[c++] What is the copy-and-swap idiom?

This answer is more like an addition and a slight modification to the answers above.

In some versions of Visual Studio (and possibly other compilers) there is a bug that is really annoying and doesn't make sense. So if you declare/define your swap function like this:

friend void swap(A& first, A& second) {

    std::swap(first.size, second.size);
    std::swap(first.arr, second.arr);

}

... the compiler will yell at you when you call the swap function:

enter image description here

This has something to do with a friend function being called and this object being passed as a parameter.


A way around this is to not use friend keyword and redefine the swap function:

void swap(A& other) {

    std::swap(size, other.size);
    std::swap(arr, other.arr);

}

This time, you can just call swap and pass in other, thus making the compiler happy:

enter image description here


After all, you don't need to use a friend function to swap 2 objects. It makes just as much sense to make swap a member function that has one other object as a parameter.

You already have access to this object, so passing it in as a parameter is technically redundant.

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What is the copy-and-swap idiom?