A copy constructor of class T
is a non-template constructor whose first parameter is T&
, const T&
, volatile T&
, or const volatile T&
, and either there are no other parameters, or the rest of the parameters all have default values.
class_name ( const class_name & ) | (1) | |
class_name ( const class_name & ) = default; | (2) | |
class_name ( const class_name & ) = delete; | (3) |
Where class_name must name the current class (or current instantiation of a class template), or, when declared at namespace scope or in a friend declaration, it must be a qualified class name.
The copy constructor is called whenever an object is initialized (by direct-initialization or copy-initialization) from another object of the same type (unless overload resolution selects a better match or the call is elided), which includes.
T a = b;
or T a(b);
, where b is of type T
; f(a);
, where a
is of type T
and f
is void f(T t)
; return a;
inside a function such as T f()
, where a
is of type T
, which has no move constructor. If no user-defined copy constructors are provided for a class type (struct
, class
, or union
), the compiler will always declare a copy constructor as a non-explicit inline public
member of its class. This implicitly-declared copy constructor has the form T::T(const T&)
if all of the following are true:
B
of T
has a copy constructor whose parameters are const B&
or const volatile B&
; M
of T
of class type or array of class type has a copy constructor whose parameters are const M&
or const volatile M&
. Otherwise, the implicitly-declared copy constructor is T::T(T&)
. (Note that due to these rules, the implicitly-declared copy constructor cannot bind to a volatile lvalue argument.).
A class can have multiple copy constructors, e.g. both | (since C++11) |
The implicitly-declared (or defaulted on its first declaration) copy constructor has an exception specification as described in dynamic exception specification (until C++17)exception specification (since C++17).
The implicitly-declared copy constructor for class T is undefined if any of the following conditions are true: | (until C++11) |
The implicitly-declared or defaulted copy constructor for class T is defined as deleted if any of the following conditions are true: | (since C++11) |
T
has non-static data members that cannot be copied (have deleted, inaccessible, or ambiguous copy constructors); T
has direct or virtual base class that cannot be copied (has deleted, inaccessible, or ambiguous copy constructors); T
has direct or virtual base class with a deleted or inaccessible destructor;
| (since C++11) |
The copy constructor for class T
is trivial if all of the following are true:
T
has no virtual member functions; T
has no virtual base classes; T
is trivial; T
is trivial; A trivial copy constructor for a non-union class effectively copies every scalar subobject (including, recursively, subobject of subobjects and so forth) of the argument and performs no other action. However, padding bytes need not be copied, and even the object representations of the copied subobjects need not be the same as long as their values are identical.
TriviallyCopyable objects can be copied by copying their object representations manually, e.g. with std::memmove
. All data types compatible with the C language (POD types) are trivially copyable.
If the implicitly-declared copy constructor is not deleted, it is defined (that is, a function body is generated and compiled) by the compiler if odr-used. For union
types, the implicitly-defined copy constructor copies the object representation (as by std::memmove
). For non-union class types (class
and struct
), the constructor performs full member-wise copy of the object's bases and non-static members, in their initialization order, using direct initialization. If this satisfies the requirements of a constexpr constructor, the generated copy constructor is constexpr
. (since C++11).
The generation of the implicitly-defined copy constructor is deprecated if | (since C++11) |
In many situations, copy constructors are optimized out even if they would produce observable side-effects, see copy elision.
struct A { int n; A(int n = 1) : n(n) { } A(const A& a) : n(a.n) { } // user-defined copy ctor }; struct B : A { // implicit default ctor B::B() // implicit copy ctor B::B(const B&) }; struct C : B { C() : B() { } private: C(const C&); // non-copyable, C++98 style }; int main() { A a1(7); A a2(a1); // calls the copy ctor B b; B b2 = b; A a3 = b; // conversion to A& and copy ctor volatile A va(10); // A a4 = va; // compile error C c; // C c2 = c; // compile error }
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
---|---|---|---|
CWG 2171 | C++14 | X(X&) = default was non-trivial | made trivial |
CWG 496 | C++11 | structs with volatile members were trivially copyable | volatile members make copy non-trivial |
CWG 2094 | C++14 | volatile members make copy non-trivial | structs with volatile members are trivially copyable |
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