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 to match existing practice.

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 was missing	added the requirement

See also

DefaultConstructible

CopyInsertable

MoveInsertable

EmplaceConstructible

Erasable