Re: initial value of reference to non-const must be an lvalue

"Jim Langston" <>
Thu, 10 Jan 2008 21:43:21 -0800
asm23 wrote:

Hi,I need some help to clarify the warning "initial value of reference
to non-const must be an lvalue".

I'm searching in this groups to find someone has the same situation
like me. I found in the Post:

#include <iostream.h>

class ostream; //Need for Overloading <<
class complex {
        double re, im; //Private members of class
        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(,; }
ostream& operator<<(ostream& os, complex& cnum) //Output a complex

operator<< is accepting a non constant ostream& and a non constant complex&

number { os << "(" << << "," << << ") "; 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

You are attempting to pass to operator<< a non constant ostream& and a non
constant *temporary* complex& The temporary is the problem. Any changes
the function made to the temporary would be lost. A very simple fix. Change
your operator<< to:

ostream& operator<<(ostream& os, const complex& cnum)

And your problem should go away.

You need to learn const correctness. A temporary rvalue can be passes as a
const. I'm not sure of all the limitations, but it seems this is one of
them. Your clue that this is a const correctness error is given in the
error itself:
"initial value of reference to ***non-const*** must be an lvalue".



I also get the same warning in my compiler.
but I add some statement like:


int a1=2;
int b1=3;

this goes very well.

What's the difference between these two code? How can I get rid of
these warnings?


Jim Langston

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