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cppdraft/polymorphic/general.md

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+[polymorphic.general]
+
+# 20 Memory management library [[mem]](./#mem)
+
+## 20.4 Types for composite class design [[mem.composite.types]](mem.composite.types#polymorphic.general)
+
+### 20.4.2 Class template polymorphic [[polymorphic]](polymorphic#general)
+
+#### 20.4.2.1 General [polymorphic.general]
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L6758)
+
+A polymorphic object manages the lifetime of an owned object[.](#1.sentence-1)
+
+A polymorphic object may own objects of
+different types at different points in its lifetime[.](#1.sentence-2)
+
+A polymorphic object is[*valueless*](#def:valueless,polymorphic_object "20.4.2.1 General [polymorphic.general]") if it has no owned object[.](#1.sentence-3)
+
+A polymorphic object may become valueless only after it has been moved from[.](#1.sentence-4)
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L6767)
+
+In every specialization polymorphic<T, Allocator>,
+if the type allocator_traits<Allocator>​::​value_type is not the same type as T, the program is ill-formed[.](#2.sentence-1)
+
+Every object of type polymorphic<T, Allocator> uses an object of type Allocator to
+allocate and free storage for the owned object as needed[.](#2.sentence-2)
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L6775)
+
+Constructing an owned object of type U with args... using the allocator a means callingallocator_traits<Allocator>​::​construct(a, *p*, args...) whereargs is an expression pack,a is an allocator, and*p* points to storage suitable for an owned object of type U[.](#3.sentence-1)
+
+[4](#4)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L6783)
+
+The member *alloc* is used for
+any memory allocation and element construction
+performed by member functions
+during the lifetime of each polymorphic value object, or
+until the allocator is replaced[.](#4.sentence-1)
+
+The allocator may be replaced only via
+assignment or swap()[.](#4.sentence-2)
+
+Allocator replacement is performed by
+copy assignment,
+move assignment, or
+swapping of the allocator
+only if ([[container.reqmts]](container.reqmts "23.2.2.2 Container requirements")):
+
+- [(4.1)](#4.1)
+
+allocator_traits<Allocator>​::​propagate_on_container_copy_assignment​::​value, or
+
+- [(4.2)](#4.2)
+
+allocator_traits<Allocator>​::​propagate_on_container_move_assignment​::​value, or
+
+- [(4.3)](#4.3)
+
+allocator_traits<Allocator>​::​propagate_on_container_swap​::​value
+
+is true within the implementation of
+the corresponding polymorphic operation[.](#4.sentence-3)
+
+[5](#5)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L6807)
+
+A program that instantiates the definition of polymorphic for
+a non-object type,
+an array type,in_place_t,
+a specialization of in_place_type_t, or
+a cv-qualified type
+is ill-formed[.](#5.sentence-1)
+
+[6](#6)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L6816)
+
+The template parameter T of polymorphic may be an incomplete type[.](#6.sentence-1)
+
+[7](#7)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L6820)
+
+The template parameter Allocator of polymorphic shall meet the requirements of *Cpp17Allocator*[.](#7.sentence-1)
+
+[8](#8)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L6824)
+
+If a program declares an explicit or
+partial specialization of polymorphic,
+the behavior is undefined[.](#8.sentence-1)