Templated return value "is being used without being initialized"
in [C++]
|
Prev: C++ Middleware Writer version 1.10 is now on line
Next: pros and cons of returning const ref to string instead of string by value
From: ManicQin on 29 Nov 2009 02:06 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 -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Goran on 29 Nov 2009 10:52 On Nov 29, 8:06 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? What, you mean, beside intializing it? No. You say "let's not dwell on the purpose..." You're right, no need for that, there __is__ no reasonable purpose to the example. How can it be useful to use uninitialized variable? What are you trying to achieve? Return supposedly random number incremented by 10? Both your compiler (I guess, through a warning), and runtime ("Run- time check failure") actually spotted a bug for you. Why not kindly fix the bug (bug being the use of an uninitialized variable)? Note also that if T is a type with a default constructor, there is no problem. You and you alone made one, by refusing to initialize, but using, a variable of an intrinsic type. Goran. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Daniel Krügler on 29 Nov 2009 10:54 On 29 Nov., 20:06, 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? I suggest to introduce a traits class that equalizes all the differences. This has the advantage that even class types with a default c'tor that corresponds to a NaN state could be adapted, e.g. start with template<typename T> struct ZeroTraits { static T zero() { return T(); } }; and specialize that only, if needed. It should work for all types where value-initialization returns the "zero" value. Now use it like this: template <class T> T foo() { T retVal(ZeroTraits<T>::zero()); //Just for an example retVal += 10; return retVal; } HTH & Greetings from Bremen, Daniel Kr�gler -- [ 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 11:10 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 = {}; It can be accessed by u.t then. In both cases, the object is value initialized, being a sensible default: constructor call for class types with a declared constructor, and recursive initialization with zero / false for non-class types. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Florian Weimer on 29 Nov 2009 11:10
> Is there a way to initialize the val? Wouldn't this work? T retVal(T()); The compiler should elide the copy-constructor call. { Note: this declares a function named retVal. -mod/sk } -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |