to that same data.
    )

$(ITEM casting_from_shared, What are the semantics of casting FROM shared TO unshared?)

    $(P Make sure there are no other shared references to that same data.
    )

$(ITEM q1, Why the name D?)

    $(P The original name was the Mars Programming Language. But my friends
    kept calling it D, and I found myself starting to call it D.
    The idea of D being a successor to C goes back at least as far as 1988,
    as in this
    $(LINK2 http://groups.google.com/groups?q=%22d+programming+language&amp;hl=en&amp;lr=&amp;ie=UTF8&amp;oe=UTF8&amp;selm=12055%40brl-adm.ARPA&amp;rnum=1, thread).
    )

$(ITEM q1_2, Is there a linux port of D?)

    $(P Yes, the D compiler includes a $(LINK2 dmd-linux.html, linux version).
    )

$(ITEM gdc, Is there a GNU version of D?)

    $(P Yes,
    $(LINK2 http://bitbucket.org/goshawk/gdc/wiki/Home, gdc - the D frontend with GCC).
    )

$(ITEM backend, How do I write my own D compiler for CPU X?)

    $(P Burton Radons has written a
    $(LINK2 http://www.opend.org/dli/DLinux.html, back end).
    You can use as a guide.
    )

$(ITEM gui, Where can I get a GUI library for D?)

    $(P Since D can call C functions, any GUI library with a C interface is 
    accessible from D. Various D GUI libraries and ports can be found at 
    <a href="http://www.prowiki.org/wiki4d/wiki.cgi?AvailableGuiLibraries">AvailableGuiLibraries</a>.
    )

$(ITEM ide, Where can I get an IDE for D?)

    $(P Try
    $(LINK2 http://www.dsource.org/projects/elephant/, Elephant),
    $(LINK2 http://www.dsource.org/projects/poseidon/, Poseidon),
    or $(LINK2 http://www.dsource.org/projects/leds/, LEDS).
    )

$(ITEM q2, What about templates?)

    $(P D now supports advanced templates.
    )

$(ITEM q3, Why emphasize implementation ease?)

    $(P Isn't ease of use for the user of the language more important? Yes,
    it is.
    But a vaporware language is useless to everyone. The easier a language
    is to implement, the more robust implementations there will be. In C's
    heyday, there were 30 different commercial C compilers for the IBM PC.
    Not many made the transition to C++. In looking at
    the C++ compilers on the market today, how many years of development
    went into each? At least 10 years? Programmers waited years
    for the various pieces of C++ to get implemented after they were
    specified.
    If C++ was not so enormously popular, it's doubtful that very complex
    features
    like multiple inheritance, templates, etc., would ever have been
    implemented.
    )

    $(P I suggest that if a language is easier to implement, then it is
    likely also easier to understand. Isn't it better to spend time learning
    to write better programs than language arcana? If a language can capture
    90% of the power of
    C++ with 10% of its complexity, I argue that is a worthwhile tradeoff.
    )


$(ITEM q4, Why is printf in D?)

    $(P $(B printf) is not part of D, it is part of C's standard
    runtime library which is accessible from D.
    D's standard runtime library has $(B std.stdio.writefln),
    which is as powerful as $(B printf) but much easier to use.
    )


$(ITEM q5, Will D be open source?)

    $(P The front end for D is open source, and the source comes with the
    $(LINK2 http://www.digitalmars.com/d/download.html, compiler).
    The runtime library is completely open source.
    David Friedman has integrated the
    $(LINK2 http://home.earthlink.net/~dvdfrdmn/d, D frontend with GCC)
    to create $(B gdc), a completely open source implementation of D.
    )

$(ITEM q6, Why fall through on switch statements?)

    $(P Many people have asked for a requirement that there be a break between
    cases in a switch statement, that C's behavior of silently falling through
    is the cause of many bugs.
    )

    $(P The reason D doesn't change this is for the same reason that integral
    promotion rules and operator precedence rules were kept the same - to
    make code that looks the same as in C operate the same. If it had subtly
    different semantics, it will cause frustratingly subtle bugs.
    )


$(ITEM q7, Why should I use D instead of Java?)

