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Last update Jan 24, 2002


Programming in D for C Programmers

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.


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.size
        (char *).size
        double.size
        Foo.size

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        =>        extended 
	_Imaginary long double =>    imaginary
	_Complex long double   =>    complex

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 

Taking the Modulus of a floating point number

The C Way

       #include <math.h> 

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

The D Way

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

       float f = x % y; 
       double d = x % y; 
       extended e = x % y; 

Dealing with NAN's 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 

Assert's 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); 

[NOTE: trace functions?]


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 the property "length".

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

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 easilly resized. D supports all the requisite memory management.
               int array[]; 

               array[array.length++] = x; 

String Concatenation

The C Way

There are several difficulties to be resolved, like when can storage be free'd, 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:
               import stdio; 

               printf("Calling all cars %d times!\n", 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's and continue's.

The C Way

Break's and continue's 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 the goto!

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.offset; 

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 is 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 a[3] = [ 3, 2, 0 ]; 
    int a[3] = [ 3, 2 ];              // unsupplied initializers are 0, just like in C 
    int a[3] = [ 2:0, 0:3, 1:2 ]; 
    int a[3] = [ 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 c[3] = [ black:3, green:2, red:5 ]; 
Nested array initializations must be explicit:
    int b[3][2] = [ [2,3], [6,5], [3,4] ]; 

    int b[3][2] = [[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[] = \" '[^\\]*(\\.[^\\]*)*' \";
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 cstring[COLORS.max + 1][] = 
    [
	COLORS.red   : "red",
	COLORS.blue  : "blue", 
	COLORS.green : "green",
    ]; 
Not perfect, but better.

Creating a new typedef'd type

The C Way

Typedef's 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 *unused; }
	typedef struct Handle__ *Handle;

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

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

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)
	    ...
	

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