D Programming Language 2.0

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Last update Wed Oct 17 15:41:31 2007

# Programming in D for C Programmers

> Et tu, D? Then fall, C! -- William Nerdspeare

Every experienced C programmer accumulates a series of idioms and techniques which become second nature. Sometimes, when learning a new language, those idioms can be so comfortable it's hard to see how to do the equivalent in the new language. So here's a collection of common C techniques, and how to do the corresponding task in D.

Since C does not have object-oriented features, there's a separate section for object-oriented issues Programming in D for C++ Programmers.

The C preprocessor is covered in The C Preprocessor vs D.

- Getting the Size of a Type
- Get the max and min values of a type
- Primitive Types
- Special Floating Point Values
- Remainder after division of floating point numbers
- Dealing with NANs in floating point compares
- Asserts
- Initializing all elements of an array
- Looping through an array
- Creating an array of variable size
- String Concatenation
- Formatted printing
- Forward referencing functions
- Functions that have no arguments
- Labelled break and continue statements
- Goto Statements
- Struct tag name space
- Looking up strings
- Setting struct member alignment
- Anonymous Structs and Unions
- Declaring struct types and variables
- Getting the offset of a struct member
- Union initializations
- Struct initializations
- Array initializations
- Escaped String Literals
- Ascii vs Wide Characters
- Arrays that parallel an enum
- Creating a new type with typedef
- Comparing structs
- Comparing strings
- Sorting arrays
- Volatile memory access
- String literals
- Data Structure Traversal
- Unsigned Right Shift
- Dynamic Closures
- Variadic Function Parameters

### Getting the Size of a Type

#### The C Way

    sizeof(int)
    sizeof(char *)
    sizeof(double)
    sizeof(struct Foo)

#### The D Way

Use the size property:

    int.sizeof
    (char *).sizeof
    double.sizeof
    Foo.sizeof

### Get the max and min values of a type

#### The C Way

    #include <limits.h>
    #include <math.h>

    CHAR_MAX
    CHAR_MIN
    ULONG_MAX
    DBL_MIN

#### The D Way

    char.max
    char.min
    ulong.max
    double.min

### Primitive Types

#### C to D types

    bool               =>        bit
    char               =>        char
    signed char        =>        byte
    unsigned char      =>        ubyte
    short              =>        short
    unsigned short     =>        ushort
    wchar_t            =>        wchar
    int                =>        int
    unsigned           =>        uint
    long               =>        int
    unsigned long      =>        uint
    long long          =>        long
    unsigned long long =>        ulong
    float              =>        float
    double             =>        double
    long double        =>        real
    _Imaginary long double =>    ireal
    _Complex long double   =>    creal

Although char is an unsigned 8 bit type, and wchar is an unsigned 16 bit type, they have their own separate types in order to aid overloading and type safety.

Ints and unsigneds in C are of varying size; not so in D.

### Special Floating Point Values

#### The C Way

    #include <fp.h>

    NAN
    INFINITY

    #include <float.h>

    DBL_DIG
    DBL_EPSILON
    DBL_MANT_DIG
    DBL_MAX_10_EXP
    DBL_MAX_EXP
    DBL_MIN_10_EXP
    DBL_MIN_EXP

#### The D Way

    double.nan
    double.infinity
    double.dig
    double.epsilon
    double.mant_dig
    double.max_10_exp
    double.max_exp
    double.min_10_exp
    double.min_exp

### Remainder after division of floating point numbers

#### The C Way

    #include <math.h>

    float f = fmodf(x,y);
    double d = fmod(x,y);
    long double r = fmodl(x,y);

#### The D Way

D supports the remainder ('%') operator on floating point operands:

    float f = x % y;
    double d = x % y;
    real r = x % y;

### Dealing with NANs in floating point compares

#### The C Way

C doesn't define what happens if an operand to a compare is NAN, and few C compilers check for it (the Digital Mars C compiler is an exception, DM's compilers do check for NAN operands).

