| title | C++ named requirements: DefaultConstructible | ||||
|---|---|---|---|---|---|
| cppdoc |
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import { DR, DRList } from "@components/defect-report"; import { Desc, DescList } from "@components/desc-list"; import Missing from "@components/Missing.astro"; import { Revision, RevisionBlock } from "@components/revision";
Specifies that an instance of the type can be default constructed.
The type T satisfies DefaultConstructible if all following statements and expressions are valid and have their specified effects:
Given
-
u, an expression of typeT. -
u, an lvalue expression of typeKey.
| Expression/Statement | Postcondition |
|---|---|
T u; |
The object u is default-initialized. |
T u{}; |
The object u is value-initialized or aggregate-initialized. |
T() |
All resources owned by u are reclaimed, no exceptions are thrown. |
T{} |
A temporary object of type T is value-initialized or aggregate-initialized. |
For objects of non-aggregate class type, a public default constructor must be defined (either user-defined or implicitly defined) to satisfy DefaultConstructible.
Non-const objects of non-class object type are always DefaultConstructible.
Const non-class types are not DefaultConstructible.
Const aggregate types are not DefaultConstructible if any of their members is an object of non-class type.
Non-object types (function types, reference types, and the (possibly cv-qualified) type void) as well as the const non-object types are never DefaultConstructible.
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
the requirements of _DefaultConstructible_ were missing added initialzing an object of a _DefaultConstructible_ type with an empty initializer could only result in value-initialization can also lead to aggregate-initialization ## See also `std::is_default_constructible` `std::is_trivially_default_constructible` `std::is_nothrow_default_constructible` checks if a type has a default constructor `default_initializable` specifies that an object of a type can be default constructed