C++ - Swap two objects *without* using an equals operator - c++

Question

So I had a coding interview a week ago for an advanced C++ position where the question was basically asking me to swap two insantiations of a class called Line. Which is basically just a container for a std::vector<int>. But the equals operator was private so I couldn't swap the variables normally. It also had a weirdly unimplemented function called "swap" that was just there. No implementation or anything. I have spent the past week going over the question in my head having no idea how to tackle this. I'm by no means a c++ expert, but I am also not a complete newbie. I however have not come up with anything useful. Was it asking me to subclass the Line class? Manipulate pointers? I have no clue.
Before closing and downvoting the question please note that the main and Line class were unmodifiable
Here is the code I got:
#include <iostream>
#include <vector>
// Unmodifiable Code:
class Line {
std::vector<int> people;
void operator=(Line& other);
public:
Line(std::vector<int> p) : people(p) {}
void print_ids() {
for (auto i = this->people.begin(); i != this->people.end(); ++i) {
std::cout<<*i;
}
}
void swap(Line& other);
};
// End Unmodifiable Code.
// THIS IS WHAT I HAD TO IMPLEMENT:
void switch_lines(Line& line1, Line& line2) {
// TODO: This.
}
// Unmodifiable Code
int main(int argc, char** argv) {
std::vector<int> people1 = std::vector<int>();
people1.push_back(12);
people1.push_back(3);
people1.push_back(4);
people1.push_back(5);
std::vector<int> people2 = std::vector<int>();
people2.push_back(7);
people2.push_back(8);
people2.push_back(9);
auto line1 = Line(people1);
auto line2 = Line(people2);
std::cout<<"The Lines Before:\n";
line1.print_ids();
line2.print_ids();
switch_lines(line1, line2);
std::cout<<"The Lines After:\n";
line1.print_ids();
line2.print_ids();
}
// End Unmodifiable Code.
Here is what I can think of and why it can't work:
Swapping the internal vectors: doesn't work because the people field is inaccessible and the Line class is unmodifiable
Swapping the Line objects themselves: I have tried swapping their internal addresses but all I can change is newly created pointers to the objects, not the original variables
Subclassing the Line class: I have tried, but it doesn't help because I can't access the people field or the = operator.

Answer 1

The challenge was to modify the switch_lines method.
Simply call the member function swap there:
line1.swap(line2);
Swap two objects without using an equals operator
If you need to implement Line::swap, you can simply std::swap each sub object:
void Line::swap(Line& other) {
std::swap(people, other.people);
}