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[obj.lifetime]
# 20 Memory management library [[mem]](./#mem)
## 20.2 Memory [[memory]](memory#obj.lifetime)
### 20.2.6 Explicit lifetime management [obj.lifetime]
[🔗](#lib:start_lifetime_as)
`template<class T>
T* start_lifetime_as(void* p) noexcept;
template<class T>
const T* start_lifetime_as(const void* p) noexcept;
template<class T>
volatile T* start_lifetime_as(volatile void* p) noexcept;
template<class T>
const volatile T* start_lifetime_as(const volatile void* p) noexcept;
`
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1056)
*Mandates*: T is an implicit-lifetime type ([[basic.types.general]](basic.types.general#term.implicit.lifetime.type "6.9.1 General"))
and not an incomplete type ([[basic.types.general]](basic.types.general#term.incomplete.type "6.9.1 General"))[.](#1.sentence-1)
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1061)
*Preconditions*: [p, (char*)p + sizeof(T)) denotes a region of allocated storage
that is
a subset of the region of storage
reachable through ([[basic.compound]](basic.compound "6.9.4 Compound types")) p and
suitably aligned for the type T[.](#2.sentence-1)
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1069)
*Effects*: Implicitly creates objects ([[intro.object]](intro.object "6.8.2 Object model")) within the denoted region
consisting of an object *a* of type T whose address is p, and
objects nested within *a*,
as follows:
The object representation of *a* is the contents of the storage prior to the call to start_lifetime_as[.](#3.sentence-1)
The value of each created object *o* of trivially copyable type ([[basic.types.general]](basic.types.general#term.trivially.copyable.type "6.9.1 General")) U is determined in the same manner as for a call
to bit_cast<U>(E) ([[bit.cast]](bit.cast "22.11.3 Function template bit_­cast")),
where E is an lvalue of type U denoting *o*,
except that the storage is not accessed[.](#3.sentence-2)
The value of any other created object is unspecified[.](#3.sentence-3)
[*Note [1](#note-1)*:
The unspecified value can be indeterminate[.](#3.sentence-4)
— *end note*]
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1089)
*Returns*: A pointer to the *a* defined in the *Effects* paragraph[.](#4.sentence-1)
[🔗](#lib:start_lifetime_as_array)
`template<class T>
T* start_lifetime_as_array(void* p, size_t n) noexcept;
template<class T>
const T* start_lifetime_as_array(const void* p, size_t n) noexcept;
template<class T>
volatile T* start_lifetime_as_array(volatile void* p, size_t n) noexcept;
template<class T>
const volatile T* start_lifetime_as_array(const volatile void* p, size_t n) noexcept;
`
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1108)
*Mandates*: T is a complete type[.](#5.sentence-1)
[6](#6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1112)
*Preconditions*: p is suitably aligned for an array of T or is null[.](#6.sentence-1)
n <= size_t(-1) / sizeof(T) is true[.](#6.sentence-2)
If n > 0 is true,
[(char*)p, (char*)p + (n * sizeof(T))) denotes
a region of allocated storage that is
a subset of the region of storage
reachable through ([[basic.compound]](basic.compound "6.9.4 Compound types")) p[.](#6.sentence-3)
[7](#7)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1122)
*Effects*: If n > 0 is true,
equivalent tostart_lifetime_as<U>(p) where U is the type “array of n T”[.](#7.sentence-1)
Otherwise, there are no effects[.](#7.sentence-2)
[8](#8)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1130)
*Returns*: A pointer to the first element of the created array,
if any;
otherwise,
a pointer that compares equal to p ([[expr.eq]](expr.eq "7.6.10 Equality operators"))[.](#8.sentence-1)
[🔗](#lib:trivially_relocate)
`template<class T>
T* trivially_relocate(T* first, T* last, T* result);
`
[9](#9)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1145)
*Mandates*: is_trivially_relocatable_v<T> && !is_const_v<T> is true[.](#9.sentence-1)
T is not an array of unknown bound[.](#9.sentence-2)