    D is distinct from Java in purpose, philosophy and reality.
    See this <a href="comparison.html">comparison</a>.
    <p>

    Java is designed to be write once, run everywhere. D is designed for writing
    efficient native system apps. Although D and Java share the notion that
    garbage collection is good and multiple inheritance is bad &lt;g&gt;, their
    different design goals mean the languages have very different feels.

$(ITEM q7_2, Doesn't C++ support strings, etc. with STL?)

    $(P In the C++ standard library are mechanisms for doing strings,
    dynamic arrays, associative arrays, bounds checked
    arrays, and complex numbers.
    )

    $(P Sure, all this stuff can be done with libraries,
    following certain coding disciplines, etc. But you can also do
    object oriented programming in C (I've seen it done).
    Isn't it incongruous that something like strings,
    supported by the simplest BASIC interpreter, requires a very
    large and complicated infrastructure to support?
    Just the implementation of a string type in STL is over two
    thousand lines of code, using every advanced feature of templates.
    How much confidence can you have that this is all working
    correctly, how do you fix it if it is not, what do you do with the
    notoriously inscrutable error messages when there's an error
    using it, how can you be sure you are using it correctly
    (so there are no memory leaks, etc.)?
    )

    $(P D's implementation of strings is simple and straightforward.
    There's little doubt about how to use it, no worries about memory leaks,
    error messages are to the point, and it isn't hard to see if it
    is working as expected or not.
    )

$(ITEM q7_3, Can't garbage collection be done in C++ with an add-on library?)

    Yes, I use one myself. It isn't part of the language, though, and
    requires some subverting of the language to make it work.
    Using gc with C++ isn't for the standard or casual C++ programmer.
    Building it into the
    language, like in D, makes it practical for everyday programming chores.
    <p>

    GC isn't that hard to implement, either, unless you're building one
    of the more advanced ones. But a more advanced one is like building
    a better optimizer - the language still works 100% correctly even
    with a simple, basic one. The programming community is better served
    by multiple implementations competing on quality of code generated
    rather than by which corners of the spec are implemented at all.

$(ITEM q7_4, Can't unit testing be done in C++ with an add-on library?)

    Sure. Try one out and then compare it with how D does it.
    It'll be quickly obvious what an improvement building it into
    the language is.

$(ITEM q8, Why have an asm statement in a portable language?)

    An asm statement allows assembly code to be inserted directly into a D
    function. Assembler code will obviously be inherently non-portable. D is
    intended, however, to be a useful language for developing systems apps.
    Systems apps almost invariably wind up with system dependent code in them
    anyway, inline asm isn't much different. Inline asm will be useful for
    things like accessing special CPU instructions, accessing flag bits, special
    computational situations, and super optimizing a piece of code.
    <p>

    Before the C compiler had an inline assembler, I used external assemblers.
    There was constant grief because many, many different versions of the
    assembler were out there, the vendors kept changing the syntax of the
    assemblers, there were many different bugs in different versions, and even
    the command line syntax kept changing. What it all meant was that users
    could not reliably rebuild any code that needed assembler. An inline
    assembler provided reliability and consistency.

$(ITEM real, What is the point of 80 bit reals?)

    More precision enables more accurate floating point computations
    to be done, especially when adding together large numbers of small
    real numbers. Prof. Kahan, who designed the Intel floating point
    unit, has an eloquent
    <a href="http://http.cs.berkeley.edu/~wkahan/JAVAhurt.pdf">paper</a>
    on the subject.

$(ITEM anonymous, How do I do anonymous struct/unions in D?)

-----------------------
import std.stdio;

struct Foo
{
    union { int a; int b; }
    struct { int c; int d; }
}

void main()
{
    writefln(
      "Foo.sizeof = %d, a.offset = %d, b.offset = %d, c.offset = %d, d.offset = %d",
      Foo.sizeof,
      Foo.a.offsetof,
      Foo.b.offsetof,
      Foo.c.offsetof,
      Foo.d.offsetof);
}
-----------------------

$(ITEM printf, How do I get printf() to work with strings?)