    #include <math.h>

    if (isnan(x) || isnan(y))
       result = FALSE;
    else
       result = (x < y);

#### The D Way

D offers a full complement of comparisons and operators that work with NAN arguments.

    result = (x < y);        // false if x or y is nan

### Asserts are a necessary part of any good defensive coding strategy

#### The C Way

C doesn't directly support assert, but does support __FILE__ and __LINE__ from which an assert macro can be built. In fact, there appears to be practically no other use for __FILE__ and __LINE__.

    #include <assert.h>

    assert(e == 0);

#### The D Way

D simply builds assert into the language:

    assert(e == 0);

### Initializing all elements of an array

#### The C Way

    #define ARRAY_LENGTH        17
    int array[ARRAY_LENGTH];
    for (i = 0; i < ARRAY_LENGTH; i++)
       array[i] = value;

#### The D Way

    int array[17];
    array[] = value;

### Looping through an array

#### The C Way

The array length is defined separately, or a clumsy sizeof() expression is used to get the length.

    #define ARRAY_LENGTH        17
    int array[ARRAY_LENGTH];
    for (i = 0; i < ARRAY_LENGTH; i++)
       func(array[i]);

or:

    int array[17];
    for (i = 0; i < sizeof(array) / sizeof(array[0]); i++)
       func(array[i]);

#### The D Way

The length of an array is accessible through the property "length".

    int array[17];
    for (i = 0; i < array.length; i++)
       func(array[i]);

or even better:

    int array[17];
    foreach (int value; array)
       func(value);

### Creating an array of variable size

#### The C Way

C cannot do this with arrays. It is necessary to create a separate variable for the length, and then explicitly manage the size of the array:

    #include <stdlib.h>

    int array_length;
    int *array;
    int *newarray;

    newarray = (int *)
       realloc(array, (array_length + 1) * sizeof(int));
    if (!newarray)
       error("out of memory");
    array = newarray;
    array[array_length++] = x;

#### The D Way

D supports dynamic arrays, which can be easily resized. D supports all the requisite memory management.

    int[] array;

    array.length = array.length + 1;
    array[array.length - 1] = x;

### String Concatenation

#### The C Way

There are several difficulties to be resolved, like when can storage be freed, dealing with null pointers, finding the length of the strings, and memory allocation:

    #include <string.h>

    char *s1;
    char *s2;
    char *s;

    // Concatenate s1 and s2, and put result in s
    free(s);
    s = (char *)malloc((s1 ? strlen(s1) : 0) +
    		  (s2 ? strlen(s2) : 0) + 1);
    if (!s)
       error("out of memory");
    if (s1)
       strcpy(s, s1);
    else
       *s = 0;
    if (s2)
       strcpy(s + strlen(s), s2);

    // Append "hello" to s
    char hello[] = "hello";
    char *news;
    size_t lens = s ? strlen(s) : 0;
    news = (char *)
       realloc(s, (lens + sizeof(hello) + 1) * sizeof(char));
    if (!news)
       error("out of memory");
    s = news;
    memcpy(s + lens, hello, sizeof(hello));

#### The D Way

D overloads the operators ~ and ~= for char and wchar arrays to mean concatenate and append, respectively:

    char[] s1;
    char[] s2;
    char[] s;

    s = s1 ~ s2;
    s ~= "hello";

### Formatted printing

#### The C Way

printf() is the general purpose formatted print routine:

    #include <stdio.h>

    printf("Calling all cars %d times!\n", ntimes);

#### The D Way

What can we say? printf() rules:

    printf("Calling all cars %d times!\n", ntimes);

writefln() improves on printf() by being type-aware and type-safe:

    import std.stdio;

    writefln("Calling all cars %s times!", ntimes);

### Forward referencing functions

#### The C Way

Functions cannot be forward referenced. Hence, to call a function not yet encountered in the source file, it is necessary to insert a function declaration lexically preceding the call.

    void forwardfunc();

    void myfunc()
    {
       forwardfunc();
    }

    void forwardfunc()
    {
       ...
    }

#### The D Way

The program is looked at as a whole, and so not only is it not necessary to code forward declarations, it is not even allowed! D avoids the tedium and errors associated with writing forward referenced function declarations twice. Functions can be defined in any order.