[10](#10)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1150)
*Preconditions*:
- [(10.1)](#10.1)
[first, last) is a valid range[.](#10.1.sentence-1)
- [(10.2)](#10.2)
[result, result + (last - first)) denotes a region of storage that
is a subset of the region reachable through result ([[basic.compound]](basic.compound "6.9.4 Compound types"))
and suitably aligned for the type T[.](#10.2.sentence-1)
- [(10.3)](#10.3)
No element in the range [first, last) is a potentially-overlapping subobject[.](#10.3.sentence-1)
[11](#11)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1163)
*Postconditions*: No effect if result == first is true[.](#11.sentence-1)
Otherwise, the range denoted by [result, result + (last - first))
contains objects (including subobjects) whose lifetime has begun and whose
object representations are the original object representations of the
corresponding objects in the source range [first, last) except
for any parts of the object representations used by the implementation to
represent type information ([[intro.object]](intro.object "6.8.2 Object model"))[.](#11.sentence-2)
If any of the objects has
union type, its active member is the same as that of the corresponding object
in the source range[.](#11.sentence-3)
If any of the aforementioned objects has a non-static
data member of reference type, that reference refers to the same entity as
does the corresponding reference in the source range[.](#11.sentence-4)
The lifetimes of the
original objects in the source range have ended[.](#11.sentence-5)
[12](#12)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1178)
*Returns*: result + (last - first)[.](#12.sentence-1)
[13](#13)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1182)
*Throws*: Nothing[.](#13.sentence-1)
[14](#14)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1186)
*Complexity*: Linear in the length of the source range[.](#14.sentence-1)
[15](#15)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1190)
*Remarks*: The destination region of storage is considered reused ([[basic.life]](basic.life "6.8.4 Lifetime"))[.](#15.sentence-1)
No constructors or destructors are invoked[.](#15.sentence-2)
[*Note [2](#note-2)*:
Overlapping ranges are supported[.](#15.sentence-3)
— *end note*]
[🔗](#lib:relocate)
`template<class T>
constexpr T* relocate(T* first, T* last, T* result);
`
[16](#16)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1207)
*Mandates*: is_nothrow_relocatable_v<T> && !is_const_v<T> is true[.](#16.sentence-1)
T is not an array of unknown bound[.](#16.sentence-2)
[17](#17)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1212)
*Preconditions*:
- [(17.1)](#17.1)
[first, last) is a valid range[.](#17.1.sentence-1)
- [(17.2)](#17.2)
[result, result + (last - first)) denotes a region of storage that is
a subset of the region reachable through result ([[basic.compound]](basic.compound "6.9.4 Compound types"))
and suitably aligned for the type T[.](#17.2.sentence-1)
- [(17.3)](#17.3)
No element in the range [first, last) is a potentially-overlapping
subobject[.](#17.3.sentence-1)
[18](#18)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1226)
*Effects*:
- [(18.1)](#18.1)
If result == first is true, no effect;
- [(18.2)](#18.2)
otherwise, if not called during constant evaluation and is_trivially_relocatable_v<T> is true, then has
effects equivalent to: trivially_relocate(first, last, result);
- [(18.3)](#18.3)
otherwise, for each integer i in [0, last - first),
* [(18.3.1)](#18.3.1)
if T is an array type, equivalent to: relocate(begin(first[i]), end(first[i]), *start_lifetime_as<T>(result + i));
* [(18.3.2)](#18.3.2)
otherwise, equivalent to: construct_at(result + i, std​::​move(first[i])); destroy_at(first + i);
[19](#19)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1247)
*Returns*: result + (last - first)[.](#19.sentence-1)
[20](#20)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L1251)
*Throws*: Nothing[.](#20.sentence-1)
[*Note [3](#note-3)*:
Overlapping ranges are supported[.](#20.sentence-2)
— *end note*]