    In C, the normal way to printf a string is to use the $(B %s)
    format:

$(CCODE
char s[8];
strcpy(s, "foo");
printf("string = '$(B %s)'\n", s);
)

    Attempting this in D, as in:

---------------------------------
char[] s;
s = "foo";
printf("string = '$(B %s)'\n", s);
---------------------------------

    usually results in garbage being printed, or an access violation.
    The cause is that in C, strings are terminated by a 0 character.
    The $(B %s) format prints until a 0 is encountered.
    In D, strings are not 0 terminated, the size is determined
    by a separate length value. So, strings are printf'd using the
    $(B %.*s) format:

---------------------------------
char[] s;
s = "foo";
printf("string = '$(B %.*s)'\n", s);
---------------------------------

    $(P which will behave as expected.
    Remember, though, that printf's $(B %.*s) will print until the length
    is reached or a 0 is encountered, so D strings with embedded 0's
    will only print up to the first 0.
    )

    $(P Of course, the easier solution is just use $(B std.stdio.writefln)
    which works correctly with D strings.
    )

$(ITEM nan, Why are floating point values default initialized to NaN rather than 0?)

    A floating point value, if no explicit initializer is given,
    is initialized to NaN (Not A Number):

---------------------------------
double d;   // d is set to double.nan
---------------------------------

    NaNs have the interesting property in that whenever a NaN is
    used as an operand in a computation, the result is a NaN. Therefore,
    NaNs will propagate and appear in the output whenever a computation
    made use of one. This implies that a NaN appearing in the output
    is an unambiguous indication of the use of an uninitialized
    variable.
    <p>

    If 0.0 was used as the default initializer for floating point
    values, its effect could easily be unnoticed in the output, and so
    if the default initializer was unintended, the bug may go
    unrecognized.
    <p>

    The default initializer value is not meant to be a useful value,
    it is meant to expose bugs. Nan fills that role well.
    <p>

    But surely the compiler can detect and issue an error message
    for variables used that are not initialized? Most of the time,
    it can, but not always, and what it can do is dependent on the
    sophistication of the compiler's internal data flow analysis.
    Hence, relying on such is unportable and unreliable.
    <p>

    Because of the way CPUs are designed, there is no NaN value for
    integers, so D uses 0 instead. It doesn't have the advantages of
    error detection that NaN has, but at least errors resulting from
    unintended default initializations will be consistent and therefore more
    debuggable.

$(ITEM assignmentoverloading, Why is overloading of the assignment operator not supported?)

    $(P Overloading of the assignment operator for structs is supported
    in D 2.0.)

$(ITEM keys, The $(SINGLEQUOTE ~) is not on my keyboard?)

    $(P On PC keyboards, hold down the [Alt] key and press the 1, 2, and 6
    keys in sequence on the numeric pad. That will generate a $(SINGLEQUOTE ~)
    character.
    )

$(ITEM omf, Can I link in C object files created with another compiler?)


    DMD produces OMF (Microsoft Object Module Format) object 
    files while other compilers such as VC++ produce COFF object
    files.
    DMD's output is designed to work with DMC, the Digital Mars C
    compiler, which also produces object files in OMF format.
    <p>

    The OMF format that DMD uses is a Microsoft defined format based on an
    earlier Intel designed one. Microsoft at one point decided to abandon it
    in favor of a Microsoft defined variant on COFF.
    <p>

    Using the same object format doesn't mean that any C library in that
    format will successfully link and run. There is a lot more compatibility
    required - such as calling conventions, name mangling, compiler helper
    functions, and hidden assumptions about the way things work. If DMD
    produced Microsoft COFF output files, there is still little chance that
    they would work successfully with object files designed and tested for
    use with VC. There were a lot of problems with this back when
    Microsoft's compilers did generate OMF.
    <p>

