A function pointer, also called a subroutine pointer or procedure pointer, is a pointer that points to a function. As opposed to referencing a data value, ...
Functionpointer
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Pointerthatpointstoafunction
Afunctionpointer,alsocalledasubroutinepointerorprocedurepointer,isapointerthatpointstoafunction.Asopposedtoreferencingadatavalue,afunctionpointerpointstoexecutablecodewithinmemory.Dereferencingthefunctionpointeryieldsthereferencedfunction,whichcanbeinvokedandpassedargumentsjustasinanormalfunctioncall.Suchaninvocationisalsoknownasan"indirect"call,becausethefunctionisbeinginvokedindirectlythroughavariableinsteadofdirectlythroughafixedidentifieroraddress.
Functionpointerscanbeusedtosimplifycodebyprovidingasimplewaytoselectafunctiontoexecutebasedonrun-timevalues.
Functionpointersaresupportedbythird-generationprogramminglanguages(suchasPL/I,COBOL,Fortran,[1]dBASEdBL,andC)andobject-orientedprogramminglanguages(suchasC++,C#,andD).[2]
Contents
1Simplefunctionpointers
1.1ExampleinC
2Functors
3Methodpointers
4InC++
4.1PointerstomemberfunctionsinC++
5AlternateCandC++Syntax
5.1CandC++
5.2C++
6Seealso
7References
8Externallinks
Simplefunctionpointers[edit]
Thesimplestimplementationofafunction(orsubroutine)pointerisasavariablecontainingtheaddressofthefunctionwithinexecutablememory.Olderthird-generationlanguagessuchasPL/IandCOBOL,aswellasmoremodernlanguagessuchasPascalandCgenerallyimplementfunctionpointersinthismanner.[3]
ExampleinC[edit]
Seealso:§ AlternateCandC++Syntax
ThefollowingCprogramillustratestheuseoftwofunctionpointers:
func1takesonedouble-precision(double)parameterandreturnsanotherdouble,andisassignedtoafunctionwhichconvertscentimetrestoinches.
func2takesapointertoaconstantcharacterarrayaswellasanintegerandreturnsapointertoacharacter,andisassignedtoaCstringhandlingfunctionwhichreturnsapointertothefirstoccurrenceofagivencharacterinacharacterarray.
#include/*forprintf*/
#include/*forstrchr*/
doublecm_to_inches(doublecm){
returncm/2.54;
}
//"strchr"ispartoftheCstringhandling(i.e.,noneedfordeclaration)
//Seehttps://en.wikipedia.org/wiki/C_string_handling#Functions
intmain(void){
double(*func1)(double)=cm_to_inches;
char*(*func2)(constchar*,int)=strchr;
printf("%f%s",func1(15.0),func2("Wikipedia",'p'));
/*prints"5.905512pedia"*/
return0;
}
Thenextprogramusesafunctionpointertoinvokeoneoftwofunctions(sinorcos)indirectlyfromanotherfunction(compute_sum,computinganapproximationofthefunction'sRiemannintegration).Theprogramoperatesbyhavingfunctionmaincallfunctioncompute_sumtwice,passingitapointertothelibraryfunctionsinthefirsttime,andapointertofunctioncosthesecondtime.Functioncompute_suminturninvokesoneofthetwofunctionsindirectlybydereferencingitsfunctionpointerargumentfuncpmultipletimes,addingtogetherthevaluesthattheinvokedfunctionreturnsandreturningtheresultingsum.Thetwosumsarewrittentothestandardoutputbymain.
#include
#include
//Functiontakingafunctionpointerasanargument
doublecompute_sum(double(*funcp)(double),doublelo,doublehi){
doublesum=0.0;
//Addvaluesreturnedbythepointed-tofunction'*funcp'
inti;
for(i=0;i<=100;i++){
//Usethefunctionpointer'funcp'toinvokethefunction
doublex=i/100.0*(hi-lo)+lo;
doubley=funcp(x);
sum+=y;
}
returnsum/101.0*(hi-lo);
}
doublesquare(doublex){
returnx*x;
}
intmain(void){
doublesum;
//Usestandardlibraryfunction'sin()'asthepointed-tofunction
sum=compute_sum(sin,0.0,1.0);
printf("sum(sin):%g\n",sum);
//Usestandardlibraryfunction'cos()'asthepointed-tofunction
sum=compute_sum(cos,0.0,1.0);
printf("sum(cos):%g\n",sum);
//Useuser-definedfunction'square()'asthepointed-tofunction
sum=compute_sum(square,0.0,1.0);
printf("sum(square):%g\n",sum);
return0;
}
Functors[edit]
Mainarticle:Functionobject
Functors,orfunctionobjects,aresimilartofunctionpointers,andcanbeusedinsimilarways.Afunctorisanobjectofaclasstypethatimplementsthefunction-calloperator,allowingtheobjecttobeusedwithinexpressionsusingthesamesyntaxasafunctioncall.Functorsaremorepowerfulthansimplefunctionpointers,beingabletocontaintheirowndatavalues,andallowingtheprogrammertoemulateclosures.Theyarealsousedascallbackfunctionsifitisnecessarytouseamemberfunctionasacallbackfunction.[4]
Many"pure"object-orientedlanguagesdonotsupportfunctionpointers.Somethingsimilarcanbeimplementedinthesekindsoflanguages,though,usingreferencestointerfacesthatdefineasinglemethod(memberfunction).CLIlanguagessuchasC#andVisualBasic.NETimplementtype-safefunctionpointerswithdelegates.
Inotherlanguagesthatsupportfirst-classfunctions,functionsareregardedasdata,andcanbepassed,returned,andcreateddynamicallydirectlybyotherfunctions,eliminatingtheneedforfunctionpointers.
