# C++ named requirements: DefaultConstructible

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< cpp‎ | named req

Specifies that an instance of the type can be default constructed.

### 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.

### 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.

### 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 an empty initializer could only result in value-initialization | can also lead to aggregate-initialization

### See also

is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible (C++11)(C++11)(C++11) | checks if a type has a default constructor (class template)
default_initializable (C++20) | specifies that an object of a type can be default constructed (concept)

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