    Having a different object file format makes it helpful in identifying
    library files that were not tested to work with DMD. If they are not,
    weird problems would result even if they successfully managed to link
    them together. It really takes an expert to get a binary built with a
    compiler from one vendor to work with the output of another vendor's
    compiler.
    <p>

    That said, the linux version of DMD produces object files in the ELF
    format which is standard on linux, and it is specifically designed to
    work with the standard linux C compiler, gcc.
    <p>

    There is one case where using existing C libraries does work - when
    those libraries come in the form of a DLL conforming to the usual C ABI
    interface. The linkable part of this is called an "import library", and
    Microsoft COFF format import libraries can be successfully converted to
    DMD OMF using the
    <a href="http://www.digitalmars.com/ctg/coff2omf.html">coff2omf</a>
    tool.

$(ITEM regexp_literals, Why not support regular expression literals
    with the $(TT /foo/g) syntax?)

    $(P There are two reasons:
    )

    $(OL

    $(LI The $(TT /foo/g) syntax would make it impossible to separate
    the lexer from the parser, as / is the divide token.)

    $(LI There are already 3 string types; adding the regex literals
    would add 3 more. This would proliferate through much of the compiler,
    debugger info, and library, and is not worth it.)

    )

$(ITEM dogfood, Why is the D front end written in C++ rather than D?)

    $(P The front end is in C++ in order to interface to the existing gcc
    and dmd back ends. 
    It's also meant to be easily interfaced to other existing back ends,
    which are likely written in C++.
    The D implementation of
    $(LINK2 http://www.digitalmars.com/dscript/index.html, DMDScript),
    which performs better than the
    $(LINK2 http://www.digitalmars.com/dscript/cppscript.html, C++ version),
    shows that there is no problem
    writing a professional quality compiler in 100% D.
    )

$(ITEM cpp_to_D, Why aren't all Digital Mars programs translated to D?)

    $(P There is little benefit to translating a complex, debugged, working
    application from one language to another. But new Digital Mars apps are
    implemented in D.)


$(ITEM foreach, When should I use a foreach loop rather than a for?)

    $(P Is it just performance or readability?
    )

    $(P By using foreach, you are letting the compiler decide on the
    optimization rather than worrying about it yourself. For example - are
    pointers or indices better?
    Should I cache the termination condition or not?
    Should I rotate the loop or not?
    The answers to these questions are not easy, and can vary from machine
    to machine. Like register assignment, let the compiler do the
    optimization.)

---
for (int i = 0; i < foo.length; i++)
---

or:

---
for (int i = 0; i < foo.length; ++i)
---

or:

---
for (T* p = &foo[0]; p < &foo[length]; p++)
---

or:

---
T* pend = &foo[length];
for (T* p = &foo[0]; p < pend; ++p)
---

or:

---
T* pend = &foo[length];
T* p = &foo[0];
if (p < pend)
{
    do
    {
    ...
    } while (++p < pend);
}
---

and, of course, should I use size_t or int?

---
for (size_t i = 0; i < foo.length; i++)
---

Let the compiler pick!

---
foreach (v; foo)
    ...
---

$(P Note that we don't even need to know what the type T needs to be, thus
avoiding bugs when T changes. I don't even have to know if foo is an array, or
an associative array, or a struct, or a collection class. This will also avoid
the common fencepost bug:)

---
for (int i = 0; i <= foo.length; i++)
---

$(P And it also avoids the need to manually create a temporary if foo is a
function call.)

$(P The only reason to use a for loop is if your loop does not fit in the
conventional form, like if you want to change
the termination condition on the fly.)


$(ITEM cpp_interface, Why doesn't D have an interface to C++ as well as C?)

$(V2
    $(P D 2.0 does have a
    $(LINK2 cpp_interface.html, limited interface to C++ code.)
    ) Here are some reasons why it isn't a full interface:
)

    $(P Attempting to have D interface with C++ is 
    nearly as complicated as writing a C++ compiler, which would destroy the 
    goal of having D be a reasonably easy language to implement.
    For people with an existing C++ code base that they must work with, they are 
    stuck with C++ (they can't move it to any other language, either).)

