D programming Language


The C Preprocessor Versus D



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The C Preprocessor Versus D


Back when C was invented, compiler technology was primitive. Installing a text macro preprocessor onto the front end was a straightforward and easy way to add many powerful features. The increasing size & complexity of programs have illustrated that these features come with many inherent problems. D doesn't have a preprocessor; but D provides a more scalable means to solve the same problems.


Header Files

The C Preprocessor Way


C and C++ rely heavilly on textual inclusion of header files. This frequently results in the compiler having to recompile tens of thousands of lines of code over and over again for every source file, an obvious source of slow compile times. What header files are normally used for is more appropriately done doing a symbolic, rather than textual, insertion. This is done with the import statement. Symbolic inclusion means the compiler just loads an already compiled symbol table. The needs for macro "wrappers" to prevent multiple #inclusion, funky #pragma once syntax, and incomprehensible fragile syntax for precompiled headers are simply unnecessary and irrelevant to D.

#include




The D Way


D uses symbolic imports:

import stdio;




#pragma once

The C Preprocessor Way


C header files frequently need to be protected against being #include'd multiple times. To do it, a header file will contain the line:

#pragma once

or the more portable:

#ifndef __STDIO_INCLUDE

#define __STDIO_INCLUDE

... header file contents

#endif


The D Way


Completely unnecessary since D does a symbolic include of import files; they only get imported once no matter how many times the import declaration appears.


#pragma pack

The C Preprocessor Way


This is used in C to adjust the alignment for structs.

The D Way


For D classes, there is no need to adjust the alignment (in fact, the compiler is free to rearrange the data fields to get the optimum layout, much as the compiler will rearrange local variables on the stack frame). For D structs that get mapped onto externally defined data structures, there is a need, and it is handled with:

struct Foo

{

align (4): // use 4 byte alignment



...

}


Macros


Preprocessor macros add powerful features and flexibility to C. But they have a downside:

  • Macros have no concept of scope; they are valid from the point of definition to the end of the source. They cut a swath across .h files, nested code, etc. When #include'ing tens of thousands of lines of macro definitions, it becomes problematicalto avoid inadvertent macro expansions.

  • Macros are unknown to the debugger. Trying to debug a program with symbolic data is undermined by the debugger only knowing about macro expansions, not themacros themselves.

  • Macros make it impossible to tokenize source code, as an earlier macro change can arbitrarilly redo tokens.

  • The purely textual basis of macros leads to arbitrary and inconsistent usage, making code using macros error prone. (Some attempt to resolve this was introduced with templates in C++.)

  • Macros are still used to make up for deficits in the language's expressive capabiltiy, such as for "wrappers" around header files.

Here's an enumeration of the common uses for macros, and the corresponding feature in D:

  1. Defining literal constants:

The C Preprocessor Way


#define VALUE 5


The D Way


const int VALUE = 5;



  1. Creating a list of values or flags:

The C Preprocessor Way


int flags:

#define FLAG_X 0x1

#define FLAG_Y 0x2

#define FLAG_Z 0x4

...

flags |= FLAGS_X;




The D Way


enum FLAGS { X = 0x1, Y = 0x2, Z = 0x4 };

FLAGS flags;

...

flags |= FLAGS.X;





  1. Distinguishing between ascii chars and wchar chars:

The C Preprocessor Way


#if UNICODE

#define dchar wchar_t

#define TEXT(s) L##s

#else


#define dchar char

#define TEXT(s) s

#endif
...

dchar h[] = TEXT("hello");




The D Way


import dchar; // contains appropriate typedef for dchar

...


dchar[] h = "hello";

D's optimizer will inline the function, and will do the conversion of the string constant at compile time.



  1. Supporting legacy compilers:

The C Preprocessor Way


#if PROTOTYPES

#define P(p) p

#else

#define P(p) ()



#endif

int func P((int x, int y));




The D Way


By making the D compiler open source, it will largely avoid the problem of syntactical backwards compatibility.

  1. Type aliasing:

The C Preprocessor Way


#define INT int


The D Way


alias int INT;



  1. Using one header file for both declaration and definition:

The C Preprocessor Way


#define EXTERN extern

#include "declations.h"

#undef EXTERN

#define EXTERN

#include "declations.h"

In declarations.h:

EXTERN int foo;


The D Way


The declaration and the definition are the same, so there is no need to muck with the storage class to generate both a declaration and a definition from the same source.

