Error in assignment operator overloading in C++ - c++

Question

class Array
{
private:
int *ptr;
int size;
public:
Array& operator = (const Array& rhs);
};
Array& Array::operator = (const Array& rhs)
{
ptr = new int [rhs.size];
size = rhs.size;
for( int i = 0; i < size; i++ )
ptr[i] = rhs.ptr[i];
return *this;
}
int main()
{
Array a;
Array b;
a = b;
}
What are problems with this code?
First thing I can observe is that size's value have not been defined. So there will be an error.
size is private so I can't assign it a value from main, yet that is a parameter of assignment operator. Where do I assign the value?
I tried to assign the value of rhs.size inside the definition of the = operator but then it threw segmentation fault. Why?
I don't understand what are they intending to do with this for loop:
for( int i = 0; i < size; i++ )
ptr[i] = rhs.ptr[i];
I want to understand what is happening with this code.

Answer 1

Before we can talk about the operator=, we need to realise that the class is fundamentally broken: it has no working constructor, and it is violating the rule of three, and is therefore not managing its resources properly.
After these two points have been fixed, (but really only after that!), we can turn to the implementation of operator=. Let’s first address your three points:
First thing I can observe is that size's value have not been defined. So there will be an error.
Once the class has a working constructor, size will have a defined value.
size is private so I can't assign it a value from main, yet that is a parameter of assignment operator. Where do I assign the value?
In the constructor.
I tried to assign the value of rhs.size inside the definition of the = operator but then it threw segmentation fault. Why?
How did you assign it? What value did you try to give it? Since rhs is const& (as it should be!), you can’t assign to its members — this will result in a compile-time error, not in a segmentation fault. In fact, the size of rhs is part of the precondition of the function. It must not be modified (hence the use of const&).
Here are some more things that are wrong with the function:
It does not guard against self-assignment (what if this == &rhs?).
It does not deallocate previously-allocated memory in ptr.

Answer 2

With no constructor there's no way to allocate memory in the first place to copy, no way to assign size, and so on. You need to add that capability to test your copy feature.
I've sketched in some more code here to get this more operational, though there's obviously still a lot of work to do:
#include <iostream>
class Array
{
private:
int *ptr;
int size;
public:
// Simple default constructor
Array(int size = 0) : ptr(new int[size]), size(size) {
}
int getSize() const { return size; };
Array& operator=(const Array& rhs);
};
Array& Array::operator=(const Array& rhs)
{
// Delete any previous allocation
delete[] ptr;
ptr = new int[rhs.size];
size = rhs.size;
for( int i = 0; i < size; i++ )
ptr[i] = rhs.ptr[i];
return *this;
}
int main()
{
Array a(5);
Array b;
std::cout << "a size=" << a.getSize() << std::endl;
b = a;
std::cout << "b size=" << b.getSize() << std::endl;
}
Keep in mind it's called operator= as one word, not operator = all spaced out. The syntax doesn't care, but programmers do as it parses visually in a whole different way.
Note that this is a long way from idiomatic C++, like using size_t is preferable for "size" type arguments, but there's a long, long list of things you'll need to pick up and learn as you go.