C++ named requirements: DefaultInsertable (since C++11)

Specifies that an object of the type can be default-constructed in-place by a given allocator.

Requirements

Given the following types, values and expressions:

Type	Definition
T	an object type
A	an allocator type
X	a container type satisfying all following conditions: X::value_type is the same as T. X::allocator_type is the same as std::allocator_traits <A> :: rebind_alloc <T>
Value	Definition
m	an lvalue of type A
p	a pointer of type T*

If the expression std::allocator_traits <A> :: construct (m, p) is well-formed, T is DefaultInsertable into X

Notes

By default, this will value-initialize the object, as by :: new ( ( void * )p) T( ) (until C++20) std::construct_at(p) (since C++20)

If value-initialization is undesirable, for example, if the object is of non-class type and zeroing out is not needed, it can be avoided by providing a custom Allocator::construct

Although it is required that customized construct is used when constructing elements of std::basic_string until C++23, all implementations only used the default mechanism. The requirement is corrected by P1072R10

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
N3346	C++11	the requirements CopyInsertable and MoveInsertable are present, but DefaultInsertable	added the requirement

See also

DefaultConstructible

CopyInsertable

MoveInsertable

EmplaceConstructible

Erasable