What Is An Implicitly-defined Copy Assignment Operator In C++?

In C++ programming language, Object-Oriented Programming (OOP) is very widely used as a way to work on data functions in a way that helps represent the real world in an abstract manner. Classes and Objects are the best way to work on properties and methods. In a modern C++ Compiler, one of the OOP features is copy assignment operator that is used with “operator=” to create a new object from an existing one. In this post, we explain an implicitly-defined copy assignment operator in C++ with examples.

What are classes and objects in C++?

Classes are defined in C++ using keyword class followed by the name of the class. Classes are the blueprint for the objects and they are user-defined data types that we can use in our program, and they work as an object constructor. Objects are an instantiation of a class, In C++ programming, most of the commands are associated with classes and objects, along with their attributes and methods. Here is a simple class example below.

then we can create our objects with this Type of myclass as shown below.

What is an implicitly-defined copy assignment operator in C++?

The Copy Assignment Operator in a class is a non-template non-static member function that is declared with the “operator=“. When you create a class, struct, or union that is copy assignable (that you can copy with the = operator symbol), it has a default copy assignment operator. The implicitly-defined copy assignment operator is defined If neither deleted nor trivial. That means this operator has a function body which is generated and compiled. This is called as Implicitly-defined copy assignment operator.

In C++, T represents a literal type, it can be function, class type (class, struct, union object types), fundamentals type (void, bool, char, wchar_t), compound types (reference, pointer, array, function, enumeration).

Since C++11,  if a class type has a user-declared destructor or user-declared copy constructor, the implicitly-defined copy assignment operator is deprecated. The implicitly-defined copy assignment operator for a class T is constexpr if,

• In general, for any non-static data member of type T 
• Since C++14, the Implicitly-defined copy assignment operator is used with a type T
• Since C++23, the assignment operator selected to copy each direct base class subobject is a constexpr function
• Since C++23, the implicitly-defined copy assignment operator for a class T is constexpr.

What are the declaration and definition of a class?

A declaration declares a unique name for the entity, along with information about its type (a type, class, struct, union) and other characteristics (parameters, options, base of it, etc.). In C++ all types, classes, structs, unions must be declared before they can be used.

A definition provides the compiler with all the information it needs to generate machine code when the entity is used later in the program. The definition means this operator has a function body which is generated and compiled.

Here is a simple syntax for default copy assignment operator with default option;

here is a declaration example in a class,

here is a definition example including declaration,

now let’s see what is implicitly-defined copy assignment operator with a simple example.

Is there an example of an implicitly-defined copy assignment operator in C++?

After these useful information above, let’s give an example of an implicitly-defined copy assignment operator. Let’s assume that we use TmyclassA as a base class and we have a new TmyclassB class. This new class can use the copy assignment operator implicitly from the TmyclassA.

Here is a TmyclassA class example.

and when you define a new TmyclassA class example as below.

As you see, here, because of : public TmyclassA part, this TmyclassB class has implicitly-defined copy assignment operator from TmyclassA. Now we can use this in the copy of class objects as shown below.

Is there a full example of an implicitly defined copy assignment operator in C++?

Here is a full example with an implicitly-defined copy assignment operator in a class.

Here is the output.

As you see, in Modern C++, we can directly use an implicitly defined copy assignment operator from other classes in C++ without any definition.

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