des@amara.uucp (Dave Steinhoff) (05/25/90)
I ran into this case in a library someone else developed with a C++1.2
version of cfront. I am using SUNC++2.0. The authors apparently got
a different result than I am getting, because their software works. I
apologize if this has been discussed before.
In the program listed below, the compiler warns me:
"conversion of lvalue needed: temporary used to initialize reference"
on the call 'f(i2);' It does indeed make a local copy of the pointer,
which I don't understand. I would have expected it to be handled as a
reference parameter with the actual argument's address passed through,
even though it is a pointer to a derived class.
When I change the call to 'f((Int*)i2);' as shown in the comment
below, the warning message goes away, but the same behavior occurs.
Can anyone explain this, or is the subclass-pointer-reference
mechanism baroquen...
------------------------------
#include <iostream.h>
class Int {};
class Int2 : public Int {};
void f (Int *& i)
{ i = new Int;}
void f2 (Int2 *& i)
{ i = new Int2;}
void f3 (Int **i)
{ *i = new Int;}
main ()
{Int2* i2 = new Int2;
Int * i1 = new Int;
cout << (long) i2 << " " << (long) i1 << "\n";
f (i2); f(i1); // f(i2) -> f((Int*)i2;
cout << (long) i2 << " " << (long) i1 << "\n";
f3 ((Int**)&i2);
cout << (long) i2 << "\n";
f2(i2);
cout << (long) i2 << "\n";
}
/* ------------------------------
Output with f(i2) and f((Int*)i2) is identical:
139832 139840
139832 140884
140892
140900
------------------------------ */
---------------------------------------------
Cheers,
Dave
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Dave Steinhoff Applied Dynamics, Int'l
des@amara.UUCP 3800 Stone School Rd.
des@adi.com Ann Arbor, Mi 48108
...uunet!amara!des (313)973-1300
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