C程序设计基础 英文版 课件 Chapter 4 The Preprocessor

Chapter4ThePreprocessorOutlines4.1HowdoesthePreprocessorWork4.2MacroDefinition4.3FileInclusion4.1HowdoesthePreprocessorWorkPreprocessordirectivestellthecompilertopreprocessthesourcecodebeforecompiling.PreprocessorDirectivesalwaysbeginwiththe‘#’(pound/hash)symbol.LinkerExecutablecodeHowdoesthePreprocessorWorkThereare4maintypesofpreprocessordirectives:Macrodefinition--------#defineFileInclusion---------#includeConditionalCompilationOtherdirectives4.2MacroDefinition1.SimpleMacros

#defineidentifier

replacement-list #definePI3.14

Thepreprocessorreplacestheidentifierwiththereplacementlistwhereveritappearslaterinthefile

#include<stdio.h>

#definePI3.14//

SimpleMacro

intmain()

{

doubler,L,S;

scanf("%lf",&r);

L=2*PI*r;

S=PI*r*r;

printf("L=%f,S=%f\n",L,S);

return0;

}

L=2*3.14*r;

S=3.14*r*r;Nestedmacrodefinition

#defineR3.0#definePI3.1415#defineL2*PI*R8

#defineMy*y+5*y

s=3*M+4*M;

S=3*y*y+5*y+4*y*y+5*y;

#defineM(y*y+5*y)

s=3*(y*y+5*y)+4*(y*y+5*y);

#definePI=3.14/***WRONG***/… L=2*PI*r;/*becomesL=2*=3.14*r;*/#defineN100;/***WRONG***/ …sum=N+5;/*becomessum=100;+5;*/Advantages:easiertoread.

easiertomodify.

GeneralPropertiesofMacrosDirectivesalwaysbeginwiththe‘#’(pound/hash)symbol.

Whenmacrosareusedasconstants,Cprogrammerscustomarilycapitalizealllettersintheirnames.Thereplacementlistsinmacrodefinitionsoftenrequireparenthesesinordertoavoidunexpectedresults.Thereisnosemicolon‘;’attheendofamacrodefinition.Amacrodefinitionnormallyremainsineffectuntiltheendofthefileinwhichitappears.The#undefdirectiveremovesthecurrentdefinitionofidentifier.

#definePI3.14159intmain(){

……}#undefPIintf1(){....}MacroDefinitions2.ParameterizedMacrosDefinitionofaparameterizedmacro

#defineidentifier(x1,x2,…,xn)replacement-list

x1,x2,…,xnareidentifiers(themacro’sparameters).MacroDefinitionsMacroDefinitions#definePI3.14#defineS(r)PI*(r)*(r)area=S(a+b);

area=3.14*(a+b)*(a+b);#defineS(r)PI*r*rarea=S(a+b);

area=3.14*a+b*a+b;

FileInclusion#includedirectiveThe#include

directivetellsthepreprocessortoinsertthecontentsofaspecifiedfile.#include<filename>#include"filename"

Typicalrulesforlocatingheaderfiles:#include

<filename>:Searchthedirectory(ordirectories)inwhichsystemheaderfilesreside.#include

"filename":Searchthecurrentdirectory,thensearchthedirectory(ordirectories)inwhichsystemheaderfilesreside.#include<math.h>AssumingthatthecompilersystemINCLUDEpathis"C:\CODEBOLCKS\MinGW\include",thecompilerwillsearchmath.hinthisdirectoryC:\CODEBOLCKS\MinGW\include#include"user.h“Assumingthattheuserpathis"D:\myhomework",thecompilerwillsearchthesystemINCLUDEpathoruserpathtofindtheheaderfile.D:\

myhomework

orC:\CODEBOLCKS\MinGW\include

Thefilenameinan#include

directivemayincludeinformationthathelpslocatethefile,suchasadirectorypathordrivespecifier: #include"c:\myproject\myhead.h“

SharingMacroDefinitionsandTypeDefinitionsSavestime.Makestheprogrameasiertomodify.Avoidsinconsistenciescausedbysourcefilescontainingdifferentdefinitionsofthesamemacroortype.

SharingVariableDeclarationsToshareafunctionamongfiles,weputitsdefinitioninonesourcefile,thenputdeclarationsinotherfilesthatneedtocallthefunction.Sharinganexternalvariableis