    void myfunc()
    {
       forwardfunc();
    }

    void forwardfunc()
    {
       ...
    }

### Functions that have no arguments

#### The C Way

    void function(void);

#### The D Way

D is a strongly typed language, so there is no need to explicitly say a function takes no arguments, just don't declare it has having arguments.

    void function()
    {
       ...
    }

### Labelled break and continue statements

#### The C Way

Break and continue statements only apply to the innermost nested loop or switch, so a multilevel break must use a goto:

        for (i = 0; i < 10; i++)
        {
           for (j = 0; j < 10; j++)
           {
    	   if (j == 3)
    	       goto Louter;
    	   if (j == 4)
    	       goto L2;
           }
         L2:
           ;
        }
    Louter:
        ;

#### The D Way

Break and continue statements can be followed by a label. The label is the label for an enclosing loop or switch, and the break applies to that loop.

    Louter:
       for (i = 0; i < 10; i++)
       {
           for (j = 0; j < 10; j++)
           {
    	   if (j == 3)
    	       break Louter;
    	   if (j == 4)
    	       continue Louter;
           }
       }
       // break Louter goes here

### Goto Statements

#### The C Way

The much maligned goto statement is a staple for professional C coders. It's necessary to make up for sometimes inadequate control flow statements.

#### The D Way

Many C-way goto statements can be eliminated with the D feature of labelled break and continue statements. But D is a practical language for practical programmers who know when the rules need to be broken. So of course D supports goto statements.

### Struct tag name space

#### The C Way

It's annoying to have to put the struct keyword every time a type is specified, so a common idiom is to use:

    typedef struct ABC { ... } ABC;

#### The D Way

Struct tag names are not in a separate name space, they are in the same name space as ordinary names. Hence:

    struct ABC { ... }

### Looking up strings

#### The C Way

Given a string, compare the string against a list of possible values and take action based on which one it is. A typical use for this might be command line argument processing.

    #include <string.h>
    void dostring(char *s)
    {
       enum Strings { Hello, Goodbye, Maybe, Max };
       static char *table[] = { "hello", "goodbye", "maybe" };
       int i;

       for (i = 0; i < Max; i++)
       {
           if (strcmp(s, table[i]) == 0)
    	   break;
       }
       switch (i)
       {
           case Hello:   ...
           case Goodbye: ...
           case Maybe:   ...
           default:      ...
       }
    }

The problem with this is trying to maintain 3 parallel data structures, the enum, the table, and the switch cases. If there are a lot of values, the connection between the 3 may not be so obvious when doing maintenance, and so the situation is ripe for bugs. Additionally, if the number of values becomes large, a binary or hash lookup will yield a considerable performance increase over a simple linear search. But coding these can be time consuming, and they need to be debugged. It's typical that such just never gets done.

#### The D Way

D extends the concept of switch statements to be able to handle strings as well as numbers. Then, the way to code the string lookup becomes straightforward:

    void dostring(char[] s)
    {
       switch (s)
       {
           case "hello":   ...
           case "goodbye": ...
           case "maybe":   ...
           default:        ...
       }
    }

Adding new cases becomes easy. The compiler can be relied on to generate a fast lookup scheme for it, eliminating the bugs and time required in hand-coding one.

### Setting struct member alignment

#### The C Way

It's done through a command line switch which affects the entire program, and woe results if any modules or libraries didn't get recompiled. To address this, `#pragma`s are used:

    #pragma pack(1)
    struct ABC
    {
       ...
    };
    #pragma pack()

But #pragmas are nonportable both in theory and in practice from compiler to compiler.

#### The D Way

Clearly, since much of the point to setting alignment is for portability of data, a portable means of expressing it is necessary.

    struct ABC
    {
       int z;               // z is aligned to the default

     align (1) int x;       // x is byte aligned
     align (4)
     {
       ...                  // declarations in {} are dword aligned
     }
     align (2):             // switch to word alignment from here on

       int y;               // y is word aligned
    }

### Anonymous Structs and Unions

Sometimes, it's nice to control the layout of a struct with nested structs and unions.

