104 lines
3.6 KiB
Markdown
104 lines
3.6 KiB
Markdown
[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)
|