C++ concepts: AllocatorAwareContainer

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An AllocatorAwareContainer is a Container that holds an instance of an Allocator and uses that instance to allocate and deallocate memory in all of its member functions.

The following rules apply to object construction

  • Copy constructors of AllocatorAwareContainers obtain their instances of the allocator by calling std::allocator_traits<allocator_type>::select_on_container_copy_construction.
  • Move constructors obtain their instances of allocators by move-constructing from the allocator belonging to the old container.
  • All other constructors take an allocator parameter.

The only way to replace an allocator is copy-assignment, move-assignment, and swap:

  • Copy-assignment will replace the allocator only if std::allocator_traits<allocator_type>::propagate_on_container_copy_assignment::value is true
  • Move-assignment will replace the allocator only if std::allocator_traits<allocator_type>::propagate_on_container_move_assignment::value is true
  • Swap will replace the allocator only if std::allocator_traits<allocator_type>::propagate_on_container_swap::value is true. Specifically, it will exchange the allocator instances through an unqualified call to the non-member function swap, see Swappable.

Note: swapping two containers with unequal allocators if propagate_on_container_swap is false is undefined behavior.

  • The accessor get_allocator() obtains a copy of the allocator that was used to construct the container or installed by the most recent allocator replacement operation.




X Container type
T Element type
A Allocator for T
a, b Objects of type X (non-const lvalue)
t Object of type T (lvalue or const rvalue)
rv Object of type T (non-const rvalue)
m Object of type A
Q Allocator type

expression return type pre/requirements post/effects complexity
allocator_type A allocator_type::value_type must be the same as X::value_type constant
get_allocator() A constant
X u; u.empty() == true && u.get_allocator() == A() constant
X u(m); u.empty() == true && u.get_allocator() == m constant
X u(t,m); u == t && u.get_allocator() == m linear
X u(rv); Move constructor of A must not throw exceptions u has the same elements and an equal allocator as rv had before the construction constant
X u(rv,m); The elements of u are the same or copies of those of rv and u.get_allocator() == m constant if m == rv.get_allocator(), otherwise linear
a = t X& a == t linear
a = rv X& All existing elements of a are either move assigned to or destroyed linear
a.swap(b) void Exchanges the contents of a and b constant

Concept requirements


Standard library

All standard library containers except std::array are AllocatorAwareContainers:

  • std::basic_string
  • std::deque
  • std::forward_list
  • std::list
  • std::vector
  • std::map
  • std::multimap
  • std::set
  • std::multiset
  • std::unordered_map
  • std::unordered_multimap
  • std::unordered_set
  • std::unordered_multiset