From cppreference.com
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 Tu;The object u is
. Tu{};The object u is
or
. T()T{}
A temporary object of type T is
or
. Notes
For objects of non-aggregate class type, a public
must be defined (either user-defined or implicitly defined) to satisfy DefaultConstructible.
Non-const objects of
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
C++98 the requirements of DefaultConstructible were missing added
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