#### The C Way

C doesn't allow anonymous structs or unions, which means that dummy tag names and dummy members are necessary:

    struct Foo
    {
       int i;
       union Bar
       {
          struct Abc { int x; long y; } _abc;
          char *p;
       } _bar;
    };

    #define x _bar._abc.x
    #define y _bar._abc.y
    #define p _bar.p

    struct Foo f;

    f.i;
    f.x;
    f.y;
    f.p;

Not only is it clumsy, but using macros means a symbolic debugger won't understand what is being done, and the macros have global scope instead of struct scope.

#### The D Way

Anonymous structs and unions are used to control the layout in a more natural manner:

    struct Foo
    {
       int i;
       union
       {
          struct { int x; long y; }
          char* p;
       }
    }

    Foo f;

    f.i;
    f.x;
    f.y;
    f.p;

### Declaring struct types and variables

#### The C Way

Is to do it in one statement ending with a semicolon:

    struct Foo { int x; int y; } foo;

Or to separate the two:

    struct Foo { int x; int y; };   // note terminating ;
    struct Foo foo;

#### The D Way

Struct definitions and declarations can't be done in the same statement:

    struct Foo { int x; int y; }    // note there is no terminating ;
    Foo foo;

which means that the terminating ; can be dispensed with, eliminating the confusing difference between struct {} and function block {} in how semicolons are used.

### Getting the offset of a struct member

#### The C Way

Naturally, another macro is used:

    #include <stddef>
    struct Foo { int x; int y; };

    off = offsetof(Foo, y);

#### The D Way

An offset is just another property:

    struct Foo { int x; int y; }

    off = Foo.y.offsetof;

### Union Initializations

#### The C Way

Unions are initialized using the "first member" rule:

    union U { int a; long b; };
    union U x = { 5 };                // initialize member 'a' to 5

Adding union members or rearranging them can have disastrous consequences for any initializers.

#### The D Way

In D, which member is being initialized is mentioned explicitly:

    union U { int a; long b; }
    U x = { a:5 };

avoiding the confusion and maintenance problems.

### Struct Initializations

#### The C Way

Members are initialized by their position within the { }s:

    struct S { int a; int b; };
    struct S x = { 5, 3 };

This isn't much of a problem with small structs, but when there are numerous members, it becomes tedious to get the initializers carefully lined up with the field declarations. Then, if members are added or rearranged, all the initializations have to be found and modified appropriately. This is a minefield for bugs.

#### The D Way

Member initialization can be done explicitly:

    struct S { int a; int b; }
    S x = { b:3, a:5 };

The meaning is clear, and there no longer is a positional dependence.

### Array Initializations

#### The C Way

C initializes array by positional dependence:

    int a[3] = { 3,2,2 };

Nested arrays may or may not have the { }:

    int b[3][2] = { 2,3, {6,5}, 3,4 };

#### The D Way

D does it by positional dependence too, but an index can be used as well. The following all produce the same result:

    int[3] a = [ 3, 2, 0 ];
    int[3] a = [ 3, 2 ];            // unsupplied initializers are 0, just like in C
    int[3] a = [ 2:0, 0:3, 1:2 ];
    int[3] a = [ 2:0, 0:3, 2 ];     // if not supplied, the index is the
    				// previous one plus one.

This can be handy if the array will be indexed by an enum, and the order of enums may be changed or added to:

    enum color { black, red, green }
    int[3] c = [ black:3, green:2, red:5 ];

Nested array initializations must be explicit:

    int[2][3] b = [ [2,3], [6,5], [3,4] ];

    int[2][3] b = [[2,6,3],[3,5,4]];            // error

### Escaped String Literals

#### The C Way

C has problems with the DOS file system because a \ is an escape in a string. To specifiy file c:\root\file.c:

    char file[] = "c:\\root\\file.c";

This gets even more unpleasant with regular expressions. Consider the escape sequence to match a quoted string:

    /"[^\\]*(\\.[^\\]*)*"/

In C, this horror is expressed as:

    char quoteString[] = "\"[^\\\\]*(\\\\.[^\\\\]*)*\"";

#### The D Way

Within strings, it is WYSIWYG (what you see is what you get). Escapes are in separate strings. So:

    char[] file = `c:\root\file.c`;
    char[] quoteString = \"  r"[^\\]*(\\.[^\\]*)*"  \";

The famous hello world string becomes:

    char[] hello = "hello world" \n;

### Ascii vs Wide Characters

Modern programming requires that wchar strings be supported in an easy way, for internationalization of the programs.

