Why do I need = copy constructor in C++

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I was reading something on copy constructors and i saw the code had two copy constructors:
number 1 copy constructor:
Vector::Vector(const Vector& a) : elem{new double[a.sz]}, sz{a.sz} {
for (int i=0; i!=sz; ++i)
elem[i] = a.elem[i];
}

number 2 copy constructor:
Vector& Vector::operator=(const Vector& a) {

double∗ p = new double[a.sz];

for (int i=0; i!=a.sz; ++i)
p[i] = a.elem[i];

delete[] elem;

elem = p;

sz = a.sz;

return ∗this;
}

The problem is that i don't understand why you need the operator= copy constructor when you have the first one. Doesn't that already do the job?. Can you please explain this to me.

What I have tried:

i have tried reading more about copy constructors to find out about this, but i haven't come across any information helping me.

Posted 23-Oct-21 23:02pm

iwanttoaskquestions

Updated 24-Oct-21 2:12am

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OriginalGriff · Accepted Answer · 2021-10-24T00:14:00

Solution 1

Operator= isn't a constructor - it's an assignment operator: operator overloading - cppreference.com[^]

Posted 24-Oct-21 0:14am

OriginalGriff

Greg Utas · Accepted Answer · 2021-10-24T02:12:00

Further to OriginalGriff's answer, there is something known as the Rule of 3[^]. It says that if a class implements a destructor, it should also implement a copy constructor and a copy assignment operator. This became the Rule of 5 in C++11, when the move constructor and move assignment operator were added. Those are more for efficiency, but it's a good idea to implement them too.

The rationale for this is that a destructor is usually implemented because an object has resources to free. So if the object is copied, copies of its resources must usually be created too.