class property specifiers (since C++26) (original) (raw)

Specifies that a class is replaceable (replaceable_if_eligible), trivially relocatable (trivially_relocatable_if_eligible), or that a class cannot be derived from (final).

Contents

[edit] Syntax

Class property specifiers appear at the beginning of the class definition, immediately after the name of the class, and cannot appear in a class declaration.

| | | | | ---------------------------------------------------------------------------------------------------- | | | | class-key attr (optional) class-head-name class-prop-specifier-seq (optional) base-clause (optional) | | | | | | |

Before (C++26), there was the class-virt-specifier (optional) in place of class-prop-specifier-seq (optional), which only allowed the final for final specifier (since C++11).

[edit] Explanation

final, replaceable_if_eligible and trivially_relocatable_if_eligible are identifiers with a special meaning when used in a class head. In other contexts, it is not reserved and may be used to name objects and functions.

[edit] final specifier

final specifies that this class may not appear in the base-specifier-list of another class definition (in other words, cannot be derived from). The program is ill-formed otherwise (a compile-time error is generated). final can also be used with a union definition, in which case it has no effect (other than on the outcome of std::is_final)(since C++14), since unions cannot be derived from.

[edit] replaceable_if_eligible specifier

replaceable_if_eligible specifies that this class is replaceable if it is eligible for replacement .

[edit] trivially_relocatable_if_eligible specifier

trivially_relocatable_if_eligible specifies that this class is trivially relocatable if it is eligible for trivial relocation .

[edit] Replaceability

A class C is replaceable if it is eligible for replacement and either:

[edit] Eligibility for replacement

A class C is eligible for replacement unless either:

[edit] Trivial relocatability

A class is trivially relocatable if it is eligible for trivial relocation and either:

[edit] Eligibility for trivial relocation

A class is eligible for trivial relocation unless it has either:

except that it is implementation-defined whether an otherwise-eligible union having one or more subobjects of polymorphic class type is eligible for trivial relocation .

[edit] Default movability

A class C is default movable if all following conditions are met:

[edit] Keywords

final,replaceable_if_eligible,trivially_relocatable_if_eligible.

[edit] Note

Feature-test macro Value Std Feature
__cpp_trivial_relocatability 202502L (C++26) Trivial relocatability

[edit] Example

struct final; // OK; declares a class named 'final', // does not use class property specifiers. struct IF final; // Ill-formed: class property specifiers // cannot appear at function declaration. struct F final {}; // OK; specifier marks class F as non-derivable. struct D: F {}; // Ill-formed: class F cannot be derived from.   // OK; specifier marks class R as 𝘳𝘦𝘱𝘭𝘢𝘤𝘦𝘢𝘣𝘭𝘦 if eligible. struct R replaceable_if_eligible {};   // OK; specifier marks class T as 𝘵𝘳𝘪𝘷𝘪𝘢𝘭𝘭𝘺 𝘳𝘦𝘭𝘰𝘤𝘢𝘵𝘢𝘣𝘭𝘦 if eligible. struct T trivially_relocatable_if_eligible {};   // OK; a class can be marked with multiple class property specifiers. struct FRT final replaceable_if_eligible trivially_relocatable_if_eligible {};   // Ill-formed: each class property specifier can appear at most once. struct FRF final replaceable_if_eligible final {};   int main() {}

[edit] References

[edit] See also

final specifier (C++11) declares that a method cannot be overridden or a class be derived from[edit]
checks if a type is a final class type (class template) [edit]