tmp/tmpvqg9q5so/{from.md → to.md}
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| 1 |
+
## Preamble <a id="class.pre">[[class.pre]]</a>
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| 3 |
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A class is a type. Its name becomes a *class-name* [[class.name]] within
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its scope.
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| 5 |
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``` bnf
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class-name:
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+
identifier
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| 9 |
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simple-template-id
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```
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+
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+
A *class-specifier* or an *elaborated-type-specifier* [[dcl.type.elab]]
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is used to make a *class-name*. An object of a class consists of a
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| 14 |
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(possibly empty) sequence of members and base class objects.
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``` bnf
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class-specifier:
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class-head '{' member-specificationₒₚₜ '}'
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```
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``` bnf
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| 22 |
+
class-head:
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+
class-key attribute-specifier-seqₒₚₜ class-head-name class-virt-specifierₒₚₜ base-clauseₒₚₜ
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class-key attribute-specifier-seqₒₚₜ base-clauseₒₚₜ
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```
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+
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``` bnf
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class-head-name:
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| 29 |
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nested-name-specifierₒₚₜ class-name
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| 30 |
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```
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| 31 |
+
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``` bnf
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class-virt-specifier:
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final
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```
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``` bnf
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class-key:
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class
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struct
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union
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```
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| 43 |
+
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| 44 |
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A class declaration where the *class-name* in the *class-head-name* is a
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| 45 |
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*simple-template-id* shall be an explicit specialization
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| 46 |
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[[temp.expl.spec]] or a partial specialization [[temp.class.spec]]. A
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*class-specifier* whose *class-head* omits the *class-head-name* defines
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an unnamed class.
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[*Note 1*: An unnamed class thus can’t be `final`. — *end note*]
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A *class-name* is inserted into the scope in which it is declared
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immediately after the *class-name* is seen. The *class-name* is also
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| 54 |
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inserted into the scope of the class itself; this is known as the
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| 55 |
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*injected-class-name*. For purposes of access checking, the
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| 56 |
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injected-class-name is treated as if it were a public member name. A
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| 57 |
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*class-specifier* is commonly referred to as a *class definition*. A
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| 58 |
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class is considered defined after the closing brace of its
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| 59 |
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*class-specifier* has been seen even though its member functions are in
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general not yet defined. The optional *attribute-specifier-seq*
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| 61 |
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appertains to the class; the attributes in the *attribute-specifier-seq*
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| 62 |
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are thereafter considered attributes of the class whenever it is named.
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| 63 |
+
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| 64 |
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If a *class-head-name* contains a *nested-name-specifier*, the
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*class-specifier* shall refer to a class that was previously declared
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| 66 |
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directly in the class or namespace to which the *nested-name-specifier*
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refers, or in an element of the inline namespace set [[namespace.def]]
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| 68 |
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of that namespace (i.e., not merely inherited or introduced by a
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| 69 |
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*using-declaration*), and the *class-specifier* shall appear in a
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namespace enclosing the previous declaration. In such cases, the
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| 71 |
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*nested-name-specifier* of the *class-head-name* of the definition shall
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not begin with a *decltype-specifier*.
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[*Note 2*: The *class-key* determines whether the class is a union
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[[class.union]] and whether access is public or private by default
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| 76 |
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[[class.access]]. A union holds the value of at most one data member at
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| 77 |
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a time. — *end note*]
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| 78 |
+
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If a class is marked with the *class-virt-specifier* `final` and it
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appears as a *class-or-decltype* in a *base-clause* [[class.derived]],
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| 81 |
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the program is ill-formed. Whenever a *class-key* is followed by a
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| 82 |
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*class-head-name*, the *identifier* `final`, and a colon or left brace,
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`final` is interpreted as a *class-virt-specifier*.
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| 84 |
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| 85 |
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[*Example 1*:
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| 86 |
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| 87 |
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``` cpp
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| 88 |
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struct A;
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| 89 |
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struct A final {}; // OK: definition of struct A,
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| 90 |
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// not value-initialization of variable final
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struct X {
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struct C { constexpr operator int() { return 5; } };
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struct B final : C{}; // OK: definition of nested class B,
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| 95 |
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// not declaration of a bit-field member final
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};
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```
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— *end example*]
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[*Note 3*: Complete objects of class type have nonzero size. Base class
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| 102 |
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subobjects and members declared with the `no_unique_address` attribute
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| 103 |
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[[dcl.attr.nouniqueaddr]] are not so constrained. — *end note*]
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| 104 |
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[*Note 4*: Class objects can be assigned ([[over.ass]],
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| 106 |
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[[class.copy.assign]]), passed as arguments to functions ([[dcl.init]],
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| 107 |
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[[class.copy.ctor]]), and returned by functions (except objects of
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| 108 |
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classes for which copying or moving has been restricted; see
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| 109 |
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[[dcl.fct.def.delete]] and [[class.access]]). Other plausible operators,
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| 110 |
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such as equality comparison, can be defined by the user; see
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| 111 |
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[[over.oper]]. — *end note*]
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| 112 |
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