Reference Member variables
in [C++]
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From: Michael Aaron Safyan on 28 Mar 2008 20:03 smawsk wrote: > Hi, > > I am writing a sample program to use a reference member variable. But > I was surprised to know that the below piece of code is not working as > I expected. > Can some one let me know what's wrong with this code? > > class X > { > public: > X(int x) : ref(x) > { > > } > void Display() const > { > > cout<<ref<<endl; > } > > private: > int& ref; > }; > > int main() > { > int c = 10; > X x(c); > x.Display(); // This function does not print 10. Instead some junk > value > > return 0; > } > > Why So? Am I missing some basic point regarding the working of > References here? > > Thanks in advance.... > > Warm Regards. > A reference must be initialized from a reference or pointer object. X::X(int) does not take a reference or pointer. What you are doing is initializing X::ref with the address of "x" in X::X(int), which is located in the stack frame of the function X::X(int) and which becomes invalid when X::X(int) has completed execution. Consider the following: int c = 10; --- c |10 | --- X x(c); --- c |10 | --- x |10 | <= Not the same address as c! --- ref|&x | <= Not pointing to c! --- // After execution of X x(c); --- c |10 | --- ref|? | <= The object "x" no longer exists! This references is invalid. --- // ... You get the point. What I think you intend to do is: class X { public: X(int& x) : _ref(&x) {} ~X(){} int get()const{ return _ref: } void print(){ std::cout<<get()<<std::endl; } private: X& operator=(const X& o){ return *this; } int& _ref; }; int main(int argc, char* argv[]) { int c = 10; X x(c); x.print(); return 0; } { Please convert tab characters to spaces before posting. -mod } -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Tom on 28 Mar 2008 20:04 On 28 Mrz., 18:09, smawsk <sk.sma...(a)gmail.com> wrote: > Why So? Am I missing some basic point regarding the working of > References here? Nice one. Didn't get it until I debugged it. Note the constructor: X(int x) : ref(x) You're intialize "ref" with "x". But here, x is already a local varible. It's not the one from the main function. And the local variable is getting destroyed soon and your reference is bogus. What you want is this: X(int &x) : ref(x) That should work. Tom ---------------------------------- Eliminar duplicados en Outlook: http://www.easy2sync.com/es/produkte/1-Click-Duplicate-Delete-Outlook.php -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Mateusz Adamczyk on 28 Mar 2008 20:14 On 28 Mar, 18:09, smawsk <sk.sma...(a)gmail.com> wrote: > X(int x) : ref(x) > { In the code above, You create reference to temporary variable x, which will be probably destroyed before destruction of a x object. If Your object should hold reference to a c variable, You should write: X(int& x) : ref(x){} and everything should be all right. Regards, Mateusz Adamczyk -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Matthew Gilbert on 28 Mar 2008 20:13 On Mar 28, 1:09 pm, smawsk <sk.sma...(a)gmail.com> wrote: > X(int x) : ref(x) x is passed by value to X's constructor. Therefore, ref(x) refers to an int that disappears at the end of the constructor. You could fix it by having X's constructor take an int&. That way, ref refers to c. Of course, then x is only valid for as long as c is valid. _matt -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: david on 28 Mar 2008 23:14
On 28 mar, 18:09, smawsk <sk.sma...(a)gmail.com> wrote: > Hi, > > I am writing a sample program to use a reference member variable. But > I was surprised to know that the below piece of code is not working as > I expected. > Can some one let me know what's wrong with this code? > > class X > { > public: > X(int x) : ref(x) > { > > } > void Display() const > { > > cout<<ref<<endl; > } > > private: > int& ref; > > }; > > int main() > { > int c = 10; > X x(c); > x.Display(); // This function does not print 10. Instead some junk > value > > return 0; > > } > > Why So? Am I missing some basic point regarding the working of > References here? { edits: quoted signature & clc++m banner removed please don't quote extraneous material -mod } Hello, X's constructor takes an int by value, so the member variable ref references a local (to the constructor) copy of the passed int. Further use of the reference outside the body of X's constructor is undefined behavior. Regards, David -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |