/ | ||||
(C++11) | ||||
(C++11) |
(C++11) | ||||
(C++20) | ||||
(C++20) |
(C++11) | ||||
expression |
pointer |
specifier | ||||
specifier (C++11) | ||||
specifier (C++11) |
(C++11) | ||||
(C++11) |
(C++11) | ||||
(C++11) |
General | ||||
/ types | ||||
types | ||||
Members | ||||
pointer | ||||
-declarations | ||||
(C++11) | ||||
specifier | ||||
specifier | ||||
Special member functions | ||||
(C++11) | ||||
(C++11) | ||||
Inheritance | ||||
specifier (C++11) | ||||
specifier (C++11) |
A copy assignment operator is a non-template non-static member function with the name operator = that can be called with an argument of the same class type and copies the content of the argument without mutating the argument.
Syntax Explanation Implicitly-declared copy assignment operator Implicitly-defined copy assignment operator Deleted copy assignment operator Trivial copy assignment operator Eligible copy assignment operator Notes Example Defect reports See also |
For the formal copy assignment operator syntax, see function declaration . The syntax list below only demonstrates a subset of all valid copy assignment operator syntaxes.
return-type parameter-list | (1) | ||||||||
return-type parameter-list function-body | (2) | ||||||||
return-type parameter-list-no-default | (3) | (since C++11) | |||||||
return-type parameter-list | (4) | (since C++11) | |||||||
return-type class-name parameter-list function-body | (5) | ||||||||
return-type class-name parameter-list-no-default | (6) | (since C++11) | |||||||
class-name | - | the class whose copy assignment operator is being declared, the class type is given as in the descriptions below |
parameter-list | - | a of only one parameter, which is of type , , const T&, volatile T& or const volatile T& |
parameter-list-no-default | - | a of only one parameter, which is of type , , const T&, volatile T& or const volatile T& and does not have a default argument |
function-body | - | the of the copy assignment operator |
return-type | - | any type, but is favored in order to allow chaining asssignments |
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.
If no user-defined copy assignment operators are provided for a class type, 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:
Otherwise the implicitly-declared copy assignment operator is declared as T & T :: operator = ( T & ) .
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.
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, the operator performs member-wise copy assignment of the object's direct bases and non-static data members, in their initialization order, using built-in assignment for the scalars, memberwise copy-assignment for arrays, and copy assignment operator for class types (called non-virtually).
The implicitly-defined copy assignment operator for a class is if is a , and that is of class type (or array thereof), the assignment operator selected to copy that member is a constexpr function. | (since C++14) (until C++23) |
The implicitly-defined copy assignment operator for a class is . | (since C++23) |
The generation of the implicitly-defined copy assignment operator is deprecated if has a user-declared destructor or user-declared copy constructor. | (since C++11) |
An implicitly-declared or explicitly-defaulted (since C++11) copy assignment operator for class T is undefined (until C++11) defined as deleted (since C++11) if any of the following conditions is satisfied:
The implicitly-declared copy assignment operator for class is defined as deleted if declares a or . | (since C++11) |
The copy assignment operator for class T is trivial if all of the following is true:
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.
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 all following conditions are satisfied: (if any) are satisfied. than any other copy assignment operator. | (since C++20) |
Triviality of eligible copy assignment operators determines whether the class is a trivially copyable type .
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.
[ edit ] defect reports.
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
---|---|---|---|
C++98 | the conditions where implicitly-declared copy assignment operators are undefined did not consider multi-dimensional array types | consider these types | |
C++11 | a volatile subobject made defaulted copy assignment operators non-trivial ( ) | triviality not affected | |
C++11 | operator=(X&) = default was non-trivial | made trivial | |
C++11 | a defaulted copy assignment operator for class was not defined as deleted if is abstract and has non-copy-assignable direct virtual base classes | the operator is defined as deleted in this case | |
C++20 | a copy assignment operator was not eligible if there is another copy assignment operator which is more constrained but does not satisfy its associated constraints | it can be eligible in this case |
This article will explain several methods of how to implement assignment operator overloading in C++.
C++ provides the feature to overload operators, a common way to call custom functions when a built-in operator is called on specific classes. These functions should have a special name starting with operator followed by the specific operator symbol itself. E.g., a custom assignment operator can be implemented with the function named operator= . Assignment operator generally should return a reference to its left-hand operand. Note that if the user does not explicitly define the copy assignment operator, the compiler generates one automatically. The generated version is quite capable when the class does not contain any data members manually allocated on the heap memory. It can even handle the array members by assigning each element to the corresponding object members. Although, it has shortcomings when dealing with dynamic memory data members, as shown in the following example code.
The above code defines only copy-constructor explicitly, which results in incorrect behavior when P1 object contents are assigned to the P3 object. Note that the second call to the P1.renamePerson function should not have modified the P3 object’s data members, but it did. The solution to this is to define an overloaded assignment operator i.e., copy-assignment operator. The next code snippet implements the version of the Person class that can copy assign the two objects of the same class correctly. Notice, though, the if statement in the copy-assignment function guarantees that the operator works correctly even when the object is assigned to itself.
Founder of DelftStack.com. Jinku has worked in the robotics and automotive industries for over 8 years. He sharpened his coding skills when he needed to do the automatic testing, data collection from remote servers and report creation from the endurance test. He is from an electrical/electronics engineering background but has expanded his interest to embedded electronics, embedded programming and front-/back-end programming.
Find centralized, trusted content and collaborate around the technologies you use most.
Q&A for work
Connect and share knowledge within a single location that is structured and easy to search.
Get early access and see previews of new features.
i want to add a overloaded assignment operator to my object class in c++ but when I do this
and with a header file like this
when compiling I get a compile error that says:
You need to declare the function in your header file so you can define it later on.
Reminder: Answers generated by artificial intelligence tools are not allowed on Stack Overflow. Learn more
Post as a guest.
Required, but never shown
By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy .
COMMENTS
21.12 — Overloading the assignment operator. Alex July 22, 2024. The copy assignment operator (operator=) is used to copy values from one object to another already existing object. As of C++11, C++ also supports "Move assignment". We discuss move assignment in lesson 22.3 -- Move constructors and move assignment .
The assignment operator,"=", is the operator used for Assignment. It copies the right value into the left value. Assignment Operators are predefined to operate only on built-in Data types. Assignment operator overloading is binary operator overloading. Overloading assignment operator in C++ copies all values of one object to another object.
There are no problems with the second version of the assignment operator. In fact, that is the standard way for an assignment operator. Edit: Note that I am referring to the return type of the assignment operator, not to the implementation itself. As has been pointed out in comments, the implementation itself is another issue.
Although the canonical implementations of the prefix increment and decrement operators return by reference, as with any operator overload, the return type is user-defined; for example the overloads of these operators for std::atomic return by value. [] Binary arithmetic operatorBinary operators are typically implemented as non-members to maintain symmetry (for example, when adding a complex ...
Overloaded operators are implemented as functions. The name of an overloaded operator is operator x, where x is the operator as it appears in the following table. For example, to overload the addition operator, you define a function called operator+. Similarly, to overload the addition/assignment operator, +=, define a function called operator+=.
It allows you to provide an intuitive interface to users of your class, plus makes it possible for templates to work equally well with classes and built-in/intrinsic types. Operator overloading allows C/C++ operators to have user-defined meanings on user-defined types (classes). Overloaded operators are syntactic sugar for function calls: class ...
Syntax. Defining an overloaded operator is like defining a function, but the name of that function is operator@, in which @ represents the operator that's being overloaded. The number of arguments in the overloaded operator's argument list depends on two factors: Whether it's a unary operator (one argument) or a binary operator (two ...
This identity cannot be relied upon for class types that overload operators. Moreover, some of the requirements implicit in the use of these operators for basic types are relaxed for overloaded operators. For example, the addition/assignment operator, +=, requires the left operand to be an l-value when applied to basic types; there is no such ...
C++ compiler implicitly provides a copy constructor, if no copy constructor is defined in the class. A bitwise copy gets created, if the Assignment operator is not overloaded. Consider the following C++ program. Explanation: Here, t2 = t1; calls the assignment operator, same as t2.operator= (t1); and Test t3 = t1; calls the copy constructor ...
You can implement C++ operator overloads by providing special member-functions on your classes that follow a particular naming convention. For example, to overload the + operator for your class, you would provide a member-function named operator+ on your class. The following set of operators is commonly overloaded for user-defined classes:
1) Do not allow assignment of one object to other object. We can create our own dummy assignment operator and make it private. 2) Write your own assignment operator that does deep copy. Same is true for Copy Constructor. Following is an example of overloading assignment operator for the above class. #include<iostream>.
1. By default, operators = and & are already overloaded in C++. For example, we can directly use the = operator to copy objects of the same class. Here, we do not need to create an operator function. 2. We cannot change the precedence and associativity of operators using operator overloading. 3. We cannot overload following operators in C++:
Different types of assignment operators are shown below: 1. "=": This is the simplest assignment operator. This operator is used to assign the value on the right to the variable on the left. Example: a = 10; b = 20; ch = 'y'; 2. "+=": This operator is combination of '+' and '=' operators. This operator first adds the current ...
You can overload the assignment operator (=) just as you can other operators and it can be used to create an object just like the copy constructor. Following example explains how an assignment operator can be overloaded. private: int feet; // 0 to infinite. int inches; // 0 to 12. public:
An overloaded assignment operator should look like this: Complex &Complex::operator=(const Complex& rhs) {. real = rhs.real; imaginary = rhs.imaginary; return *this; }; You should also note, that if you overload the assignment operator you should overload the copy constructor for Complex in the same manner:
The assignment operator (operator =) has special properties: ... The best known example of a canonical overloaded operator& is the Microsoft class CComPtr. An example of its use in EDSL can be found in boost.spirit. The boolean logic operators, operator && and operator ||. Unlike the built-in versions, the overloads cannot implement short ...
Triviality of eligible copy assignment operators determines whether the class is a trivially copyable type. [] NoteIf 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 ...
The solution to this is to define an overloaded assignment operator i.e., copy-assignment operator. The next code snippet implements the version of the Person class that can copy assign the two objects of the same class correctly. Notice, though, the if statement in the copy-assignment function guarantees that the operator works correctly even ...
Using friend operator overload should do the trick for you and is a common way to define binary operators, just add: friend sample operator+(const sample& a, const sample& b); //in class sample operator+(const sample& a, const sample& b) { //outside the class return sample(a.x + b.x); }
This is a list of operators in the C and C++ programming languages.All the operators (except typeof) listed exist in C++; the column "Included in C", states whether an operator is also present in C. Note that C does not support operator overloading.. When not overloaded, for the operators &&, ||, and , (the comma operator), there is a sequence point after the evaluation of the first operand.
4. Correct me if I'm wrong: I understand that when having a class with members that are pointers, a copy of a class object will result in that the pointers representing the same memory address. This can result in changes done to one class object to affect all copies of this object. A solution to this can be to overload the = operator.
First, don't tag this as C, since this is C++. Second, you need a copy constructor to go along with the assignment operator. I suggest you write the copy constructor first. Once you do that, the assignment operator becomes a very simple 5 lines of std::swap calls. -