Extensivelyusingfunctionpointerstocallfunctionsmayproduceaslow-downforthecodeonmodernprocessors,becausebranchpredictormaynotbeabletofigureoutwheretobranchto(itdependsonthevalueofthefunctionpointeratruntime)althoughthiseffectcanbeoverstatedasitisoftenamplycompensatedforbysignificantlyreducednon-indexedtablelookups.
Methodpointers[edit]
C++includessupportforobject-orientedprogramming,soclassescanhavemethods(usuallyreferredtoasmemberfunctions).Non-staticmemberfunctions(instancemethods)haveanimplicitparameter(thethispointer)whichisthepointertotheobjectitisoperatingon,sothetypeoftheobjectmustbeincludedaspartofthetypeofthefunctionpointer.Themethodisthenusedonanobjectofthatclassbyusingoneofthe"pointer-to-member"operators:.*or->*(foranobjectorapointertoobject,respectively).
AlthoughfunctionpointersinCandC++canbeimplementedassimpleaddresses,sothattypicallysizeof(Fx)==sizeof(void*),memberpointersinC++aresometimesimplementedas"fatpointers",typicallytwoorthreetimesthesizeofasimplefunctionpointer,inordertodealwithvirtualmethodsandvirtualinheritance[citationneeded].
InC++[edit]
InC++,inadditiontothemethodusedinC,itisalsopossibletousetheC++standardlibraryclasstemplatestd::function,ofwhichtheinstancesarefunctionobjects:
#include
#include
staticdoublederivative(conststd::function&f,doublex0,doubleeps){
doubleeps2=eps/2;
doublelo=x0-eps2;
doublehi=x0+eps2;
return(f(hi)-f(lo))/eps;
}
staticdoublef(doublex){
returnx*x;
}
intmain(){
doublex=1;
std::cout<
usingnamespacestd;
classFoo{
public:
intadd(inti,intj){
returni+j;
}
intmult(inti,intj){
returni*j;
}
staticintnegate(inti){
return-i;
}
};
intbar1(inti,intj,Foo*pFoo,int(Foo::*pfn)(int,int)){
return(pFoo->*pfn)(i,j);
}
typedefint(Foo::*Foo_pfn)(int,int);
intbar2(inti,intj,Foo*pFoo,Foo_pfnpfn){
return(pFoo->*pfn)(i,j);
}
typedefint(*PFN)(int);
intbar3(inti,PFNpfn){
returnpfn(i);
}
intmain(){
Foofoo;
cout<*m)('A');
//Thisdefines'Ref',afunctionthatacceptsareference-to-'C',
//apointer-to-member-of-'C'oftype'Fn',anda'char',
//callsthefunctionandreturnstheresult
intRef(C&r,FnC::*m,charc){
return(r.*m)(c);
}//Ref(r,m,c)
//Thisdefines'Ptr',afunctionthatacceptsapointer-to-'C',
//apointer-to-member-of-'C'oftype'Fn',anda'char',
//callsthefunctionandreturnstheresult
intPtr(C*p,FnC::*m,charc){
return(p->*m)(c);
}//Ptr(p,m,c)
//LEGACY:Notethattomaintainexistingcodebases,theabovedefinitionstylecanstillbeusedfirst;
//thentheoriginaltypecanbedefinedintermsofitusingthenewstyle.
//Thisdefines'FnC',atypeofpointer-to-member-of-class-'C'oftype'Fn'
typedefFnC::*FnC;
//'FnC'canbeusedwherever'FnC::*'can
FnCfnC=&C::Member;
intRefP(C&p,FnCm,charc);
Seealso[edit]
Delegation(computing)
Functionobject
Higher-orderfunction
Proceduralparameter
References[edit]
^AndrewJ.Miller."FortranExamples".http://www.esm.psu.edu/~ajm138/fortranexamples.html.Retrieved2013-09-14.{{citeweb}}:Externallinkin|location=(help)CS1maint:location(link)
^"TheFunctionPointerTutorials".http://www.newty.de/:logo.Retrieved2011-04-13.FunctionPointersarepointers,i.e.variables,whichpointtotheaddressofafunction{{citeweb}}:Externallinkin|location=(help)
^"TheFunctionPointerTutorials".http://www.newty.de/:logo.Retrieved2011-04-13.Importantnote:Afunctionpointeralwayspointstoafunctionwithaspecificsignature!Thusallfunctions,youwanttousewiththesamefunctionpointer,musthavethesameparametersandreturn-type!{{citeweb}}:Externallinkin|location=(help)
^"Expertise:IntermediateLanguage:C++:UseFunctorforCallbacksinC++".http://www.devx.com/:DevX.com.2005-01-31.Retrieved2011-04-13.Ifyouwanttouseamemberfunctionasacallbackfunction,thenthememberfunctionneedstobeassociatedwithanobjectoftheclassbeforeitcanbecalled.Inthiscase,youcanusefunctor[withanexampleonthispage].{{citeweb}}:Externallinkin|location=(help)
Externallinks[edit]
FAQonFunctionPointers,thingstoavoidwithfunctionpointers,someinformationonusingfunctionobjects
FunctionPointerTutorials,aguidetoC/C++functionpointers,callbacks,andfunctionobjects(functors)
MemberFunctionPointersandtheFastestPossibleC++Delegates,CodeProjectarticlebyDonClugston
PointerTutorials,C++documentationandtutorials
CpointersexplainedavisualguideofpointersinC
SecureFunctionPointerandCallbacksinWindowsProgramming,CodeProjectarticlebyR.Selvam
TheCBook,FunctionPointersinCby"TheCBook"
FunctionPointersindBASEdBL,FunctionPointerindBASEdBL
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