assignment operator c inheritance

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Assignment Operators in C

In C, the assignment operator stores a certain value in an already declared variable. A variable in C can be assigned the value in the form of a literal, another variable or an expression. The value to be assigned forms the right hand operand, whereas the variable to be assigned should be the operand to the left of = symbol, which is defined as a simple assignment operator in C. In addition, C has several augmented assignment operators.

The following table lists the assignment operators supported by the C language −

Simple assignment operator (=)

The = operator is the most frequently used operator in C. As per ANSI C standard, all the variables must be declared in the beginning. Variable declaration after the first processing statement is not allowed. You can declare a variable to be assigned a value later in the code, or you can initialize it at the time of declaration.

You can use a literal, another variable or an expression in the assignment statement.

Once a variable of a certain type is declared, it cannot be assigned a value of any other type. In such a case the C compiler reports a type mismatch error.

In C, the expressions that refer to a memory location are called "lvalue" expressions. A lvalue may appear as either the left-hand or right-hand side of an assignment.

On the other hand, the term rvalue refers to a data value that is stored at some address in memory. A rvalue is an expression that cannot have a value assigned to it which means an rvalue may appear on the right-hand side but not on the left-hand side of an assignment.

Variables are lvalues and so they may appear on the left-hand side of an assignment. Numeric literals are rvalues and so they may not be assigned and cannot appear on the left-hand side. Take a look at the following valid and invalid statements −

Augmented assignment operators

In addition to the = operator, C allows you to combine arithmetic and bitwise operators with the = symbol to form augmented or compound assignment operator. The augmented operators offer a convenient shortcut for combining arithmetic or bitwise operation with assignment.

For example, the expression a+=b has the same effect of performing a+b first and then assigning the result back to the variable a.

Similarly, the expression a<<=b has the same effect of performing a<<b first and then assigning the result back to the variable a.

Here is a C program that demonstrates the use of assignment operators in C:

When you compile and execute the above program, it produces the following result −

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assignment operators

Assignment operators, what is “self assignment”.

Self assignment is when someone assigns an object to itself. For example,

Obviously no one ever explicitly does a self assignment like the above, but since more than one pointer or reference can point to the same object (aliasing), it is possible to have self assignment without knowing it:

This is only valid for copy assignment. Self-assignment is not valid for move assignment.

Why should I worry about “self assignment”?

If you don’t worry about self assignment , you’ll expose your users to some very subtle bugs that have very subtle and often disastrous symptoms. For example, the following class will cause a complete disaster in the case of self-assignment:

If someone assigns a Fred object to itself, line #1 deletes both this->p_ and f.p_ since *this and f are the same object. But line #2 uses *f.p_ , which is no longer a valid object. This will likely cause a major disaster.

The bottom line is that you the author of class Fred are responsible to make sure self-assignment on a Fred object is innocuous . Do not assume that users won’t ever do that to your objects. It is your fault if your object crashes when it gets a self-assignment.

Aside: the above Fred::operator= (const Fred&) has a second problem: If an exception is thrown while evaluating new Wilma(*f.p_) (e.g., an out-of-memory exception or an exception in Wilma ’s copy constructor ), this->p_ will be a dangling pointer — it will point to memory that is no longer valid. This can be solved by allocating the new objects before deleting the old objects.

Okay, okay, already; I’ll handle self-assignment. How do I do it?

You should worry about self assignment every time you create a class . This does not mean that you need to add extra code to all your classes: as long as your objects gracefully handle self assignment, it doesn’t matter whether you had to add extra code or not.

We will illustrate the two cases using the assignment operator in the previous FAQ :

If self-assignment can be handled without any extra code, don’t add any extra code. But do add a comment so others will know that your assignment operator gracefully handles self-assignment:

Example 1a:

Example 1b:

If you need to add extra code to your assignment operator, here’s a simple and effective technique:

Or equivalently:

By the way: the goal is not to make self-assignment fast. If you don’t need to explicitly test for self-assignment, for example, if your code works correctly (even if slowly) in the case of self-assignment, then do not put an if test in your assignment operator just to make the self-assignment case fast. The reason is simple: self-assignment is almost always rare, so it merely needs to be correct - it does not need to be efficient. Adding the unnecessary if statement would make a rare case faster by adding an extra conditional-branch to the normal case, punishing the many to benefit the few.

In this case, however, you should add a comment at the top of your assignment operator indicating that the rest of the code makes self-assignment is benign, and that is why you didn’t explicitly test for it. That way future maintainers will know to make sure self-assignment stays benign, or if not, they will need to add the if test.

I’m creating a derived class; should my assignment operators call my base class’s assignment operators?

Yes (if you need to define assignment operators in the first place).

If you define your own assignment operators, the compiler will not automatically call your base class’s assignment operators for you. Unless your base class’s assignment operators themselves are broken, you should call them explicitly from your derived class’s assignment operators (again, assuming you create them in the first place).

