Input/Output with files

C++providesthefollowingclassestoperformoutputandinputofcharactersto/fromfiles:ofstream:Streamclasstowriteonfilesifstream:Streamclasstoreadfromfilesfstream:Streamclasstobothreadandwritefrom/tofiles.Theseclassesarederiveddirectlyorindirectlyfromtheclassesistream,andostream.Wehavealreadyusedobjectswhosetypesweretheseclasses:cinisanobjectofclassistreamandcoutisanobjectofclassostream.Therefore,wehavealreadybeenusingclassesthatarerelatedtoourfilestreams.Andinfact,wecanuseourfilestreamsthesamewaywearealreadyusedtousecinandcout,withtheonlydifferencethatwehavetoassociatethesestreamswithphysicalfiles.Let'sseeanexample:

//basicfileoperations#include<iostream>#include<fstream>usingnamespacestd;intmain(){ofstreammyfile;myfile.open("example.txt");myfile<<"Writingthistoafile.\n";myfile.close();return0;}

[fileexample.txt]Writingthistoafile.

Thiscodecreatesafilecalledexample.txtandinsertsasentenceintoitinthesamewayweareusedtodowithcout,butusingthefilestreammyfileinstead.Butlet'sgostepbystep:

Openafile

Thefirstoperationgenerallyperformedonanobjectofoneoftheseclassesistoassociateittoarealfile.Thisprocedureisknownastoopenafile.Anopenfileisrepresentedwithinaprogrambyastreamobject(aninstantiationofoneoftheseclasses,inthepreviousexamplethiswasmyfile)andanyinputoroutputoperationperformedonthisstreamobjectwillbeappliedtothephysicalfileassociatedtoit.Inordertoopenafilewithastreamobjectweuseitsmemberfunctionopen():open(filename,mode);Wherefilenameisanull-terminatedcharactersequenceoftypeconstchar*(thesametypethatstringliteralshave)representingthenameofthefiletobeopened,andmodeisanoptionalparameterwithacombinationofthefollowingflags:

ios::in	Openforinputoperations.
ios::out	Openforoutputoperations.
ios::binary	Openinbinarymode.
ios::ate	Settheinitialpositionattheendofthefile.Ifthisflagisnotsettoanyvalue,theinitialpositionisthebeginningofthefile.
ios::app	Alloutputoperationsareperformedattheendofthefile,appendingthecontenttothecurrentcontentofthefile.Thisflagcanonlybeusedinstreamsopenforoutput-onlyoperations.
ios::trunc	Ifthefileopenedforoutputoperationsalreadyexistedbefore,itspreviouscontentisdeletedandreplacedbythenewone.

AlltheseflagscanbecombinedusingthebitwiseoperatorOR(|).Forexample,ifwewanttoopenthefileexample.bininbinarymodetoadddatawecoulddoitbythefollowingcalltomemberfunctionopen():

12	ofstreammyfile;myfile.open("example.bin",ios::out|ios::app|ios::binary);

Eachoneoftheopen()memberfunctionsoftheclassesofstream,ifstreamandfstreamhasadefaultmodethatisusedifthefileisopenedwithoutasecondargument:

class	defaultmodeparameter
ofstream	ios::out
ifstream	ios::in
fstream	ios::in|ios::out

Forifstreamandofstreamclasses,ios::inandios::outareautomaticallyandrespectivelyassumed,evenifamodethatdoesnotincludethemispassedassecondargumenttotheopen()memberfunction.Thedefaultvalueisonlyappliedifthefunctioniscalledwithoutspecifyinganyvalueforthemodeparameter.Ifthefunctioniscalledwithanyvalueinthatparameterthedefaultmodeisoverridden,notcombined.Filestreamsopenedinbinarymodeperforminputandoutputoperationsindependentlyofanyformatconsiderations.Non-binaryfilesareknownastextfiles,andsometranslationsmayoccurduetoformattingofsomespecialcharacters(likenewlineandcarriagereturncharacters).Sincethefirsttaskthatisperformedonafilestreamobjectisgenerallytoopenafile,thesethreeclassesincludeaconstructorthatautomaticallycallstheopen()memberfunctionandhastheexactsameparametersasthismember.Therefore,wecouldalsohavedeclaredthepreviousmyfileobjectandconductedthesameopeningoperationinourpreviousexamplebywriting:

ofstreammyfile("example.bin",ios::out|ios::app|ios::binary);

