C++ named requirements: DefaultInsertable

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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 type
m an lvalue of type A
p the pointer of type T* prepared by the container

where X::allocator_type is identical to std::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 call placement-new, as by ::new((void*)p) T() (until C++20)std::construct_at(p) (since C++20) (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.

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.

See also

DefaultConstructible
CopyInsertable
MoveInsertable
EmplaceConstructible
Erasable