C++ named requirements: DefaultInsertable (since C++11) (original) (raw)
Specifies that an object of the type can be default-constructed in-place by a given allocator.
[edit] 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.
[edit] 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.
[edit] 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 MoveInsertableare present, but DefaultInsertable was missing | added the requirement |