callbacks description

7/25/2019 Callbacks description

1/12

NATIONALENERGYRESEARCHSCIENTIFICCOMPUTINGCENTER

harles Le ett

CallbacksCallbacks

May 2 2003


2/12

2Charles Le ett

CallbacksCallbacks

In C, callbacks are easy - one can store the address

of a callback functon n a !arable.

In C"", t#s trcker - $hle one can get the addressof global functons, statc ob%ects, and statc

&e&ber functons of ob%ects, t#s &ore dffcult toe'tract the actual address of a non-statc &e&berfuncton of a non-statc ob%ect.

Most general(ed callback fra&e$orks n C"" &akeuse of nhertance to defne an access )ont to anob%ect#s &e&ber functons.


3/12

3Charles Le ett

libsigc++libsigc++

C"" lbrary *not %ust header fles+

Can handle &e&ber functon of any ob%ect der!ed fro& gCb%ect functon ob%ect der!ed fro& gCb%ect statc, global or frend functon statc functon ob%ect &e&ber functon to a statc ob%ect

three se)arate enttes n!ol!ed n &echans& sender rece!er so&eone &akng connecton bet$een the t$o


4/12


5/12

5Charles Le ett

Signal ImplementationSignal Implementation

gnals are C"" functon ob%ects

1 connecton fro& a sgnal to a *&e&ber+ functon&atchng sgnal5s nterface can be &ade

If the functon s a &e&ber functon

Callng a sgnal looks e'actly lke callng a nor&al C""functon

signal buttonPressed;

void my_function(int param1, float param2);buttonPressed.connect(slot(my_function));

!y"lass myob#ect;

buttonPressed.connect(slot(myob#ect,!y"lass$$my_function));

buttonPressed(1%, 2%.%);


6/12

6Charles Le ett

Mismatched argument types and countsMismatched argument types and counts

Can also connect functons to sgnals $th dfferent

argu&ents or return ty)es usng ada)ters bygeneratng ne$ slot ob%ects that $ra) around ane'stng slot

&

slot1, 2 etra);

slot2);

slot2);

-


7/127Charles Le ett

Handling Return ValuesHandling Return Values

6hat s the return !alue of a gnal that n turn

calls &any dfferent callbacks that each return adfferent !alue7 lbrary offers a Marshal ob%ect, taken as an e'tra

te&)late argu&ent to a sgnal, that allo$s)rocessng of return codes.

/he &arshallng functon $ll be called $th thereturn !alue of each slot connected to the sgnal asthey are called. /he e&sson )rocess can besto))ed by the &arshallng functon returnng true.


8/128Charles Le ett

Callbacks Without InheritanceCallbacks Without Inheritance

6hat f you need to )erfor& a callback to a &e&ber

functon of a class that sn#t statc, and doesn#tnhert fro& a base class7 /he )roble& s that there s no $ay n C"" to

dentfy the e'act address of a &e&ber functon.

ee&s $erd doesn#t t7 /he co&)ler kno$s theaddress - t has to be able to call t, but there s no$ay to ds)lay ths !alue.

r s there7 6e can use bnd*+, to bnd a &e&ber functon

declaraton to an nstantated ob%ect, and then callt, but $e need a $ay of un8uely dentfyng each

such bound functon.


9/129Charles Le ett

Binding FunctionsBinding Functions

It#s easer to use the boostbnd*+ functon than

the stl bnd, but t essentally does the sa&e thngclass "/ &public$float fcn(int _i) & return _i0.; --;

& "/ cb1; boost$$function< float,int > bfcn

(boost$$bind("/$$fcn, cb1, _1));

cout


10/1210Charles Le ett

Binding FunctionsBinding Functions

9ut once t#s bound to a boostfuncton, there#s no

$ay to dentfy the ob%ects that $ent nto t.

6e need to fnd a $ay to un8uely dentfy anfuncton:ob%ect )ar, and assocate t $th the

boostfuncton.

;asy to dentfy the ob%ect - %ust use the address. 9ut $hat about the functon7 Can#t )rnt out

C9=fcn - t#s not statc& cout



Function ddressesFunction ddresses

/urns out, that )rntf s &uch s&arter than cout

/hs s actually de)endent on the co&)ler used. gcc2.? can#t dstngush bet$een the non-!rtual

functons, but gcc 3.2 can

class "/ &public$ float fcn1(int) &-; float fcn2(int) &-; virtual float fcn(int) &-; virtual float fcn3(int) &-;

-;& printf (45 5 5 56n7, "/$$fcn1, "/$$fcn2 "/$$fcn, "/$$fcn3);-

output>1 8 9%323 9%3283



!utting it all together!utting it all together

o$ $e can un8uely dentfy any &e&ber functon,

usng the address of the ob%ect, and the address ofthe functon returned by s)rntf. 6e can thenassocate ths nfor&aton $th the address of theboostfuncton, and nd!dually call any functon at$ll, $thout ha!ng to resort to nhertance or

statc functons.

callbacks description

Documents