Combiningobjectconstructionandstreamopeninginasinglestatement.Bothformstoopenafilearevalidandequivalent.Tocheckifafilestreamwassuccessfulopeningafile,youcandoitbycallingtomemberis_open()withnoarguments.Thismemberfunctionreturnsaboolvalueoftrueinthecasethatindeedthestreamobjectisassociatedwithanopenfile,orfalseotherwise:

if(myfile.is_open()){/*ok,proceedwithoutput*/}

Closingafile

Whenwearefinishedwithourinputandoutputoperationsonafileweshallcloseitsothatitsresourcesbecomeavailableagain.Inordertodothatwehavetocallthestream'smemberfunctionclose().Thismemberfunctiontakesnoparameters,andwhatitdoesistoflushtheassociatedbuffersandclosethefile:

myfile.close();

Oncethismemberfunctioniscalled,thestreamobjectcanbeusedtoopenanotherfile,andthefileisavailableagaintobeopenedbyotherprocesses.Incasethatanobjectisdestructedwhilestillassociatedwithanopenfile,thedestructorautomaticallycallsthememberfunctionclose().

Textfiles

Textfilestreamsarethosewherewedonotincludetheios::binaryflagintheiropeningmode.Thesefilesaredesignedtostoretextandthusallvaluesthatweinputoroutputfrom/tothemcansuffersomeformattingtransformations,whichdonotnecessarilycorrespondtotheirliteralbinaryvalue.Dataoutputoperationsontextfilesareperformedinthesamewayweoperatedwithcout:

//writingonatextfile#include<iostream>#include<fstream>usingnamespacestd;intmain(){ofstreammyfile("example.txt");if(myfile.is_open()){myfile<<"Thisisaline.\n";myfile<<"Thisisanotherline.\n";myfile.close();}elsecout<<"Unabletoopenfile";return0;

[fileexample.txt]Thisisaline.Thisisanotherline.

Datainputfromafilecanalsobeperformedinthesamewaythatwedidwithcin:

//readingatextfile#include<iostream>#include<fstream>#include<string>usingnamespacestd;intmain(){stringline;ifstreammyfile("example.txt");if(myfile.is_open()){while(getline(myfile,line)){cout<<line<<endl;}myfile.close();}elsecout<<"Unabletoopenfile";return0;}

Thisisaline.Thisisanotherline.

Thislastexamplereadsatextfileandprintsoutitscontentonthescreen.Wehavecreatedawhileloopthatreadsthefilelinebyline,usinggetline.Thevaluereturnedbygetlineisareferencetothestreamobjectitself,whichwhenevaluatedasabooleanexpression(asinthiswhile-loop)istrueifthestreamisreadyformoreoperations,andfalseifeithertheendofthefilehasbeenreachedorifsomeothererroroccurred.

Checkingstateflags

Inadditiontogood(),whichcheckswhetherthestreamisreadyforinput/outputoperations,othermemberfunctionsexisttocheckforspecificstatesofastream(allofthemreturnaboolvalue):bad()Returnstrueifareadingorwritingoperationfails.Forexampleinthecasethatwetrytowritetoafilethatisnotopenforwritingorifthedevicewherewetrytowritehasnospaceleft.fail()Returnstrueinthesamecasesasbad(),butalsointhecasethataformaterrorhappens,likewhenanalphabeticalcharacterisextractedwhenwearetryingtoreadanintegernumber.eof()Returnstrueifafileopenforreadinghasreachedtheend.good()Itisthemostgenericstateflag:itreturnsfalseinthesamecasesinwhichcallinganyofthepreviousfunctionswouldreturntrue.Inordertoresetthestateflagscheckedbyanyofthesememberfunctionswehavejustseenwecanusethememberfunctionclear(),whichtakesnoparameters.

