non-lvalue pointer error

[Dave82]

Hi,

I'm attempting to write a simple program using an array class implemented in a header file. Whenever I compile the program, I get a "non-lvalue in assignment" error. The details are as follows:

Compiler: Bloodshed Dev-Cpp Version 4.9.9.2
Error Message: non-lvalue in assignment (location indicated in file)
Header file: arraykm.h
Main .cpp file: arraytest.cpp

Header file:
#include <iostream>
using namespace std;

//Array Class*******************************************************************

//Documentation*****************************************************************
class array
{
private:
int elements;
double *pt;

public:
static int total;
array(int size);
int getsize();
double element(int i);
static int totalarrays();

};
//Documentation*****************************************************************

//Implementation****************************************************************
array::array(int size)
{
elements = size;
pt = new double[elements

if(pt == NULL) {cout<<"failed to allocate array\n";}

total++;
}

int array::getsize() {return elements;}

double array::element(int i)
{
if(i<1 || i>elements) {cout<<"array index out of bounds\n";}
return pt[i-1];
}

int array::totalarrays() {return total;}
//Implementation****************************************************************

//Array Class*******************************************************************

Main program:
#include <iostream>
#include <arraykm.h>
using namespace std;

int main()
{
int arraysize = 30;
array a1(arraysize);

int var1 = a1.getsize();
cout<<"The array has "<<var1<<" elements.\n";

for(int i=1; i<=arraysize; i++) {a1.element(i) = i*250;} //error occurs here

for(int i=1; i<=arraysize; i++)
{double var2 = a1.element(i);
cout<<"Element "<<i<<" = "<<var2;}

return 0;
}

As far as I understand, I'm assigning the value of i*250 to a pointer pointing to the location i-1 in the allocated array. Why does the error occur?

Any help would be greatly appreciated.

Thanks,
Dave

[Disch]

As far as I understand, I'm assigning the value of i*250 to a pointer pointing to the location i-1

Not quite. element returns a double, not a pointer. That is, it returns a copy of pt[i-1]


If you want to return an assignable value, you want to return a reference. Change the function to this:

double& element(int i)  // note the & 

Also -- ew @ 1-based indexes.

[Dave82]

Ok that removed the lvalue error.
So to clarify, the function double& element(int i) returning pt[i-1] returns a pointer to its address correct?

This may be unrelatd, but when I made the following changes to the class function prototype & implementation:

Prototype change:
double& element(int i);

Implementation change:
double& array::element(int i)
{if(i<1 || i>elements) {cout<<"array index out of bounds\n";}
return pt[i-1];}

I received a linker error stating an undefined reference to array::total. The problem is no such function exists in my array class, although a public variable total does exist. What could be causing this problem?

[Disch]

returning pt[i-1] returns a pointer to its address correct?

No, it returns a reference.

Similar to a pointer, but not quite the same thing.

You could accomplish the same thing with a pointer, but then the syntax would be much uglier because you'd have to dereference the pointer.

I received a linker error stating an undefined reference to array::total. The problem is no such function exists in my array class, although a public variable total does exist. What could be causing this problem?

It's kind of weird, but static members need to be instantiated in a single source file.

Put this line in your cpp file for the class:

int array::total = 0;  // don't put it in any function 

It's similar to how you have function prototypes and function bodies. A prototype will let you call the function, but you'll get a linker error if you don't have a function body.

Similarly, static int total; is like the variable prototype, but not the variable body. You need to do the above code to give it a body.

[Dave82]

Ok, just flipped through a text on reference returns, think I get it now.

I've shifted the static variable total & the totalarray() implementation into my .cpp file & it works.

Thank you for all the help! :D

Cheers,
Dave