C++ named requirements: DefaultConstructible - cppreference.com (original) (raw)
Specifies that an instance of the type can be default constructed.
[edit] Requirements
The type T satisfies DefaultConstructible if all following statements and expressions are valid and have their specified effects:
| Expression/Statement | Postcondition |
|---|---|
| T u; | The object u is default-initialized. |
| T u{}; | The object u is value-initialized or aggregate-initialized. |
| T() T{} | A temporary object of type T is value-initialized or aggregate-initialized. |
[edit] Notes
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.
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 724 | C++98 | the requirements of DefaultConstructible were missing | added |
| LWG 2170 | C++98 | initialzing an object of a DefaultConstructible type with anempty initializer could only result in value-initialization | can also lead toaggregate-initialization |