C++ named requirements: DefaultInsertable
Specifies that an instance of the type can be default-constructed in-place by a given allocator.
T is DefaultInsertable into the Container
value_type is identical to
T if, given
||an allocator type|
|| an lvalue of type |
|| the pointer of type |
X::allocator_type is identical to std::allocator_traits<A>::rebind_alloc<T>,
the following expression is well-formed:
X is not allocator-aware, the term is defined as if
A were std::allocator<T>, except that no allocator object needs to be created, and user-defined specializations of std::allocator are not instantiated.
By default, this will call placement-new, as by ::new((void*)p) T() (that is, value-initialize the object pointed to by
p). 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.
 See also