[boost] Re: safe_dereference()

From: Anthony Williams (anthwil_at_[hidden])
Date: 2002-09-05 05:04:35

 Hillel Y. Sims writes:
 > 
 > "Anthony Williams" <anthwil_at_[hidden]> wrote in message
 > news:15733.50724.159000.285126_at_gargle.gargle.HOWL...
 > > David B. Held writes:
 > >  >     // Requirements:
 > >  >     //   T must be default-constructible
 > >  >     template <typename T>
 > >  >     T const& safe_dereference(T* p)
 > >  >     {
 > >  >         return p ? *p : T();
 > >  >     }
 > >  >
 > >
 > > I can see the point, but this is undefined behaviour --- you are returning
 > a
 > > reference to a temporary, which as gone out of scope.
 > >
 > 
 > Why is it ok to say
 > 
 >    int main()
 >    {
 >        const string& s = func();
 >        cout << s << endl;
 >    }
 > 
 > when func is defined as
 >    string func() { return string("hello"); }
 > 
 > but then not ok if func is
 >    const string& func() { return string("hello"); } ?
 > 
 > Is it not possible for a compiler to bind the temporary directly to the
 > const & in both cases?

No. The second case is _identical_ to

const string& func()
{
    string temp("hello");
    return temp;
}

Which, returns a reference to a local variable, and which I hope you agree is
therefore undefined behaviour. Anywhere you use

const X&someRef=some_expression_resulting_in_a_temporary_X;

to extend the life of a temporary, you can write

X someVar=some_expression_resulting_in_a_temporary_X;
const X& someRef=someVar;

and you cannot tell the difference, give or take some invocations of the copy
constructor, which are optional in both cases anyway.

Anthony