#### The C Way

C uses the wchar_t and the L prefix on strings:

    #include <wchar.h>
    char foo_ascii[] = "hello";
    wchar_t foo_wchar[] = L"hello";

Things get worse if code is written to be both ascii and wchar compatible. A macro is used to switch strings from ascii to wchar:

    #include <tchar.h>
    tchar string[] = TEXT("hello");

#### The D Way

The type of a string is determined by semantic analysis, so there is no need to wrap strings in a macro call:

    char[] foo_ascii = "hello";        // string is taken to be ascii
    wchar[] foo_wchar = "hello";       // string is taken to be wchar

### Arrays that parallel an enum

#### The C Way

Consider:

    enum COLORS { red, blue, green, max };
    char *cstring[max] = {"red", "blue", "green" };

This is fairly easy to get right because the number of entries is small. But suppose it gets to be fairly large. Then it can get difficult to maintain correctly when new entries are added.

#### The D Way

    enum COLORS { red, blue, green }

    char[][COLORS.max + 1] cstring =
    [
        COLORS.red   : "red",
        COLORS.blue  : "blue",
        COLORS.green : "green",
    ];

Not perfect, but better.

### Creating a new type with typedef

#### The C Way

Typedefs in C are weak, that is, they really do not introduce a new type. The compiler doesn't distinguish between a typedef and its underlying type.

    typedef void *Handle;
    void foo(void *);
    void bar(Handle);

    Handle h;
    foo(h);			// coding bug not caught
    bar(h);			// ok

The C solution is to create a dummy struct whose sole purpose is to get type checking and overloading on the new type.

    struct Handle__ { void *value; }
    typedef struct Handle__ *Handle;
    void foo(void *);
    void bar(Handle);

    Handle h;
    foo(h);			// syntax error
    bar(h);			// ok

Having a default value for the type involves defining a macro, a naming convention, and then pedantically following that convention:

    #define HANDLE_INIT ((Handle)-1)

    Handle h = HANDLE_INIT;
    h = func();
    if (h != HANDLE_INIT)
        ...

For the struct solution, things get even more complex:

    struct Handle__ HANDLE_INIT;

    void init_handle()	// call this function upon startup
    {
        HANDLE_INIT.value = (void *)-1;
    }

    Handle h = HANDLE_INIT;
    h = func();
    if (memcmp(&h,&HANDLE_INIT,sizeof(Handle)) != 0)
        ...

There are 4 names to remember: `Handle, HANDLE_INIT, struct Handle__, value`.

#### The D Way

No need for idiomatic constructions like the above. Just write:

    typedef void* Handle;
    void foo(void*);
    void bar(Handle);

    Handle h;
    foo(h);
    bar(h);

To handle a default value, add an initializer to the typedef, and refer to it with the `.init` property:

    typedef void* Handle = cast(void*)(-1);
    Handle h;
    h = func();
    if (h != Handle.init)
        ...

There's only one name to remember: `Handle`.

### Comparing structs

#### The C Way

While C defines struct assignment in a simple, convenient manner:

    struct A x, y;
    ...
    x = y;

it does not for struct comparisons. Hence, to compare two struct instances for equality:

    #include <string.h>

    struct A x, y;
    ...
    if (memcmp(&x, &y, sizeof(struct A)) == 0)
        ...

Note the obtuseness of this, coupled with the lack of any kind of help from the language with type checking.

There's a nasty bug lurking in the memcmp(). The layout of a struct, due to alignment, can have 'holes' in it. C does not guarantee those holes are assigned any values, and so two different struct instances can have the same value for each member, but compare different because the holes contain different garbage.

#### The D Way

D does it the obvious, straightforward way:

    A x, y;
    ...
    if (x == y)
        ...

### Comparing strings

#### The C Way

The library function strcmp() is used:

    char string[] = "hello";

    if (strcmp(string, "betty") == 0)	// do strings match?
        ...

C uses 0 terminated strings, so the C way has an inherent inefficiency in constantly scanning for the terminating 0.

#### The D Way

Why not use the == operator?

    char[] string = "hello";

    if (string == "betty")
        ...

D strings have the length stored separately from the string. Thus, the implementation of string compares can be much faster than in C (the difference being equivalent to the difference in speed between the C memcmp() and strcmp()).

D supports comparison operators on strings, too:

    char[] string = "hello";

    if (string < "betty")
        ...

which is useful for sorting/searching.

### Sorting arrays

#### The C Way

Although many C programmers tend to reimplmement bubble sorts over and over, the right way to sort in C is to use qsort():

    int compare(const void *p1, const void *p2)
    {
        type *t1 = (type *)p1;
        type *t2 = (type *)p2;

        return *t1 - *t2;
    }

    type array[10];
    ...
    qsort(array, sizeof(array)/sizeof(array[0]),
    	sizeof(array[0]), compare);

A compare() must be written for each type, and much careful typo-prone code needs to be written to make it work.

#### The D Way

Sorting couldn't be easier:

    type[] array;
    ...
    array.sort;      // sort array in-place

### Volatile memory access

#### The C Way

To access volatile memory, such as shared memory or memory mapped I/O, a pointer to volatile is created:

    volatile int *p = address;

    i = *p;

#### The D Way

D has volatile as a statement type, not as a type modifier:

    int* p = address;

    volatile { i = *p; }

### String literals

#### The C Way

String literals in C cannot span multiple lines, so to have a block of text it is necessary to use \ line splicing:

    "This text spans\n\
    multiple\n\
    lines\n"

If there is a lot of text, this can wind up being tedious.

#### The D Way

String literals can span multiple lines, as in:

    "This text spans
    multiple
    lines
    "

So blocks of text can just be cut and pasted into the D source.

### Data Structure Traversal

#### The C Way

Consider a function to traverse a recursive data structure. In this example, there's a simple symbol table of strings. The data structure is an array of binary trees. The code needs to do an exhaustive search of it to find a particular string in it, and determine if it is a unique instance.

To make this work, a helper function `membersearchx` is needed to recursively walk the trees. The helper function needs to read and write some context outside of the trees, so a custom `struct Paramblock` is created and a pointer to it is used to maximize efficiency.

    struct Symbol
    {
       char *id;
       struct Symbol *left;
       struct Symbol *right;
    };

    struct Paramblock
    {
       char *id;
       struct Symbol *sm;
    };

    static void membersearchx(struct Paramblock *p, struct Symbol *s)
    {
       while (s)
       {
          if (strcmp(p->id,s->id) == 0)
          {
             if (p->sm)
                error("ambiguous member %s\n",p->id);
             p->sm = s;
          }

          if (s->left)
             membersearchx(p,s->left);
          s = s->right;
       }
    }

    struct Symbol *symbol_membersearch(Symbol *table[], int tablemax, char *id)
    {
       struct Paramblock pb;
       int i;

       pb.id = id;
       pb.sm = NULL;
       for (i = 0; i < tablemax; i++)
       {
          membersearchx(pb, table[i]);
       }
       return pb.sm;
    }

#### The D Way

This is the same algorithm in D, and it shrinks dramatically. Since nested functions have access to the lexically enclosing function's variables, there's no need for a Paramblock or to deal with its bookkeeping details. The nested helper function is contained wholly within the function that needs it, improving locality and maintainability.

