C++ named requirements: DefaultInsertable (od C++11)
Specifies that an instance of the type can be default-constructed in-place by a given allocator.
Requirements
The type T is DefaultInsertable into the Container X whose value_type is identical to T if, given
A
an allocator typem
an lvalue of typeA
p
the pointer of typeT*
prepared by the container where X::allocator_type is identical tostd::allocator_traits<A>::rebind_alloc<T>
,
the following expression is well-formed:
std::allocator_traits<A>::construct(m, p);
If X is not allocator-aware or is a std::basic_string specialization, 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.
Notes
By default, this will value-initialize the object,
as by ::new((void*)p) T()
(do C++20) std::construct_at(p)
(od 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.