too many parameters for this operator function - c++

Question

First, I'm not good at english and also first time in StackOverflow, but I try to explain about my code's problem.
I was asked to make my own Vector(similar thing) from my professer, and there's a problem in fuction which returns a reference to the element at the requested position in the vector container. If the requested position is out of range, it should output some messages and terminate the program.
I should make this to Operator overloading, and this is my code.
double operator [](int n, const MyDoubleVector& _mV)//The arror come out at this line.
{
if(n > num)//'num' is private reference in class to count array. it typed int.
{
return 0;
}
return &_mV.data[n];//'data' is private reference in class. It declare like 'double *data = new double [num];'
}
I saw that sometimes 'friend' solve this, but when I put 'friend' in this line, it said me like "operator[] must be a member function."
Finally, Ihave no idea how to do. Would you please help me?

Answer 1

You need to implement the overload of operator[] as a member function of your class MyDoubleVector.
Here's the definition :
double & MyDoubleVector::operator[](int index);

Answer 2

operator [] must be defined as a member of the class.
example:
#include <iostream>
#include <cstdlib>
#include <algorithm>
struct MyDoubleVector
{
MyDoubleVector()
{}
MyDoubleVector(MyDoubleVector const& other)
{
// very naiive copy constructor
if (other.data)
{
std::for_each(other.data, other.data + other.num, [&](double val)
{
this->push(val);
});
}
}
MyDoubleVector& operator=(MyDoubleVector const& other)
{
auto temp = other; // invoke copy constructor
std::swap(num, temp.num);
std::swap(capacity, temp.capacity);
std::swap(data, temp.data);
return *this;
}
~MyDoubleVector()
{
delete [] data;
}
double& operator [](int n);
/** either define the method inline like this...
{
if(n > num)
{
std::cerr << "MyDoubleVector::operator[]: index " << n << " out of range" << std::endl;
std::exit(100);
}
return data[n];
}
**/
void push(double val)
{
if (num == capacity)
{
more();
}
data[num++] = val;
}
private:
void more()
{
if (!data)
{
data = new double [10];
capacity = 16;
}
else
{
auto newcapacity = capacity * 2;
auto newdata = new double [newcapacity];
std::copy(data, data + capacity, newdata);
std::swap(data, newdata);
capacity = newcapacity;
delete [] newdata;
}
}
int num = 0;
int capacity = 0;
double* data = nullptr;
};
/** ...
** or out of line like this
**/
double& MyDoubleVector::operator [](int n)
{
if(n > num)
{
std::cerr << "MyDoubleVector::operator[]: index " << n << " out of range" << std::endl;
std::exit(100);
}
return data[n];
}
int main()
{
MyDoubleVector v;
v.push(10.0);
v[1];
}