  1. Lightweight inline functions:

The C Preprocessor Way


#define X(i) ((i) = (i) / 3)


The D Way


int X(inout int i) { return i = i / 3; }

The compiler optimizer will inline it; no efficiency is lost.



  1. Assert function file and line number information:

The C Preprocessor Way


#define assert(e) ((e) || _assert(__LINE__, __FILE__))


The D Way


assert() is a built-in expression primitive. Giving the compiler such knowledge of assert() also enables the optimizer to know about things like the _assert() function never returns.

  1. Setting function calling conventions:

The C Preprocessor Way


#ifndef _CRTAPI1

#define _CRTAPI1 __cdecl

#endif

#ifndef _CRTAPI2



#define _CRTAPI2 __cdecl

#endif
int _CRTAPI2 func();




The D Way


Calling conventions can be specified in blocks, so there's no need to change it for every function:

extern (Windows)

{

int onefunc();



int anotherfunc();

}



  1. Hiding __near or __far pointer wierdness:

The C Preprocessor Way


#define LPSTR char FAR *


The D Way


D doesn't support 16 bit code, mixed pointer sizes, and different kinds of pointers, and so the problem is just irrelevant.

  1. Simple generic programming:

The C Preprocessor Way


Selecting which function to use based on text substitution:

#ifdef UNICODE

int getValueW(wchar_t *p);

#define getValue getValueW

#else

int getValueA(char *p);



#define getValue getValueA

#endif



The D Way


D enables declarations of symbols that are aliases of other symbols:

version (UNICODE)

{

int getValueW(wchar[] p);



alias getValueW getValue;

}

else



{

int getValueA(char[] p);

alias getValueA getValue;

}


Conditional Compilation

The C Preprocessor Way


Conditional compilation is a powerful feature of the C preprocessor, but it has its downside:

  • The preprocessor has no concept of scope. #if/#endif can be interleaved with code in a completely unstructured and disorganized fashion, making things difficult to follow.

  • Conditional compilation triggers off of macros - macros that can conflict with identifiers used in the program.

  • #if expressions are evaluated in subtly different ways than C expressions are.

  • The preprocessor language is fundamentally different in concept than C, for example, whitespace and line terminators mean things to the preprocessor that they do not in C.

The D Way


D supports conditional compilation:

  1. Separating version specific functionality into separate modules.

  2. The debug statement for enabling/disabling debug harnesses, extra printing, etc.

  3. The version statement for dealing with multiple versions of the program generated from a single set of sources.

  4. The if (0) statement.

  5. The /+ +/ nesting comment can be used to comment out blocks of code.


Code Factoring

The C Preprocessor Way


It's common in a function to have a repetitive sequence of code to be executed in multiple places. Performance considerations preclude factoring it out into a separate function, so it is implemented as a macro. For example, consider this fragment from a byte code interpreter:

unsigned char *ip; // byte code instruction pointer

int *stack;

int spi; // stack pointer

...

#define pop() (stack[--spi])



#define push(i) (stack[spi++] = (i))

while (1)

{

switch (*ip++)



{

case ADD:

op1 = pop();

op2 = pop();

result = op1 + op2;

push(result);

break;
case SUB:

...


}

}

This suffers from numerous problems:



  1. The macros must evaluate to expressions and cannot declare any variables. Consider the difficulty of extending them to check for stack overflow/underflow.

  2. The macros exist outside of the semantic symbol table, so remain in scope even outside of the function they are declared in.

  3. Parameters to macros are passed textually, not by value, meaning that the macro implementation needs to be careful to not use the parameter more than once, and must protect it with ().

  4. Macros are invisible to the debugger, which sees only the expanded expressions.

The D Way


D neatly addresses this with nested functions:

ubyte* ip; // byte code instruction pointer

int[] stack; // operand stack

int spi; // stack pointer

...
int pop() { return stack[--spi]; }

void push(int i) { stack[spi++] = i; }


while (1)

{

switch (*ip++)



{

case ADD:

op1 = pop();

op2 = pop();

push(op1 + op2);

break;
case SUB:

...

}

}



The problems addressed are:

  1. The nested functions have available the full expressive power of D functions. The array accesses already are bounds checked (adjustable by compile time switch).

  2. Nested function names are scoped just like any other name.

  3. Parameters are passed by value, so need to worry about side effects in the parameter expressions.

  4. Nested functions are visible to the debugger.

Additionally, nested functions can be inlined by the implementation resulting in the same high performance that the C macro version exhibits.



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