    $(P There are many issues that would have to be resolved in order for D
    code to call some arbitrary C++ code that is presumed to be unmodifiable. This
    list certainly isn't complete, it's just to show the scope of the
    difficulties involved.
    )

    $(OL
    $(LI D source code is unicode, C++'s is ASCII with code pages. Or not.
    It's unspecified. This impacts the contents of string literals.)

    $(LI std::string cannot deal with multibyte UTF.)

    $(LI C++ has a tag name space. D does not. Some sort of renaming would
    have to happen.)

    $(LI C++ code often relies on compiler specific extensions.)

    $(LI C++ has namespaces. D has modules. There is no obvious mapping
    between the two.)

    $(LI C++ views source code as one gigantic file (after preprocessing). D
    sees source code as a hierarchy of modules and packages.)

    $(LI Enum name scoping rules behave differently.)

    $(LI C++ code, despite decades of attempts to replace macro features
    with inbuilt ones, relies more heavily than ever on layer after layer of
    arbitrary macros. There is no D analog for token pasting or
    stringizing.)

    $(LI Macro names have global scope across #include files, but are local
    to the gigantic source files.)

    $(LI C++ has arbitrary multiple inheritance and virtual base classes. D
    does not.)

    $(LI C++ does not distinguish between in, out and ref (i.e. inout) parameters.)

    $(LI The C++ name mangling varies from compiler to compiler.)

    $(LI C++ throws exceptions of arbitrary type, not just descendants of
    Object.)

    $(LI C++ overloads based on const and volatile. D overloads
    based on const and immutable.)

    $(LI C++ overloads operators in significantly different ways - for
    example, operator[]() overloading for lvalue and rvalue is based on
    const overloading and a proxy class.)

    $(LI C++ overloads operators like &lt; completely independently of
    &gt;.)

    $(LI C++ does not distinguish between a class and a struct object.)

    $(LI The vtbl[] location and layout is different between C++ and D.)

    $(LI The way RTTI is done is completely different. C++ has no
    classinfo.)

    $(LI D does not have two phase lookup, nor does it have Koenig (ADL)
    lookup.)

    $(LI C++ relates classes with the 'friend' system, D uses packages and
    modules.)

    $(LI C++ class design tends to revolve around explicit memory allocation
    issues, D's do not.)

    $(LI D's template system is very different.)

    $(LI C++ has 'exception specifications'.)

    $(LI C++ has global operator overloading.)

    $(LI C++ name mangling depends on const and volatile being type
    modifiers.
    D name mangling depends on const and immutable being type modifiers.
    D's const is also transitive, unlike C++. One cannot have a const
    pointer to mutable in D.
    )

    )

    $(P The bottom line is the language features affect the design of the code. C++
    designs just don't fit with D. Even if you could find a way to automatically
    adapt between the two, the result will be about as enticing as the left side of
    a honda welded to the right side of a camaro. 
    )

$(ITEM reference-counting, Why doesn't D use reference counting for garbage collection?)

    $(P Reference counting has its advantages, but some severe
    disadvantages:
    )

    $(UL

    $(LI Cyclical data structures won't get freed.)

    $(LI Every pointer copy requires an increment and a corresponding
    decrement - including when simply passing a reference to a function.)

    $(LI In a multithreaded app, the incs and decs must be synchronized.)

    $(LI Exception handlers (finally blocks) must be inserted to handle all the
    decs so there are no leaks. Contrary to assertions otherwise, there is
    no such thing as "zero overhead exceptions.")

    $(LI In order to support slicing and interior pointers, as well as
    supporting reference counting on arbitrary allocations of non-object
    data, a separate "wrapper" object must be allocated for each allocation
    to be ref counted. This essentially doubles the number of allocations
    needed.)

    $(LI The wrapper object will mean that all pointers will need to be
    double-dereferenced to access the data.)

    $(LI Fixing the compiler to hide all this stuff from the programmer will
    make it difficult to interface cleanly with C.)

