Skip to main content

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 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 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.

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 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 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.