@@ -4490,7 +4490,7 @@ By default, C++ treats classes as value-like types, but not all types are value-
Set of default operations rules:
* [C.20: If you can avoid defining any default operations, do](#Rc-zero)
* [C.21: If you define or `=delete` any default operation, define or `=delete` them all](#Rc-five)
* [C.21: If you define or `=delete` any copy, move, or destructor function, define or `=delete` them all](#Rc-five)
* [C.22: Make default operations consistent](#Rc-matched)
Destructor rules:
...
...
@@ -4578,29 +4578,25 @@ This is known as "the rule of zero".
(Not enforceable) While not enforceable, a good static analyzer can detect patterns that indicate a possible improvement to meet this rule.
For example, a class with a (pointer, size) pair of member and a destructor that `delete`s the pointer could probably be converted to a `vector`.
### <a name="Rc-five"></a>C.21: If you define or `=delete` any default operation, define or `=delete` them all
### <a name="Rc-five"></a>C.21: If you define or `=delete` any copy, move, or destructor function, define or `=delete` them all
##### Reason
The *special member functions* are the default constructor, copy constructor,
copy assignment operator, move constructor, move assignment operator, and
destructor.
The semantics of copy, move, and destruction are closely related, so if one needs to be declared, the odds are that others need consideration too.
The semantics of the special functions are closely related, so if one needs to be declared, the odds are that others need consideration too.
Declaring any special member function except a default constructor,
Declaring any copy/move/destructor function,
even as `=default` or `=delete`, will suppress the implicit declaration
of a move constructor and move assignment operator.
Declaring a move constructor or move assignment operator, even as
`=default` or `=delete`, will cause an implicitly generated copy constructor
or implicitly generated copy assignment operator to be defined as deleted.
So as soon as any of the special functions is declared, the others should
So as soon as any of these are declared, the others should
all be declared to avoid unwanted effects like turning all potential moves
into more expensive copies, or making a class move-only.
##### Example, bad
struct M2 { // bad: incomplete set of default operations
struct M2 { // bad: incomplete set of copy/move/destructor operations
public:
// ...
// ... no copy or move operations ...
...
...
@@ -4622,12 +4618,12 @@ Given that "special attention" was needed for the destructor (here, to deallocat
##### Note
This is known as "the rule of five" or "the rule of six", depending on whether you count the default constructor.
This is known as "the rule of five."
##### Note
If you want a default implementation of a default operation (while defining another), write `=default` to show you're doing so intentionally for that function.
If you don't want a default operation, suppress it with `=delete`.
If you want a default implementation (while defining another), write `=default` to show you're doing so intentionally for that function.
If you don't want a generated default function, suppress it with `=delete`.
##### Example, good
...
...
@@ -4672,7 +4668,7 @@ Relying on an implicitly generated copy operation in a class with a destructor i
##### Note
Writing the six special member functions can be error prone.
Writing these functions can be error prone.
Note their argument types:
class X {
...
...
@@ -4690,7 +4686,7 @@ To avoid the tedium and the possibility of errors, try to follow the [rule of ze
##### Enforcement
(Simple) A class should have a declaration (even a `=delete` one) for either all or none of the special functions.
(Simple) A class should have a declaration (even a `=delete` one) for either all or none of the copy/move/destructor functions.
### <a name="Rc-matched"></a>C.22: Make default operations consistent
