Just wanted to add why this idiom is used at all, which seemed like the spirit of the original question. This idiom is used within many std library classes where swap is implemented. From http://www.cplusplus.com/reference/algorithm/swap/: Many components of the standard library (within std) call swap in an unqualified manner to allow custom overloads for non-fundamental … Read more
The copy constructor exists to make copies. In theory when you write a line like: CLASS c(foo()); The compiler would have to call the copy constructor to copy the return of foo() into c. Copy elision is a technique to skip calling the copy constructor so as not to pay for the overhead. For example, … Read more
If you define the assignment operator to take a value, you should not (need not and cannot) define the assignment operator taking an rvalue-reference. There is no point to it. In general, you only need to provide an overload taking an rvalue-reference when you need to differentiate an lvalue from an rvalue, but in this … Read more
It’s my fault. (half-kidding, half-not). When I first showed example implementations of move assignment operators, I just used swap. Then some smart guy (I can’t remember who) pointed out to me that the side effects of destructing the lhs prior to the assignment might be important (such as the unlock() in your example). So I … Read more
There are several ways to write swap, some better than others. Over time, though, it was found a single definition works best. Let’s consider how we might think about writing a swap function. We first see that containers like std::vector<> have a single-argument member function swap, such as: struct vector { void swap(vector&) { /* … Read more
Overview Why do we need the copy-and-swap idiom? Any class that manages a resource (a wrapper, like a smart pointer) needs to implement The Big Three. While the goals and implementation of the copy-constructor and destructor are straightforward, the copy-assignment operator is arguably the most nuanced and difficult. How should it be done? What pitfalls … Read more