C++ named requirements: MoveInsertable (since C++11)
Specifies that an object of the type can be constructed into uninitialized storage from an rvalue of that type by a given allocator.
Requirements
Given the following types, values and expressions:
| Type | Definition |
T
|
an object type |
A
|
an allocator type |
X
|
a container type satisfying all following conditions:
|
| Value | Definition |
| m |
an lvalue of type
A
|
| p |
a pointer of type
T*
|
| Expression | Definition |
| rv |
an expression denoting an rvalue of type
T
|
| expr | std:: allocator_traits < A > :: construct ( m, p, rv ) |
T
is
MoveInsertable
into
X
if all following conditions are satisfied:
- expr is well-formed.
- Right after the evaluation of expr , the value of * p is equivalent to the value of rv before the evaluation.
Notes
If
A
is
std::
allocator
<
T
>
, then this will call placement
new
, as by
::
new
(
(
void
*
)
p
)
T
(
rv
)
(until C++20)
std::
construct_at
(
p, rv
)
(since C++20)
. This effectively requires
T
to be move constructible.
If
std::
allocator
<
T
>
or a similar allocator is used, a class does not have to implement a
move constructor
to satisfy this type requirement: a
copy constructor
that takes a
const
T
&
argument can bind rvalue expressions. If a
MoveInsertable
class implements a move constructor, it may also implement
move semantics
to take advantage of the fact that the value of
rv
after construction is unspecified.
Although it is required that customized
construct
is used when constructing elements of
std::basic_string
until C++23, all implementations only used the default mechanism. The requirement is corrected by
P1072R10
to match existing practice.
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 2177 | C++11 | evaluting expr did not have any postcondition | added |
See also
| CopyInsertable |