When and why to use swap
What is so important about the swap functionality that it deserves its own chapter? For that matter, why even use swap, and not continue to refer to an object by its original name?
Mostly, it has to do with exception safety, which is also why we keep mentioning when swap can and cannot throw an exception.
Swap and exception safety
The most important application of swap in C++ has to do with writing exception-safe code, or, more generally, error-safe code. Here is the problem, in a nutshell—in an exception-safe program, throwing an exception should never leave the program in an undefined state.
More generally, an error condition should never leave the program in an undefined state. Note that the error does not need to be handled by means of an exception—for example, returning an error code from a function should also be handled without creating undefined behavior. In particular, if an operation causes an error, the resources already...
