bernt@wolfen.cc.uow.oz (Bernt Ribbum) (09/24/90)
Hello Networld!
Anyone who could help a newcomer to C++?
I'm trying to set up an array of pointers to functions within a class,
much like the example here (for simplicity no array here):
#include <stream.h>
class test {
void f1(int i) // function to be called
{ cout<<"ok"<<i<<"\n"; }
void (test::*ptf)(int); // pointer-to-function
B
public:
test() { ptf=test::f1; } // constructor - assigns &f1 to ptf
void execute (int i)
{ (*ptf)(i); } // try to call f1()
};
main() {
test variable;
variable.execute(1);
}
My compiler (Turbo C++ V1.0) complains about the call (*ptf)(i):
-- Temporary used for parameter 'this' in function test::execute(int)
-- Type mismatch in parameter 'this' in function test::execute(int)
-- Parameter 'i' is never used in function test::execute(int)
What am I doing wrong?? It is a perfectly legal setup outside of classes
in any case! Or maybe I should use another way to do this?
Please email replies - I'll summarise here if someone has a solution!
Thanks,
--
_
/_) _ _ _ _/ Bernt Ribbum, Dept of Elec & Comp Eng, Univ of Wollongong
/_) (-'/ '/ > ( PO.Box 1144, Wollongong NSW 2500, AUSTRALIA ------------
---------------- bernt@wolfen.cc.uow.edu.au ----------------------------- landauer@morocco.Eng.Sun.COM (Doug Landauer) (09/30/90)
> Please email replies - I'll summarise here if someone has a solution! I tried to, but I got an error message back <<< 554 <bernt@wolfen.cc.uow.oz>... Unknown domain : cc . uow . oz 554 <bernt@wolfen.cc.uow.oz>... Service unavailable So, this time I'll just post it. > I'm trying to set up an array of pointers to functions within a class, > much like the example here (for simplicity no array here): > > #include <stream.h> > > class test { > void f1(int i) // function to be called > { cout<<"ok"<<i<<"\n"; } > > void (test::*ptf)(int); // pointer-to-function Whoops, your comment shows what's missing here. ptf is *not* a "pointer-to-function". Instead, it is a "pointer-to-MEMBER-function", which is a different beast altogether. > > public: > test() { ptf=test::f1; } // constructor - assigns &f1 to ptf > void execute (int i) > { (*ptf)(i); } // try to call f1() > }; > > main() { > test variable; > variable.execute(1); > } > > My compiler (Turbo C++ V1.0) complains about the call (*ptf)(i): > > -- Temporary used for parameter 'this' in function test::execute(int) > -- Type mismatch in parameter 'this' in function test::execute(int) > -- Parameter 'i' is never used in function test::execute(int) cfront 2.1 shows a little more clearly what's happening: "bernt.cc", line XX: error: pointer to member dereferenced Read the E&S ARM section about pointers to members, and your compiler should be much happier about compiling your code. In order to make your little test acceptable to cfront, all I had to do was change the call to give the ptr-to-member-fn an explicit "this" for which to call the function that it pointed at. Like so: Change the line > { (*ptf)(i); } // try to call f1() to a line like > { (this->*ptf)(i); } // try to call f1() > What am I doing wrong?? All pointers-to-members must be given an object for which to call their pointed-at functions. Even if the call is from within another member function. Ptrs-to-members are usually implemented as OFFSETs within the class, rather than as true pointers. I hope this helps.... -- Doug Landauer - Sun Microsystems, Inc. - Languages - landauer@eng.sun.com Matt Groening on C++: "Our language is one great salad."
wai@swift.cs.tcd.ie (10/02/90)
In article <1069@exodus.Eng.Sun.COM>, landauer@morocco.Eng.Sun.COM (Doug Landauer) writes: >> I'm trying to set up an array of pointers to functions within a class, >> much like the example here (for simplicity no array here): >> >> #include <stream.h> >> >> class test { >> void f1(int i) // function to be called >> { cout<<"ok"<<i<<"\n"; } >> >> void (test::*ptf)(int); // pointer-to-function > > Whoops, your comment shows what's missing here. > ptf is *not* a "pointer-to-function". Instead, > it is a "pointer-to-MEMBER-function", which is > a different beast altogether. >> >> public: >> test() { ptf=test::f1; } // constructor - assigns &f1 to ptf Is there any difference between { ptf = &test::f1; } Both of them work okay. > Read the E&S ARM section about pointers to members, and your compiler > should be much happier about compiling your code. In order to make > your little test acceptable to cfront, all I had to do was change the > call to give the ptr-to-member-fn an explicit "this" for which to call > the function that it pointed at. Like so: > > Change the line > >> { (*ptf)(i); } // try to call f1() > > to a line like > >> { (this->*ptf)(i); } // try to call f1() > >> What am I doing wrong?? > > All pointers-to-members must be given an object for which to call > their pointed-at functions. Even if the call is from within another > member function. Ptrs-to-members are usually implemented as > OFFSETs within the class, rather than as true pointers. > > I hope this helps.... > -- > Doug Landauer - Sun Microsystems, Inc. - Languages - landauer@eng.sun.com > Matt Groening on C++: "Our language is one great salad." Is there any way to creat a generic pointer to member function class and yet be able to avoid all the signature type checking stuff? Wai.