I want to refresh my memory on the conditions under which a compiler typically auto generates a default constructor, copy constructor and assignment operator.
I recollect there were some rules, but I don't remember, and also can't find a reputable resource online. Can anyone help?
In the following, "auto-generated" means "implicitly declared as defaulted, but not defined as deleted". There are situations where the special member functions are declared, but defined as deleted.
const members in the class will prevent the constructor from being auto-generated...
I've found the diagram below very useful.
= delete (obvious) or = default (less obvious to me). Am I right?The following graphic by Howard Hinnant sums all the rules up nicely. Red cells are deprecated behavior (see [depr.impldec]) and may be changed to deleted at some point.
C++17 N4659 standard draft
For a quick cross standard reference, have a look at the "Implicitly-declared" sections of the following cppreference entries:
The same information can of course be obtained from the standard. E.g. on C++17 N4659 standard draft:
15.8.1 "Copy/move constructors" says for for copy constructor:
6 If the class definition does not explicitly declare a copy constructor, a non-explicit one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy constructor is defined as deleted; otherwise, it is defined as defaulted (11.4). The latter case is deprecated if the class has a user-declared copy assignment operator or a user-declared destructor.
and for move constructor:
8 If the definition of a class X does not explicitly declare a move constructor, a non-explicit one will be implicitly declared as defaulted if and only if
(8.1) — X does not have a user-declared copy constructor,
(8.2) — X does not have a user-declared copy assignment operator,
(8.3) — X does not have a user-declared move assignment operator, and
(8.4) — X does not have a user-declared destructor.
15.8.2 "Copy/move assignment operator" says for copy assignment:
2 If the class definition does not explicitly declare a copy assignment operator, one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy assignment operator is defined as deleted; otherwise, it is defined as defaulted (11.4). The latter case is deprecated if the class has a user-declared copy constructor or a user-declared destructor.
and for move assignment:
4 If the definition of a class X does not explicitly declare a move assignment operator, one will be implicitly declared as defaulted if and only if
- (4.1) — X does not have a user-declared copy constructor,
- (4.2) — X does not have a user-declared move constructor,
- (4.3) — X does not have a user-declared copy assignment operator, and
- (4.4) — X does not have a user-declared destructor.
15.4 "Destructors" says it for destructors:
4 If a class has no user-declared destructor, a destructor is implicitly declared as defaulted (11.4). An implicitly-declared destructor is an inline public member of its class.
