C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  declaring a function pointer variable          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message lou zion Guest Posted: Fri Feb 25, 2005 10:31 pm    Post subject: declaring a function pointer variable hi all, i've got a class that takes a parameterless function pointer as a parameter. i want to store that function pointer in a variable and i'm trying to figure out the syntax. i came up with the stuff below, but it's telling me it can't convert from void *(void) to void(void), which i didn't think i was doing. what's the proper syntax for defining and initializing a function pointer like this? thnks a bunch, lou class TabWatcher : public QObject { public: TabWatcher( const char * name = 0, QWidget * NewFocusWidget = 0, void *funcptr()=0 ) ; ~TabWatcher(); protected: void (*FuncPtr)(); } TabWatcher::TabWatcher( const char * name, QWidget * NewFocusWidget, void *funcptr()) : QObject(parent,0), newfocus(NewFocusWidget), FuncPtr(funcptr) { } Back to top Howard Guest Posted: Fri Feb 25, 2005 11:06 pm    Post subject: Re: declaring a function pointer variable "lou zion" <illuzioner (AT) adelphia (DOT) net> wrote Quote: hi all, i've got a class that takes a parameterless function pointer as a parameter. i want to store that function pointer in a variable and i'm trying to figure out the syntax. i came up with the stuff below, but it's telling me it can't convert from void *(void) to void(void), which i didn't think i was doing. what's the proper syntax for defining and initializing a function pointer like this? thnks a bunch, lou class TabWatcher : public QObject { public: TabWatcher( const char * name = 0, QWidget * NewFocusWidget = 0, void *funcptr()=0 ) ; ~TabWatcher(); protected: void (*FuncPtr)(); } TabWatcher::TabWatcher( const char * name, QWidget * NewFocusWidget, void *funcptr()) : QObject(parent,0), newfocus(NewFocusWidget), FuncPtr(funcptr) { } I find it quite useful to define a type for my function pointers. It makes constructing the declarations much easier. For example, typedef void (*VOIDFUNC_TYPE)(); .... void bar() {...} .... void foo( VOIDFUNC_TYPE funptr ) {...} .... foo(bar); -Howard Back to top Rolf Magnus Guest Posted: Sat Feb 26, 2005 12:35 am    Post subject: Re: declaring a function pointer variable lou zion wrote: Quote: hi all, i've got a class that takes a parameterless function pointer as a parameter. i want to store that function pointer in a variable and i'm trying to figure out the syntax. i came up with the stuff below, but it's telling me it can't convert from void *(void) to void(void), which i didn't think i was doing. what's the proper syntax for defining and initializing a function pointer like this? You defined it correctly as a member variable., but the parameter looks different. Why? Quote: thnks a bunch, lou class TabWatcher : public QObject { public: TabWatcher( const char * name = 0, QWidget * NewFocusWidget = 0, void *funcptr()=0 ) ; TabWatcher( const char * name = 0, QWidget * NewFocusWidget = 0, void (*funcptr)() = 0); Quote: ~TabWatcher(); protected: void (*FuncPtr)(); } ; Back to top lou zion Guest Posted: Sat Feb 26, 2005 12:38 am    Post subject: Re: declaring a function pointer variable Quote: I find it quite useful to define a type for my function pointers. It makes constructing the declarations much easier. For example, typedef void (*VOIDFUNC_TYPE)(); ... void bar() {...} ... void foo( VOIDFUNC_TYPE funptr ) {...} ... foo(bar); i like this approach, so i gave it a whirl. class TabWatcher { public: TabWatcher(const char * name = 0, QWidget * NewFocusWidget = 0, VOIDFUNC_TYPE funcptr=0 ) ; ~TabWatcher(); protected: QWidget *newfocus; VOIDFUNC_TYPE FuncPtr; } TabWatcher::TabWatcher(const char * name, QWidget * NewFocusWidget, VOIDFUNC_TYPE funcptr) : newfocus(NewFocusWidget), FuncPtr(funcptr) { } this compiles and runs fine so long as i don't make a tabwatcher class adding an actual function pointer. in another class 'mainclass' i have a function: void mainclass::passfunc() { } again, compiles and runs fine. in my constructor of mainclass i call: TabWatcher *newptr = new TabWatcher( 0, rbHomePriceLocked, passfunc ); this doesn't compile, spitting up the error: cannot convert parameter from 'void (void)' to 'VOIDFUNC_TYPE' if i try casting it like this: TabWatcher *newptr = new TabWatcher( 0, rbHomePriceLocked, (VOIDFUNC_TYPE) passfunc ); i then get the error: cannot convert from 'overloaded-function' to 'VOIDFUNC_TYPE' well, the function is not overloaded. what can i try now? thnx again! lou Back to top lou zion Guest Posted: Sat Feb 26, 2005 1:07 am    Post subject: Re: declaring a function pointer variable "Rolf Magnus" <ramagnus (AT) t-online (DOT) de> wrote Quote: i've got a class that takes a parameterless function pointer as a parameter. i want to store that function pointer in a variable and i'm trying to figure out the syntax. i came up with the stuff below, but it's telling me it can't convert from void *(void) to void(void), which i didn't think i was doing. what's the proper syntax for defining and initializing a function pointer like this? You defined it correctly as a member variable., but the parameter looks different. Why? i tried several ways of doing it and nothing was compatible when i actually tried to invoke a call to a function with a function-pointer parameter. that never seems to