Copy constructors

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

C++ language

General topics
Preprocessor Comments	Keywords Escape sequences
Flow control
Conditional execution statements
if	switch
Iteration statements (loops)
for range-for (C++11)	while do-while
Jump statements
continue - break	goto - return
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until C++17*)
noexcept specifier (C++11)
Exceptions
throw-expression try block	handler
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Class/struct types Union types
Specifiers
const/volatile decltype (C++11) auto (C++11)	constexpr (C++11) consteval (C++20) constinit (C++20)
Storage duration specifiers
Initialization
Default-initialization Value-initialization Zero-initialization Copy-initialization Direct-initialization	Aggregate initialization List-initialization (C++11)   Constant initialization Reference initialization

Expressions
Value categories Order of evaluation	Operators Operator precedence
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (C++11)
User-defined (C++11)
Utilities
Attributes (C++11)
Types
typedef declaration
Type alias declaration (C++11)
Casts
Implicit conversions static_cast const_cast	Explicit conversions dynamic_cast reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Constructors this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier (C++11)   final specifier (C++11)	explicit (C++11) static
Special member functions
Default constructor Copy constructor Move constructor (C++11)	Copy assignment Move assignment (C++11) Destructor
Templates
Class template Function template	Template specialization Parameter packs (C++11)
Miscellaneous
Inline assembly	History of C++

 

Classes

General

Overview

class/struct types

union types

Injected-class-name

Members

Data members

Static members

The this pointer

Nested classes

Member templates

Bit-fields

using-declarations

Member functions

Member access specifiers

Constructors and member initializer lists

Default member initializer (C++11)

friend specifier

explicit specifier

Converting constructor

Special member functions

Default constructor

Copy constructor

Move constructor (C++11)

Copy assignment operator

Move assignment operator (C++11)

Destructor

Inheritance

Base and derived classes

Empty base optimization (EBO)

Virtual member functions

Pure virtual functions and abstract classes

override specifier (C++11)

final specifier (C++11)

 

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.

Syntax

class-name ( cv class-name & )	(1)
class-name ( const(optional) class-name & ) = default;	(2)	(since C++11)
class-name ( cv class-name & ) = delete;	(3)	(since C++11)

Where cv is const(optional) volatile(optional), and 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.

Explanation

1) Typical declaration of a copy constructor.

2) Forcing a copy constructor to be generated by the compiler.

3) Avoiding implicit generation of the copy constructor.

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

• initialization: T a = b; or T a(b);, where b is of type T;
• function argument passing: f(a);, where a is of type T and f is void f(T t);
• function return: return a; inside a function such as T f(), where a is of type T, which has no move constructor.

Implicitly-declared copy 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:

• each direct and virtual base B of T has a copy constructor whose parameters are const B& or const volatile B&;
• each non-static data member 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 T::T(const T&) and T::T(T&).

If some user-defined copy constructors are present, the user may still force the generation of the implicitly declared copy constructor with the keyword default.

(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)noexcept specification(since C++17).

Deleted implicitly-declared copy constructor

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 or a non-static data member with a deleted or inaccessible destructor;

T is a union-like class and has a variant member with non-trivial copy constructor; T has a data member of rvalue reference type; T has a user-defined move constructor or move assignment operator (this condition only causes the implicitly-declared, not the defaulted, copy constructor to be deleted).

(since C++11)

Trivial copy constructor

The copy constructor for class T is trivial if all of the following are true:

• it is not user-provided (that is, it is implicitly-defined or defaulted);
• T has no virtual member functions;
• T has no virtual base classes;
• the copy constructor selected for every direct base of T is trivial;
• the copy constructor selected for every non-static class type (or array of class type) member of 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.

Eligible copy constructor

A copy constructor is eligible if it is either user-declared or both implicitly-declared and definable.	(until C++11)
A copy constructor is eligible if it is not deleted.	(since C++11) (until C++20)
A copy constructor is eligible if it is not deleted, and its associated constraints, if any, are satisfied, and no copy constructor with the same first parameter type is more constrained than it.	(since C++20)

Triviality of eligible copy constructors determines whether the class is an implicit-lifetime type, and whether the class is a trivially copyable type.

Implicitly-defined copy constructor

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 or needed for constant evaluation(since C++11). 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(until C++23)constexpr function(since C++23), the generated copy constructor is constexpr. The generation of the implicitly-defined copy constructor is deprecated if T has a user-defined destructor or user-defined copy assignment operator.

(since C++11)

Notes

In many situations, copy constructors are optimized out even if they would produce observable side-effects, see copy elision.

Example

struct A
{
    int n;
    A(int n = 1) : n(n) {}
    A(const A& a) : n(a.n) {} // user-defined copy constructor
};
 
struct B : A
{
    // implicit default constructor B::B()
    // implicit copy constructor 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 constructor
 
    B b;
    B b2 = b;
    A a3 = b; // conversion to A& and copy constructor
 
    volatile A va(10);
    // A a4 = va; // compile error
 
    C c;
    // C c2 = c; // compile error
}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
CWG 2094	C++11	volatile members make copy non-trivial (CWG issue 496)	triviality not affected
CWG 2171	C++11	X(X&) = default was non-trivial	made trivial

See also

• converting constructor
• copy assignment
• copy elision
• default constructor
• destructor
• explicit
• initialization
  • aggregate initialization
  • constant initialization
  • copy initialization
  • default initialization
  • direct initialization
  • initializer list
  • list initialization
  • reference initialization
  • value initialization
  • zero initialization
• move assignment
• move constructor
• new