This section discusses automatically generated copy-assignment operators, it's important to know that the generated function does not have strong exception guarantees. Therefore, if an exception is thrown during the copy-assignment, the object might end up in a state where it is only partially copied.
As with the copy-constructor and copy-assignment, the move-constructor and move-assignment can be generated by the compiler. Although some compilers allow themselves to automatically generate these functions under certain conditions (more about this later), we can simply force the compiler to generate them by using the default keyword.
In the case of the Bird class, we simply extend it like this: ...
