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Specifies that an object of the type can be default-constructed in-place by a given allocator.
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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:
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| Value | Definition |
| m | an lvalue of typeA |
| p | a pointer of typeT* |
If the expressionstd::allocator_traits<A>::construct(m, p) is well-formed,T isDefaultInsertable intoX.
By default, this willvalue-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 acustomAllocator::construct.
Although it is required that customizedconstruct is used when constructing elements ofstd::basic_string until C++23, all implementations only used the default mechanism. The requirement is corrected byP1072R10 to match existing practice.
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 requirementsCopyInsertable andMoveInsertable are present, butDefaultInsertable was missing | added the requirement |
| DefaultConstructible | |
| CopyInsertable | |
| MoveInsertable | |
| EmplaceConstructible | |
| Erasable |
