[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)