An interesting follow-up is why ?
In C++98 was the Rule of Three:
Should you define any one of the following, you should define all three:
- destructor
- copy constructor
- copy assignment operator
This rule of thumb was created because many people only thought about releasing the resource held within the destructor, and forgot the consequences this special behavior had on copies.
When C++11 came around the corner, a number of people argued that this issue was caused by the default definition provided by the language, and that it would have been better, in hindsight, not to provide them by default. Of course, C provides them by default (for struct
) so...
... some suggested that in fact the Rule of Three could be enforced by the compiler; or at least, since changing the existing behavior might break existing code, that a pendant to the Rule of Three could be enforced by the compiler whenever talking about move constructor or move assignment operator (which guarantees new C++11 code).
The Rule of Five:
Should you define any one of the following, you should define all five:
- destructor
- move constructor
- move assignment operator
- copy constructor
- copy assignment operator
was thus nearly fully implemented as:
- if you define a move constructor or move assignment operator, the other 4 methods are implicitly deleted (unless you provide them)
- if you define a destructor, copy constructor or copy assignment operator, the move constructor and move assignment operator are implicitly deleted (unless you provide them)
the second statement is slightly incomplete for backward compatibility reasons with existing C++98 code (which should compile as C++11 without a change of behavior).