...must be lavalue warning
Thelma Lubkin
warning: initial value of reference to non-const must be an lvalue?
I include a very short piece of code that demonstrates the problem.
I want to do arithmetic in a formulaic manner, w/o having to provide
an lvalue for each intermediate calculation: I have lots of code that
generates such warnings but works fine; I'd rather not have pages of
warnings for every program run and the fear that a 'warning' will
someday turn on me and cause problems.
--thanks
thelma
//*********************************************************************
// CODE
//*********************************************************************
#include <iostream.h>
class ostream; //Need for Overloading <<
class complex {
double re, im; //Private members of class
public:
complex() { re=0.0; im=0.0; } //Empty Constructor
complex(double r, double i=0.0) //Constructor from 2 doubles
{ re=r; im=i; }
friend ostream& operator<<(ostream&, complex&);
friend inline complex operator+(complex, complex);
};
inline complex operator+(complex a1, complex a2) //Add 2 complex numbers
{ return complex(a1.re+a2.re, a1.im+a2.im); }
ostream& operator<<(ostream& os, complex& cnum) //Output a complex number
{ os << "(" << cnum.re << "," << cnum.im << ") "; return os; };
int main(void)
{ complex a(1,2), b(3,4); //Define complex numbers
cout <<"a=" << a <<",b=" << b <<" a+b = " << a+b << endl; //Print sum
}
//*********************************************************************
// MAKEFILE
//*********************************************************************
ctest.cc -o ctest
//*********************************************************************
// WARNING on RUNNING MAKEFILE
//*********************************************************************
Script started on Sun Dec 6 14:02:02 1998329
/u/thelma/eliwork/longtrain/Robin/Robin_clean> makctest
"ctest.cc", line 24: warning: initial value of reference to non-const must be an lvalue
cout <<"a=" << a <<",b=" << b <<" a+b = " << a+b << endl; //Print sum
^
//*********************************************************************
// OUTPUT of RUN
//*********************************************************************
a=(1,2) ,b=(3,4) a+b = (4,6)
331 /u/thelma/eliwork/longtrain/Robin/Robin_clean> exit
script done on Sun Dec 6 14:02:13 1998
Biju Thomas
>
> What can I do to get rid of this warning ---
>
> warning: initial value of reference to non-const must be an lvalue?
>
This warning means that you are taking a non-const reference to a
temporary variable. This temporary is going to die shortly, and, after
that, the reference will be invalid. For example, the following is
illegal:
int a = 0, b = 0;
int& i = a + b;
For standard conforming compilers, this should give an error.
> I want to do arithmetic in a formulaic manner, w/o having to provide
> an lvalue for each intermediate calculation: I have lots of code that
> generates such warnings but works fine;
It may work in case if you are not using the reference after the death
of the temporary. But, don't depend upon this construct.
> I'd rather not have pages of
> warnings for every program run and the fear that a 'warning' will
> someday turn on me and cause problems.
It will surely cause problems some day.
> friend ostream& operator<<(ostream&, complex&);
> ostream& operator<<(ostream& os, complex& cnum) //Output a complex number
In this function, the last parameter should be 'const complex&'. This
will make the warning go away.
> "ctest.cc", line 24: warning: initial value of reference to non-const must be an lvalue
> cout <<"a=" << a <<",b=" << b <<" a+b = " << a+b << endl; //Print sum
> ^
The warning comes becase (a+b) is a temporary. Once you make the change
suggested above, it should work, since it is correct to bind a const
reference to a temporary.
Regards,
Biju Thomas