    $(LI Ref counting can fragment the heap thereby consuming more memory
    just like the gc can, though the gc typically will consume more memory
    overall.)

    $(LI Ref counting does not eliminate latency problems, it just reduces
    them.)

    )

    $(P The proposed C++ shared_ptr&lt;&gt;, which implements ref counting,
    suffers from all these faults. I haven't seen a heads up benchmark of
    shared_ptr&lt;&gt; vs mark/sweep, but I wouldn't be surprised if shared_ptr&lt;&gt;
    turned out to be a significant loser in terms of both performance and
    memory consumption.
    )

    $(P That said, D may in the future optionally support some form of ref
    counting, as rc is better for managing scarce resources like file
    handles. 
    )

$(ITEM gc_1, Isn't garbage collection slow and non-deterministic?)

    $(P Yes, but $(B all) dynamic memory management is slow and
    non-deterministic, including malloc/free.
    If you talk to the people who actually do real time
    software, they don't use malloc/free precisely because they are not
    deterministic. They preallocate all data.
    However, the use of GC instead of malloc enables advanced language
    constructs (especially, more powerful array syntax), which greatly
    reduce the number of memory allocations which need to be made.
    This can mean that GC is actually faster than explict management.
    )

$(ITEM pure, Can't a sufficiently smart compiler figure out that a function is pure automatically?)

    $(P The problems with the
    $(LINK2 http://c2.com/cgi/wiki?SufficientlySmartCompiler, sufficently smart compiler)
    argument are:
    )

    $(OL
    $(LI most functions call other functions, which call other functions,
    until one of them calls a library routine whose source is not available
    to the compiler. So it will have to assume it is not pure.
    With a pure function attribute, external library functions that are pure
    can be marked as pure, and then the analysis can work for enough cases
    to be useful.
    )

    $(LI Since virtual functions (and by extension delegates and function
    pointers) can be extended by the user at times the compiler won't see
    it, they have to be assumed to be impure.
    )

    $(LI If the programmer intends for a particular function to be pure
    and the compiler detects it is not, this may go unnoticed by the
    programmer. Even worse, if the programmer does notice it, it may
    be arbitrarily difficult to determine why the compiler thinks it is
    impure - is it a programming mistake or a compiler bug?
    )

    )

$(COMMENT
    > Single inheritance may be easier to implement, but you are losing
    >something.  It's a little concerning how often folks here take the
    >opinion that "Feature X has problems and I never use it anyway, so no
    >body else 'really' needs it."  I'm not specificly blaming you, but i've
    >lost track of how many time if seen that reasoning tonight.  I'm afraid
    >I'll see it a lot in the 275 I still have to read.  


    Your reasoning has merit. The counterargument (and I've discussed
    this at length with my colleagues) is that C++ gives you a dozen ways
    and styles to do X. Programmers tend to develop specific styles and do
    things in certain ways. This leads to one programmer's use of C++ to
    be radically different than another's, almost to the point where
    they are different languages.  C++ is a huge language, and C++
    programmers tend to learn particular "islands" in the language
    and not be too familiar with the rest of it.

    Hence one idea behind D is to *reduce* the number of ways X can be
    accomplished, and reduce the balkanization of programmer expertise.
    Then, one programmer's coding style will look more like another's,
    with the intended result that legacy D code will be more maintainable.
    For example, over the  years I've seen dozens of different ways that
    debug code was inserted into a program, all very different. D has one
    way - with the debug attribute/statement. C++ has a dozen string
    classes plus the native C way of doing strings. D has one way of
    doing strings.

    I intend to further help this along by writing a D style guide,
    "The D Way". There's a start on it all ready with the document
    on how to do error handling: 

        www.digitalmars.com/d/errors.html

)

)

Macros:
    TITLE=D 2.0 FAQ
    WIKI=FAQ
    ITEMR=$(LI $(LINK2 #$1, $+))
    ITEM=<hr><h3><a name="$1">$+</a></h3>