Re: [JSON[review] Andrzej's review

From: Peter Dimov (pdimov_at_[hidden])
Date: 2020-09-17 21:02:49

> Vinnie Falco wrote:
>
> > Just open an issue and explain how you think self-swap, self-move, and 
> > self-assignment should work and I will make the changes.
>
> They should be no-ops.

More specifically, self-swap should be a no-op, and assignments should be 
equivalent to:

T& operator=( T const& rhs )
{
    T(rhs).swap(*this); return *this;
}

T& operator=( T&& rhs )
{
    T(std::move(rhs)).swap(*this); return *this;
}

This is important for classes such as json::value, object, array, where 
*this may own rhs, so it's important not to destroy rhs too early. E.g.

    v = v.at(0);
    v = std::move(v.at(0));

    a = a.at(0).as_array();
    a = std::move(a.at(0)).as_array();

    o = o.at("key").as_object();
    o = std::move(o.at("key").as_object());

This is not self-assignment, but it has the same lifetime pitfalls - if you 
destroy the old contents of *this, rhs is invalidated, so you can no longer 
copy (or move) it.