Templated return value "is being used without being initialized"
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From: red floyd on 29 Nov 2009 11:11 ManicQin wrote: > Hi, > It would be nice if somebody would be able to give be a pointer here. > I have a function with a templated return type. > > for example: > template <class T> > T foo() > { > T retVal; > > //Just for an example > retVal += 10; > > return retVal; > } [redacted] > > Is there a way to initialize the val? > Is there a different way to work in order to overcome this problem? > T retval = T(); -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: CornedBee on 29 Nov 2009 11:13 On Nov 29, 8:06 pm, ManicQin <manic...(a)gmail.com> wrote: > I cannot predict what type will be used, it could be a P.O.D. or any > class that implements +=, and I cant write > T retval(0); in order to initialize the variable. > > Is there a way to initialize the val? T retval = T(); Sebastian -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Johannes Schaub (litb) on 29 Nov 2009 21:15 Johannes Schaub (litb) wrote: > ManicQin wrote: > >> Hi, >> It would be nice if somebody would be able to give be a pointer here. >> I have a function with a templated return type. >> >> for example: >> template <class T> >> T foo() >> { >> T retVal; >> >> //Just for an example >> retVal += 10; >> >> return retVal; >> } >> >> Let's not dwell on the purpose of the function please, >> Basically we can use ... almost all types that "allow" the operator += >> When running the code the compiler will scream (and rightfully) >> Run-Time Check Failure #3 - The variable 'retVal' is being used >> without being initialized. >> >> I cannot predict what type will be used, it could be a P.O.D. or any >> class that implements +=, and I cant write >> T retval(0); in order to initialize the variable. >> >> Is there a way to initialize the val? >> Is there a different way to work in order to overcome this problem? >> > > Quick one, that fails if the type is non-copyable. But this doesn't > matter, as you need to copy "T" anyway if you wanna return by value: > > T retVal((T())); > > Fixing it involves some workaround to overcome the impossible "T > retVal();" object declaration syntax. > > struct { T t; } u = {}; > Haha, the irony. Should have tested that "trick". Shortly afterwards i was reminded of this bug report to GCC http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40239 . In fact the Standard requires a copy constructor for T also here when read extremely narrowly. I guess you need to write a bit more now if you don't want to imply the copy constructible requirement. Like boost::value_initialized<>: struct u_ { u_():t() {} T t; } u; That will now correctly not require a copy constructor. Have fun, and sorry for my double post :) -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Kenshin on 29 Nov 2009 21:13 On Nov 29, 10:06 pm, ManicQin <manic...(a)gmail.com> wrote: > Hi, > It would be nice if somebody would be able to give be a pointer here. > I have a function with a templated return type. > > for example: > template <class T> > T foo() > { > T retVal; > > //Just for an example > retVal += 10; > > return retVal; > > } > > Let's not dwell on the purpose of the function please, > Basically we can use ... almost all types that "allow" the operator += > When running the code the compiler will scream (and rightfully) > Run-Time Check Failure #3 - The variable 'retVal' is being used > without being initialized. > > I cannot predict what type will be used, it could be a P.O.D. or any > class that implements +=, and I cant write > T retval(0); in order to initialize the variable. > > Is there a way to initialize the val? > Is there a different way to work in order to overcome this problem? > > thanks > Do you get this error at point-of-definition, or at point-of-call? If at point of call, make sure you call it like: foo<Data-Type>(); Where Data_Type is self-explanatory. In addition CornedBee below suggests, T retval = T(), which the std libs also uses. Try that. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: ManicQin on 29 Nov 2009 21:14
On Nov 30, 6:11 am, red floyd <no.spam.h...(a)its.invalid> wrote: > > T retval = T(); > Thank you all, The thing is that I have a function that executes a "command" class on an unknown set of arguments and accumulates the results in a templated fashion... //RetMod is the method of returning the values, I can AND\XOR them like I do with my booleans //and I can concat them like I do with my strings (and some other operations I do with my objetcs //The CALL_MEMBER_FN is a macro of mine to help me extract the command from the queue and inVal is off course the val to be used. //RetMod()(retVal,CALL_MEMBER_FN(*(*iter),Functor)(inVal1)); I think I'll adopt Daniel Kr�gler's trait, A silly question: Is there any reason for me to prefer a copy constructor or assignment? // T retVal(ZeroTraits<T>::zero()); // T retVal = ZeroTraits<T>::zero(); Thanks! -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |