vector - swap two elements when elements have const members

Question

I have the following code

class a {
public:
    const int aa;
    a(int aa) : aa(aa){}
};
int main() {
    std::vector<a> v;
    v.emplace_back(1);
    v.emplace_back(2);
    v.emplace_back(3);
    v.emplace_back(4);

    std::iter_swap(v.begin() + 1, v.rbegin());

    system("pause");
    return 0;
}

I get an error when I try to swap two elements of the vector.

Error   C2280   'a &a::operator =(const a &)': attempting to reference a deleted function

I understand it's because a has a constant member, but I am not able to figure out how to get this to work.

Answer 1

The problem is that the swap changes the value of existing element. Since the elements are const, they cannot be changed hence the compiler error.

If you insist on const elements, your vector should be vector<a*> or event better vector<unique_ptr<a>>. This will allow you to swap elements without mutating the actual elements of a

Answer 2

Based on this thread following works:

#include <vector>
#include <iostream>

class Foo {
public:
    const int value;
    Foo(const int &&from) : value(std::move(from)){}
    Foo(const Foo &&other) : value(std::move(other.value)){}
    Foo & operator=(Foo && source) {
        this -> ~ Foo ();
        new (this) Foo(std::move(source));
        return *this;
    }
};

int main() {
    std::vector<Foo> v;
    v.emplace_back(1);
    v.emplace_back(2);
    v.emplace_back(3);
    v.emplace_back(4);

    std::cout << (v.begin() + 1)->value << "," <<v.rbegin()->value << std::endl;
    std::iter_swap(v.begin() + 1, v.rbegin());
    std::cout << (v.begin() + 1)->value << "," <<v.rbegin()->value << std::endl;
    return 0;
}