C++ named requirements: DefaultInsertable (since 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
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an allocator type |
m
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an lvalue of type A
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p
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the pointer of type T* prepared by the container
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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 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.
See also
| DefaultConstructible | |
| CopyInsertable | |
| MoveInsertable | |
| EmplaceConstructible | |
| Erasable |