The easy way to handle error - initial value of reference to non-const must be an lvalue

Question

I am tired of fixing the problem: initial value of reference to non-const must be an lvalue in the cpp; even if I get the reference from another forum, it still returns an error:

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

To identify the problem, I will show you the detail here:

#include "stdafx.h"
 #include <iostream>
 
 using namespace std;
 
 void test(float *&x){
  
  *x = 1000;
 }
 
 int main(){
  float nKByte = 100.0;
  test(&nKByte);
  cout << nKByte << " megabytes" << endl;
  cin.get();
 }

How do I do that? Could you support me in improving this problem?

dttutoria · Accepted Answer

The cause: You are informing the compiler that you intend to change the value of a pointer when you send it by a non-const reference. The compiler believes that even if your code does not currently accomplish that, it will do it. That is why the error: initial value of reference to non-const must be an lvalue occurs. Solution: Declare x a constant to correct this issue:

// This tells the compiler that you are not planning to modify the pointer
// passed by reference
void test(float * const &x){
*x = 1000;
}

Another way, prior to running test, create a variable to which you attach a pointer to nKByte:

float nKByte = 100.0;
// If "test()" decides to modify `x`, the modification will be reflected in nKBytePtr
float *nKBytePtr = &nKByte;
test(nKBytePtr);

Julia Huguet · Answer

The &nKByte creates an unconstrained temporary value that cannot be bound to any non-const.

You can change void test(float *&x) into void test(float * const &x), or drop the pointer entirely and use void test(float &x); /*...*/ test(nKByte);.

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