C++ named requirements: CopyConstructible
Specifies that an instance of the type can be copy-constructed from an lvalue expression .
Requirements
The type
T
satisfies
CopyConstructible
if
-
The type
Tsatisfies MoveConstructible , and
Given
-
v
, an
lvalue
expression of type
Tor const T or an rvalue expression of type const T , - u , an arbitrary identifier.
The following expressions must be valid and have their specified effects:
| Expression | Post-conditions |
|---|---|
| T u = v ; |
The value of
u
is equivalent to the value of
v
.
The value of v is unchanged. |
| T ( v ) |
The value of
T
(
v
)
is equivalent to the value of
v
.
The value of v is unchanged. |
|
The expression
v.~T
(
)
also must be valid, and, for lvalue
v
, the expression
&
v
must have the type
|
(until C++11) |
Notes
Until C++11, classes that overloaded operator & were not CopyConstructible and thus were not usable in the standard library containers . This is a design decision in C++98 (instead of a defect, see LWG issue 390 ).
Since C++11, the standard library uses std::addressof whenever the address of an object is needed.
| Extended content |
|---|
|
Being a CopyConstructible class implies std::is_copy_constructible but not vice versa since std::is_copy_constructible will only check for the ability to call the constructor with the correct arguments, and, e.g., not a MoveConstructible requirement.
Run this code
#include <type_traits> #include <utility> struct S { S() = default; S(S&&) = delete; S(const S&) = default; }; static_assert(std::is_copy_constructible_v<S>); int main() { S s1; // Class `S` doesn't satisfy MoveConstructible requirement, // hence doesn't satisfy CopyConstructible requirement [[maybe_unused]] S s2{std::move(s1)}; // ill-formed, use of deleted function } |
References
| Extended content |
|---|
|
See also
|
(C++11)
(C++11)
(C++11)
|
checks if a type has a copy constructor
(class template) |
|
(C++20)
|
specifies that an object of a type can be copy constructed and move constructed
(concept) |