However if you do not create your own assignment operators, the ones that the compiler create for you will automatically call your base class’s assignment operators.

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The assignment operator ( = ) is, strictly speaking, a binary operator. Its declaration is identical to any other binary operator, with the following exceptions:

• It must be a nonstatic member function. No operator= can be declared as a nonmember function.
• It is not inherited by derived classes.
• A default operator= function can be generated by the compiler for class types, if none exists.

The following example illustrates how to declare an assignment operator:

The supplied argument is the right side of the expression. The operator returns the object to preserve the behavior of the assignment operator, which returns the value of the left side after the assignment is complete. This allows chaining of assignments, such as:

The copy assignment operator is not to be confused with the copy constructor. The latter is called during the construction of a new object from an existing one:

It is advisable to follow the rule of three that a class which defines a copy assignment operator should also explicitly define copy constructor, destructor, and, starting with C++11, move constructor and move assignment operator.

• Operator Overloading
• Copy Constructors and Copy Assignment Operators (C++)

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The capability of a class to derive properties and characteristics from another class is called Inheritance . Inheritance is one of the most important features of Object-Oriented Programming. 

Inheritance is a feature or a process in which, new classes are created from the existing classes. The new class created is called “derived class” or “child class” and the existing class is known as the “base class” or “parent class”. The derived class now is said to be inherited from the base class.

When we say derived class inherits the base class, it means, the derived class inherits all the properties of the base class, without changing the properties of base class and may add new features to its own. These new features in the derived class will not affect the base class. The derived class is the specialized class for the base class.

• Sub Class: The class that inherits properties from another class is called Subclass or Derived Class. 
• Super Class: The class whose properties are inherited by a subclass is called Base Class or Superclass. 

The article is divided into the following subtopics:  

Why and when to use inheritance?

• Modes of Inheritance
• Types of Inheritance

Consider a group of vehicles. You need to create classes for Bus, Car, and Truck. The methods fuelAmount(), capacity(), applyBrakes() will be the same for all three classes. If we create these classes avoiding inheritance then we have to write all of these functions in each of the three classes as shown below figure: 

You can clearly see that the above process results in duplication of the same code 3 times. This increases the chances of error and data redundancy. To avoid this type of situation, inheritance is used. If we create a class Vehicle and write these three functions in it and inherit the rest of the classes from the vehicle class, then we can simply avoid the duplication of data and increase re-usability. Look at the below diagram in which the three classes are inherited from vehicle class:  

Using inheritance, we have to write the functions only one time instead of three times as we have inherited the rest of the three classes from the base class (Vehicle). Implementing inheritance in C++ : For creating a sub-class that is inherited from the base class we have to follow the below syntax. 

Derived Classes: A Derived class is defined as the class derived from the base class. Syntax : 

Where class      — keyword to create a new class derived_class_name   — name of the new class, which will inherit the base class access-specifier  — either of private, public or protected. If neither is specified, PRIVATE is taken as default base-class-name  — name of the base class Note : A derived class doesn’t inherit access to private data members. However, it does inherit a full parent object, which contains any private members which that class declares.

Example: 1. class ABC : private XYZ              //private derivation             {                } 2. class ABC : public XYZ              //public derivation             {               } 3. class ABC : protected XYZ              //protected derivation             {              } 4. class ABC: XYZ                            //private derivation by default {            }

o When a base class is privately inherited by the derived class, public members of the base class becomes the private members of the derived class and therefore, the public members of the base class can only be accessed by the member functions of the derived class. They are inaccessible to the objects of the derived class. o On the other hand, when the base class is publicly inherited by the derived class, public members of the base class also become the public members of the derived class. Therefore, the public members of the base class are accessible by the objects of the derived class as well as by the member functions of the derived class.  

In the above program, the ‘Child’ class is publicly inherited from the ‘Parent’ class so the public data members of the class ‘Parent’ will also be inherited by the class ‘Child’. Modes of Inheritance: There are 3 modes of inheritance.

• Public Mode : If we derive a subclass from a public base class. Then the public member of the base class will become public in the derived class and protected members of the base class will become protected in the derived class.
• Protected Mode : If we derive a subclass from a Protected base class. Then both public members and protected members of the base class will become protected in the derived class.
• Private Mode : If we derive a subclass from a Private base class. Then both public members and protected members of the base class will become Private in the derived class.

Note: The private members in the base class cannot be directly accessed in the derived class, while protected members can be directly accessed. For example, Classes B, C, and D all contain the variables x, y, and z in the below example. It is just a question of access.  

