Copy assignment operator

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A copy assignment operator of class T is a non-template non-static member function with the name operator= that takes exactly one parameter of type T, T&, const T&, volatile T&, or const volatile T&. For a type to be CopyAssignable, it must have a public copy assignment operator.

Syntax

class-name & class-name :: operator= ( class-name ) (1)
class-name & class-name :: operator= ( const class-name & ) (2)
class-name & class-name :: operator= ( const class-name & ) = default; (3) (since C++11)
class-name & class-name :: operator= ( const class-name & ) = delete; (4) (since C++11)

Explanation

1) Typical declaration of a copy assignment operator when copy-and-swap idiom is used.
2) Typical declaration of a copy assignment operator when copy-and-swap idiom is not used.
3) Forcing a copy assignment operator to be generated by the compiler.
4) Avoiding implicit copy assignment.

The copy assignment operator is called whenever selected by overload resolution, e.g. when an object appears on the left side of an assignment expression.

Implicitly-declared copy assignment operator

If no user-defined copy assignment operators are provided for a class type (struct, class, or union), the compiler will always declare one as an inline public member of the class. This implicitly-declared copy assignment operator has the form T& T::operator=(const T&) if all of the following is true:

  • each direct base B of T has a copy assignment operator whose parameters are B or 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 assignment operator whose parameters are M or const M& or const volatile M&.

Otherwise the implicitly-declared copy assignment operator is declared as T& T::operator=(T&). (Note that due to these rules, the implicitly-declared copy assignment operator cannot bind to a volatile lvalue argument.)

A class can have multiple copy assignment operators, e.g. both T& T::operator=(T&) and T& T::operator=(T). If some user-defined copy assignment operators are present, the user may still force the generation of the implicitly declared copy assignment operator with the keyword default. (since C++11)

The implicitly-declared (or defaulted on its first declaration) copy assignment operator has an exception specification as described in dynamic exception specification (until C++17)noexcept specification (since C++17)

Because the copy assignment operator is always declared for any class, the base class assignment operator is always hidden. If a using-declaration is used to bring in the assignment operator from the base class, and its argument type could be the same as the argument type of the implicit assignment operator of the derived class, the using-declaration is also hidden by the implicit declaration.

Deleted implicitly-declared copy assignment operator

An implicitly-declared copy assignment operator for class T is defined as deleted if any of the following is true:

  • T has a user-declared move constructor;
  • T has a user-declared move assignment operator.

Otherwise, it is defined as defaulted.

A defaulted copy assignment operator for class T is defined as deleted if any of the following is true:

  • T has a non-static data member of non-class type (or array thereof) that is const;
  • T has a non-static data member of a reference type;
  • T has a non-static data member or a direct base class that cannot be copy-assigned (overload resolution for the copy assignment fails, or selects a deleted or inaccessible function);
  • T is a union-like class, and has a variant member whose corresponding assignment operator is non-trivial.

Trivial copy assignment operator

The copy assignment operator for class T is trivial if all of the following is true:

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

A trivial copy assignment operator makes a copy of the object representation as if by std::memmove. All data types compatible with the C language (POD types) are trivially copy-assignable.

Eligible copy assignment operator

A copy assignment operator is eligible if it is either user-declared or both implicitly-declared and definable.

(until C++11)

A copy assignment operator is eligible if it is not deleted.

(since C++11)
(until C++20)

A copy assignment operator is eligible if

  • it is not deleted, and
  • its associated constraints, if any, are satisfied, and
  • no copy assignment operator with the same first parameter type and the same cv/ref-qualifiers (if any) is more constrained than it.
(since C++20)

Triviality of eligible copy assignment operators determines whether the class is a trivially copyable type.

Implicitly-defined copy assignment operator

If the implicitly-declared copy assignment operator is neither deleted nor trivial, 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++14). For union types, the implicitly-defined copy assignment copies the object representation (as by std::memmove). For non-union class types (class and struct), the operator performs member-wise copy assignment of the object's bases and non-static members, in their initialization order, using built-in assignment for the scalars and copy assignment operator for class types.

The implicitly-defined copy assignment operator for a class T is constexpr if

  • T is a literal type, and
  • the assignment operator selected to copy each direct base class subobject is a constexpr function, and
  • for each non-static data member of T that is of class type (or array thereof), the assignment operator selected to copy that member is a constexpr function.
(since C++14)

The generation of the implicitly-defined copy assignment operator is deprecated if T has a user-declared destructor or user-declared copy constructor.

(since C++11)

Notes

If both copy and move assignment operators are provided, overload resolution selects the move assignment if the argument is an rvalue (either a prvalue such as a nameless temporary or an xvalue such as the result of std::move), and selects the copy assignment if the argument is an lvalue (named object or a function/operator returning lvalue reference). If only the copy assignment is provided, all argument categories select it (as long as it takes its argument by value or as reference to const, since rvalues can bind to const references), which makes copy assignment the fallback for move assignment, when move is unavailable.

It is unspecified whether virtual base class subobjects that are accessible through more than one path in the inheritance lattice, are assigned more than once by the implicitly-defined copy assignment operator (same applies to move assignment).

See assignment operator overloading for additional detail on the expected behavior of a user-defined copy-assignment operator.

Example

Run this code
#include <iostream>
#include <memory>
#include <string>
#include <algorithm>
 
struct A
{
    int n;
    std::string s1;
    A() = default;
    A(A const&) = default;
    // user-defined copy assignment (copy-and-swap idiom)
    A& operator=(A other)
    {
        std::cout << "copy assignment of A\n";
        std::swap(n, other.n);
        std::swap(s1, other.s1);
        return *this;
    }
};
 
struct B : A
{
    std::string s2;
    // implicitly-defined copy assignment
};
 
struct C
{
    std::unique_ptr<int[]> data;
    std::size_t size;
    // user-defined copy assignment (non copy-and-swap idiom)
    // note: copy-and-swap would always reallocate resources
    C& operator=(const C& other)
    {
        if (this != &other) // not a self-assignment
        {
            if (size != other.size) // resource cannot be reused
            {
                data.reset(new int[other.size]);
                size = other.size;
            }
            std::copy(&other.data[0], &other.data[0] + size, &data[0]);
        }
        return *this;
    }
};
 
int main()
{
    A a1, a2;
    std::cout << "a1 = a2 calls ";
    a1 = a2; // user-defined copy assignment
 
    B b1, b2;
    b2.s1 = "foo";
    b2.s2 = "bar";
    std::cout << "b1 = b2 calls ";
    b1 = b2; // implicitly-defined copy assignment
    std::cout << "b1.s1 = " << b1.s1 << "; b1.s2 = " << b1.s2 << '\n';
}

Output: 

a1 = a2 calls copy assignment of A
b1 = b2 calls copy assignment of A
b1.s1 = foo; b1.s2 = bar

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 a volatile subobject made defaulted copy
assignment operators non-trivial (CWG issue 496) 		triviality not affected
CWG 2171 C++11 operator=(X&) = default was non-trivial made trivial
CWG 2180 C++11 a defaulted copy assignment operator for class T was not defined as deleted
if T is abstract and has non-copy-assignable direct virtual base classes 		the operator is defined
as deleted in this case

See also

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