getandputstreampointers

Alli/ostreamsobjectshave,atleast,oneinternalstreampointer:ifstream,likeistream,hasapointerknownasthegetpointerthatpointstotheelementtobereadinthenextinputoperation.ofstream,likeostream,hasapointerknownastheputpointerthatpointstothelocationwherethenextelementhastobewritten.Finally,fstream,inheritsboth,thegetandtheputpointers,fromiostream(whichisitselfderivedfrombothistreamandostream).Theseinternalstreampointersthatpointtothereadingorwritinglocationswithinastreamcanbemanipulatedusingthefollowingmemberfunctions:

tellg()andtellp()

Thesetwomemberfunctionshavenoparametersandreturnavalueofthemembertypepos_type,whichisanintegerdatatyperepresentingthecurrentpositionofthegetstreampointer(inthecaseoftellg)ortheputstreampointer(inthecaseoftellp).

seekg()andseekp()

Thesefunctionsallowustochangethepositionofthegetandputstreampointers.Bothfunctionsareoverloadedwithtwodifferentprototypes.Thefirstprototypeis:

seekg(position);seekp(position);

Usingthisprototypethestreampointerischangedtotheabsolutepositionposition(countingfromthebeginningofthefile).Thetypeforthisparameteristhesameastheonereturnedbyfunctionstellgandtellp:themembertypepos_type,whichisanintegervalue.Theotherprototypeforthesefunctionsis:

seekg(offset,direction);seekp(offset,direction);

Usingthisprototype,thepositionofthegetorputpointerissettoanoffsetvaluerelativetosomespecificpointdeterminedbytheparameterdirection.offsetisofthemembertypeoff_type,whichisalsoanintegertype.Anddirectionisoftypeseekdir,whichisanenumeratedtype(enum)thatdeterminesthepointfromwhereoffsetiscountedfrom,andthatcantakeanyofthefollowingvalues:

ios::beg	offsetcountedfromthebeginningofthestream
ios::cur	offsetcountedfromthecurrentpositionofthestreampointer
ios::end	offsetcountedfromtheendofthestream

Thefollowingexampleusesthememberfunctionswehavejustseentoobtainthesizeofafile:

//obtainingfilesize#include<iostream>#include<fstream>usingnamespacestd;intmain(){longbegin,end;ifstreammyfile("example.txt");begin=myfile.tellg();myfile.seekg(0,ios::end);end=myfile.tellg();myfile.close();cout<<"sizeis:"<<(end-begin)<<"bytes.\n";return0;} [/code]

sizeis:40bytes.

Binaryfiles

Inbinaryfiles,toinputandoutputdatawiththeextractionandinsertionoperators(<<and>>)andfunctionslikegetlineisnotefficient,sincewedonotneedtoformatanydata,anddatamaynotusetheseparationcodesusedbytextfilestoseparateelements(likespace,newline,etc...).Filestreamsincludetwomemberfunctionsspecificallydesignedtoinputandoutputbinarydatasequentially:writeandread.Thefirstone(write)isamemberfunctionofostreaminheritedbyofstream.Andreadisamemberfunctionofistreamthatisinheritedbyifstream.Objectsofclassfstreamhavebothmembers.Theirprototypesare:

write(memory_block,size);read(memory_block,size);

Wherememory_blockisoftype"pointertochar"(char*),andrepresentstheaddressofanarrayofbyteswherethereaddataelementsarestoredorfromwherethedataelementstobewrittenaretaken.Thesizeparameterisanintegervaluethatspecifiesthenumberofcharacterstobereadorwrittenfrom/tothememoryblock.