compile. so, i played around. i can get it to compile if i declare the parameter exactly like the variable i.e. void (*func)(), but i still can't seem to get it to compile when i try to use it in a call. i always get this: cannot convert parameter 3 from 'void (void)' to 'void (__cdecl *)(void)' using vc++ .net (without any .net). i don't believe this is a vc++ thing, though. i'm still lost. i don't know if it makes any difference, but i'm using Qt. thanks for any help. lou Back to top lou zion Guest Posted: Sat Feb 26, 2005 1:28 am    Post subject: Re: declaring a function pointer variable "lou zion" <illuzioner (AT) adelphia (DOT) net> wrote Quote: "Rolf Magnus" <ramagnus (AT) t-online (DOT) de> wrote in message news:cvofv9$l63$02$2 (AT) news (DOT) t-online.com... i've got a class that takes a parameterless function pointer as a parameter. i want to store that function pointer in a variable and i'm trying to figure out the syntax. i came up with the stuff below, but it's telling me it can't convert from void *(void) to void(void), which i didn't think i was doing. what's the proper syntax for defining and initializing a function pointer like this? You defined it correctly as a member variable., but the parameter looks different. Why? i tried several ways of doing it and nothing was compatible when i actually tried to invoke a call to a function with a function-pointer parameter. that never seems to compile. so, i played around. i can get it to compile if i declare the parameter exactly like the variable i.e. void (*func)(), but i still can't seem to get it to compile when i try to use it in a call. i always get this: cannot convert parameter 3 from 'void (void)' to 'void (__cdecl *)(void)' using vc++ .net (without any .net). i don't believe this is a vc++ thing, though. i'm still lost. i don't know if it makes any difference, but i'm using Qt. thanks for any help. lou another twist on this. if i declare a global function and pass it, it compiles fine. there's something about passing a class function that's messing it up. lou Back to top Rennie deGraaf Guest Posted: Sat Feb 26, 2005 1:37 am    Post subject: Re: declaring a function pointer variable lou zion wrote: Quote: hi all, i've got a class that takes a parameterless function pointer as a parameter. i want to store that function pointer in a variable and i'm trying to figure out the syntax. i came up with the stuff below, but it's telling me it can't convert from void *(void) to void(void), which i didn't think i was doing. what's the proper syntax for defining and initializing a function pointer like this? I've tried to do the same thing, and decided that it's a lot easier to pass fuctors (objects with the call operator overloaded) around, rather than proper functions. I other words, define a class like this (untested): class Func { public: void operator() () const; } void Func::operator() () const { // do some stuff here } Now, you can pass objects of type Func around like objects, and call them like functions. This shouldn't cause any significant performance hit, and is much easier to code. Rennie Back to top Rolf Magnus Guest Posted: Mon Feb 28, 2005 12:12 pm    Post subject: Re: declaring a function pointer variable lou zion wrote: Quote: "lou zion" <illuzioner (AT) adelphia (DOT) net> wrote in message news:5OudnaVf4tQrVoLfRVn-jA (AT) adelphia (DOT) com... "Rolf Magnus" <ramagnus (AT) t-online (DOT) de> wrote in message news:cvofv9$l63$02$2 (AT) news (DOT) t-online.com... i've got a class that takes a parameterless function pointer as a parameter. i want to store that function pointer in a variable and i'm trying to figure out the syntax. i came up with the stuff below, but it's telling me it can't convert from void *(void) to void(void), which i didn't think i was doing. what's the proper syntax for defining and initializing a function pointer like this? You defined it correctly as a member variable., but the parameter looks different. Why? i tried several ways of doing it and nothing was compatible when i actually tried to invoke a call to a function with a function-pointer parameter. that never seems to compile. so, i played around. i can get it to compile if i declare the parameter exactly like the variable i.e. void (*func)(), but i still can't seem to get it to compile when i try to use it in a call. i always get this: cannot convert parameter 3 from 'void (void)' to 'void (__cdecl *)(void)' using vc++ .net (without any .net). i don't believe this is a vc++ thing, though. i'm still lost. i don't know if it makes any difference, but i'm using Qt. thanks for any help. lou another twist on this. if i declare a global function and pass it, it compiles fine. there's something about passing a class function that's messing it up. You mean a non-static member function? Those are different from other functions, because they need an object they are called for, even when called through a pointer. Therefore, a pointer a to non-static member function is not compatible to a regular function pointer. An example of how to use them: #include <iostream> struct Foo { void print(int x) { std::cout << "The value is " << (i == x ? "equal to " : "not equal to ") << x << 'n'; } int i; }; int main() { Foo obj = { 42 }; void (Foo::* func)(int) = &Foo::print; (obj.*func)(10); } Btw: what do you need the function pointers for? 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