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C++ named requirements: MoveInsertable (since C++11)

Specifies that an object of the type can be constructed into uninitialized storage from an rvalue of that type by a given allocator.

Requirements​

The type T is MoveInsertable 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
  • rv rvalue expression of type T 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, rv);

And after evaluation, the value of *p is equivalent to the value of rv (rv remains valid, but is in an unspecified state.)

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​

If A is std::allocator<T>, then this will call placement-new, as by ::new((void*)p) T(rv) (until C++20) std::construct_at(p, rv) (since C++20) This effectively requires T to be move constructible.

If std::allocator<T> or a similar allocator is used, a class does not have to implement a move constructor to satisfy this type requirement: a copy constructor that takes a const T& argument can bind rvalue expressions. If a MoveInsertable class implements a move constructor, it may also implement move semantics to take advantage of the fact that the value of rv after construction is unspecified.

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: MoveInsertable (since C++11)

Specifies that an object of the type can be constructed into uninitialized storage from an rvalue of that type by a given allocator.

Requirements​

The type T is MoveInsertable 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
  • rv rvalue expression of type T 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, rv);

And after evaluation, the value of *p is equivalent to the value of rv (rv remains valid, but is in an unspecified state.)

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​

If A is std::allocator<T>, then this will call placement-new, as by ::new((void*)p) T(rv) (until C++20) std::construct_at(p, rv) (since C++20) This effectively requires T to be move constructible.

If std::allocator<T> or a similar allocator is used, a class does not have to implement a move constructor to satisfy this type requirement: a copy constructor that takes a const T& argument can bind rvalue expressions. If a MoveInsertable class implements a move constructor, it may also implement move semantics to take advantage of the fact that the value of rv after construction is unspecified.

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.