//readingacompletebinaryfile#include<iostream>#include<fstream>usingnamespacestd;ifstream::pos_typesize;char*memblock;intmain(){ifstreamfile("example.bin",ios::in|ios::binary|ios::ate);if(file.is_open()){size=file.tellg();memblock=newchar[size];file.seekg(0,ios::beg);file.read(memblock,size);file.close();cout<<"thecompletefilecontentisinmemory";delete[]memblock;}elsecout<<"Unabletoopenfile";return0;}

thecompletefilecontentisinmemory

Inthisexampletheentirefileisreadandstoredinamemoryblock.Let'sexaminehowthisisdone:First,thefileisopenwiththeios::ateflag,whichmeansthatthegetpointerwillbepositionedattheendofthefile.Thisway,whenwecalltomembertellg(),wewilldirectlyobtainthesizeofthefile.Noticethetypewehaveusedtodeclarevariablesize:

ifstream::pos_typesize;

ifstream::pos_typeisaspecifictypeusedforbufferandfilepositioningandisthetypereturnedbyfile.tellg().Thistypeisdefinedasanintegertype,thereforewecanconductonitthesameoperationsweconductonanyotherintegervalue,andcansafelybeconvertedtoanotherintegertypelargeenoughtocontainthesizeofthefile.Forafilewithasizeunder2GBwecoulduseint:

12	intsize;size=(int)file.tellg();

Oncewehaveobtainedthesizeofthefile,werequesttheallocationofamemoryblocklargeenoughtoholdtheentirefile:

memblock=newchar[size];

Rightafterthat,weproceedtosetthegetpointeratthebeginningofthefile(rememberthatweopenedthefilewiththispointerattheend),thenreadtheentirefile,andfinallycloseit:

123	file.seekg(0,ios::beg);file.read(memblock,size);file.close();

Atthispointwecouldoperatewiththedataobtainedfromthefile.Ourprogramsimplyannouncesthatthecontentofthefileisinmemoryandthenterminates.

BuffersandSynchronization

Whenweoperatewithfilestreams,theseareassociatedtoaninternalbufferoftypestreambuf.Thisbufferisamemoryblockthatactsasanintermediarybetweenthestreamandthephysicalfile.Forexample,withanofstream,eachtimethememberfunctionput(whichwritesasinglecharacter)iscalled,thecharacterisnotwrittendirectlytothephysicalfilewithwhichthestreamisassociated.Insteadofthat,thecharacterisinsertedinthatstream'sintermediatebuffer.Whenthebufferisflushed,allthedatacontainedinitiswrittentothephysicalmedium(ifitisanoutputstream)orsimplyfreed(ifitisaninputstream).Thisprocessiscalledsynchronizationandtakesplaceunderanyofthefollowingcircumstances:Whenthefileisclosed:beforeclosingafileallbuffersthathavenotyetbeenflushedaresynchronizedandallpendingdataiswrittenorreadtothephysicalmedium.Whenthebufferisfull:Buffershaveacertainsize.Whenthebufferisfullitisautomaticallysynchronized.Explicitly,withmanipulators:Whencertainmanipulatorsareusedonstreams,anexplicitsynchronizationtakesplace.Thesemanipulatorsare:flushandendl.Explicitly,withmemberfunctionsync():Callingstream'smemberfunctionsync(),whichtakesnoparameters,causesanimmediatesynchronization.Thisfunctionreturnsanintvalueequalto-1ifthestreamhasnoassociatedbufferorincaseoffailure.Otherwise(ifthestreambufferwassuccessfullysynchronized)itreturns0.使用seekg,seekp时要小心，如f.seekg(n);当n大于文件大小时会出现莫名其妙错误，正确用法是用tellg或者tellp求出文件大小，如下例：

stringstr1="HelloWorld.\n";stringstr2="WelcometoBosch.yumen\n";f.open("liaoStr.txt",fstream::out);f<<str1;f<<str2;f.clear();f.close();f.open("liaoStr.txt",fstream::in);intbegin=0;f.seekg(0,ios_base::end);//定位到文件末尾intlen=f.tellg();//计算文件大小while(!f.eof())//用f.good()更好{intoffset=begin*12;if(offset>len)break;f.seekg(offset,ios_base::beg);f>>str1;cout<<str1<<endl;begin++;}f.close();

来源：http://www.cplusplus.com/doc/tutorial/files/参考中文博客：http://www.cppblog.com/saga/archive/2007/06/19/26652.htmlhttp://blog.csdn.net/typecool/article/details/5863247