The performance of the two versions is indistinguishable.

    class Symbol
    {   char[] id;
        Symbol left;
        Symbol right;
    }

    Symbol symbol_membersearch(Symbol[] table, char[] id)
    {   Symbol sm;

        void membersearchx(Symbol s)
        {
    	while (s)
    	{
    	    if (id == s.id)
    	    {
    		if (sm)
    		    error("ambiguous member %s\n", id);
    		sm = s;
    	    }

    	    if (s.left)
    		membersearchx(s.left);
    	    s = s.right;
    	}
        }

        for (int i = 0; i < table.length; i++)
        {
    	membersearchx(table[i]);
        }
        return sm;
    }

### Unsigned Right Shift

#### The C Way

The right shift operators >> and >>= are signed shifts if the left operand is a signed integral type, and are unsigned right shifts if the left operand is an unsigned integral type. To produce an unsigned right shift on an int, a cast is necessary:

    int i, j;
    ...
    j = (unsigned)i >> 3;

If `i` is an `int`, this works fine. But if `i` is of a type created with typedef,

    myint i, j;
    ...
    j = (unsigned)i >> 3;

and `myint` happens to be a `long int`, then the cast to unsigned will silently throw away the most significant bits, corrupting the answer.

#### The D Way

D has the right shift operators >> and >>= which behave as they do in C. But D also has explicitly unsigned right shift operators >>> and >>>= which will do an unsigned right shift regardless of the sign of the left operand. Hence,

    myint i, j;
    ...
    j = i >>> 3;

avoids the unsafe cast and will work as expected with any integral type.

### Dynamic Closures

#### The C Way

Consider a reusable container type. In order to be reusable, it must support a way to apply arbitrary code to each element of the container. This is done by creating an apply function that accepts a function pointer to which is passed each element of the container contents.

A generic context pointer is also needed, represented here by `void *p`. The example here is of a trivial container class that holds an array of ints, and a user of that container that computes the maximum of those ints.

    struct Collection
    {
        int array[10];

        void apply(void *p, void (*fp)(void *, int))
        {
    	for (int i = 0; i < sizeof(array)/sizeof(array[0]); i++)
    	    fp(p, array[i]);
        }
    };

    void comp_max(void *p, int i)
    {
        int *pmax = (int *)p;

        if (i > *pmax)
    	*pmax = i;
    }

    void func(Collection *c)
    {
        int max = INT_MIN;

        c->apply(&max, comp_max);
    }

The C way makes heavy use of pointers and casting. The casting is tedious, error prone, and loses all type safety.

#### The D Way

The D version makes use of delegates to transmit context information for the apply function, and nested functions both to capture context information and to improve locality.

    class Collection
    {
        int[10] array;

        void apply(void delegate(int) fp)
        {
    	for (int i = 0; i < array.length; i++)
    	    fp(array[i]);
        }
    }

    void func(Collection c)
    {
        int max = int.min;

        void comp_max(int i)
        {
    	if (i > max)
    	    max = i;
        }

        c.apply(comp_max);
    }

Pointers are eliminated, as well as casting and generic pointers. The D version is fully type safe. An alternate method in D makes use of function literals:

    void func(Collection c)
    {
        int max = int.min;

        c.apply(delegate(int i) { if (i > max) max = i; } );
    }

eliminating the need to create irrelevant function names.

### Variadic Function Parameters

The task is to write a function that takes a varying number of arguments, such as a function that sums its arguments.

#### The C Way

    #include <stdio.h>
    #include <stdarg.h>

    int sum(int dim, ...)
    {   int i;
        int s = 0;
        va_list ap;

        va_start(ap, dim);
        for (i = 0; i < dim; i++)
    	s += va_arg(ap, int);
        va_end(ap);
        return s;
    }

    int main()
    {
        int i;

        i = sum(3, 8,7,6);
        printf("sum = %d\n", i);

        return 0;
    }

There are two problems with this. The first is that the `sum` function needs to know how many arguments were supplied. It has to be explicitly written, and it can get out of sync with respect to the actual number of arguments written. The second is that there's no way to check that the types of the arguments provided really were ints, and not doubles, strings, structs, etc.

#### The D Way

The ... following an array parameter declaration means that the trailing arguments are collected together to form an array. The arguments are type checked against the array type, and the number of arguments becomes a property of the array:

    import std.stdio;

    int sum(int[] values ...)
    {
        int s = 0;

        foreach (int x; values)
    	s += x;
        return s;
    }

    int main()
    {
        int i;

        i = sum(8,7,6);
        writefln("sum = %d", i);

        return 0;
    }

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