Confusion about an error: lvalue required as unary '&' operand

Question

I have been trying to understand pointer concepts by writing simple code, and I got an error problem, and it seems like I couldn't solve it or understand it.

#include <stdio.h>

int *foo(void);

int main(void) {
    printf("%d\n", *foo());

    return 0;
}

int *foo(void) {
    static int num = 1;
    ++num;
    return &(++num);

}

Here is the error message.

error: lvalue required as unary ‘&’ operand
  return &(++num);

Function 'foo()' returns a pointer to int, and main is supposed to be print the returned int by using * operator. For static num within foo(), I thought that by putting static qualifier, num is not temporary variable anymore, so '&' can be used to num.

same question : [Not able to understand error condition wrt lvalues](https://stackoverflow.com/questions/24471063/not-able-to-understand-error-condition-wrt-lvalues/24471125) — Sander De Dycker, Jun 24 '19 at 08:58
To fix the problem you can change the line to `++num; return &num;` — M.M, Jun 24 '19 at 09:31

score 11 · Answer 1 · answered Jun 24 '19 at 08:55

11

The result of ++num is the new value, not the variable, so you're trying to take the address of something that is not actually stored anywhere. That's the layman's explanation.

answered Jun 24 '19 at 08:55

unwind

391,730
64
469
606

Vlad from Moscow · Accepted Answer · 2019-06-24T09:12:07.547

There is a difference between C and C++ relative to the prefix increment operator ++.

In C the result is the new value of the operand after incrementation. So in this expression &(++num) there is an atttempt to get the address of a temporary object (rvalue).

In C++ the program will be correct because in C++ the result is the updated operand; it is an lvalue.

That is in C the result is a new value while in C++ the result is the updated operand.

So in C you may not for example write

++++++x;

while in C++ this expression

++++++x;

is correct and you may apply the unary operator & to the expression like

&++++++x;

To make the function correct in C you have to separate the applied operators like

int *foo(void) {
    static int num = 1;
    ++num;
    //return &(++num);
    ++num;
    return &num;

}

Confusion about an error: lvalue required as unary '&' operand

2 Answers2

Linked

Related