The below table summarizes the above three modes and shows the access specifier of the members of the base class in the subclass when derived in public, protected and private modes: 

Types Of Inheritance:-

• Single inheritance
• Multilevel inheritance
• Multiple inheritance
• Hierarchical inheritance
• Hybrid inheritance

Types of Inheritance in C++

1. Single Inheritance : In single inheritance, a class is allowed to inherit from only one class. i.e. one subclass is inherited by one base class only.

Output:- Enter the Value of A= 3 3 Enter the Value of B= 5 5 Product of 3 * 5 = 15

2. Multiple Inheritance: Multiple Inheritance is a feature of C++ where a class can inherit from more than one class. i.e one subclass is inherited from more than one base class .

Here, the number of base classes will be separated by a comma (‘, ‘) and the access mode for every base class must be specified. 

To know more about it, please refer to the article Multiple Inheritances . 

3. Multilevel Inheritance : In this type of inheritance, a derived class is created from another derived class.

4. Hierarchical Inheritance : In this type of inheritance, more than one subclass is inherited from a single base class. i.e. more than one derived class is created from a single base class.

5. Hybrid (Virtual) Inheritance : Hybrid Inheritance is implemented by combining more than one type of inheritance. For example: Combining Hierarchical inheritance and Multiple Inheritance.  Below image shows the combination of hierarchical and multiple inheritances:

6. A special case of hybrid inheritance: Multipath inheritance :  A derived class with two base classes and these two base classes have one common base class is called multipath inheritance. Ambiguity can arise in this type of inheritance.  Example:

In the above example, both ClassB and ClassC inherit ClassA, they both have a single copy of ClassA. However Class-D inherits both ClassB and ClassC, therefore Class-D has two copies of ClassA, one from ClassB and another from ClassC.  If we need to access the data member of ClassA through the object of Class-D, we must specify the path from which a will be accessed, whether it is from ClassB or ClassC, bcoz compiler can’t differentiate between two copies of ClassA in Class-D.

There are 2 Ways to Avoid this Ambiguity:  

1) Avoiding ambiguity using the scope resolution operator:  Using the scope resolution operator we can manually specify the path from which data member a will be accessed, as shown in statements 3 and 4, in the above example. 

Note: Still, there are two copies of ClassA in Class-D. 2) Avoiding ambiguity using the virtual base class:  

According to the above example, Class-D has only one copy of ClassA, therefore, statement 4 will overwrite the value of a, given in statement 3.

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21.12 — Overloading the assignment operator

The copy assignment operator (operator=) is used to copy values from one object to another already existing object .

Related content

As of C++11, C++ also supports “Move assignment”. We discuss move assignment in lesson 22.3 -- Move constructors and move assignment .

Copy assignment vs Copy constructor

The purpose of the copy constructor and the copy assignment operator are almost equivalent -- both copy one object to another. However, the copy constructor initializes new objects, whereas the assignment operator replaces the contents of existing objects.

The difference between the copy constructor and the copy assignment operator causes a lot of confusion for new programmers, but it’s really not all that difficult. Summarizing:

• If a new object has to be created before the copying can occur, the copy constructor is used (note: this includes passing or returning objects by value).
• If a new object does not have to be created before the copying can occur, the assignment operator is used.

Overloading the assignment operator

Overloading the copy assignment operator (operator=) is fairly straightforward, with one specific caveat that we’ll get to. The copy assignment operator must be overloaded as a member function.

This prints:

This should all be pretty straightforward by now. Our overloaded operator= returns *this, so that we can chain multiple assignments together:

Issues due to self-assignment

Here’s where things start to get a little more interesting. C++ allows self-assignment:

This will call f1.operator=(f1), and under the simplistic implementation above, all of the members will be assigned to themselves. In this particular example, the self-assignment causes each member to be assigned to itself, which has no overall impact, other than wasting time. In most cases, a self-assignment doesn’t need to do anything at all!

However, in cases where an assignment operator needs to dynamically assign memory, self-assignment can actually be dangerous:

First, run the program as it is. You’ll see that the program prints “Alex” as it should.

Now run the following program:

You’ll probably get garbage output. What happened?

Consider what happens in the overloaded operator= when the implicit object AND the passed in parameter (str) are both variable alex. In this case, m_data is the same as str.m_data. The first thing that happens is that the function checks to see if the implicit object already has a string. If so, it needs to delete it, so we don’t end up with a memory leak. In this case, m_data is allocated, so the function deletes m_data. But because str is the same as *this, the string that we wanted to copy has been deleted and m_data (and str.m_data) are dangling.

Later on, we allocate new memory to m_data (and str.m_data). So when we subsequently copy the data from str.m_data into m_data, we’re copying garbage, because str.m_data was never initialized.

Detecting and handling self-assignment

Fortunately, we can detect when self-assignment occurs. Here’s an updated implementation of our overloaded operator= for the MyString class:

By checking if the address of our implicit object is the same as the address of the object being passed in as a parameter, we can have our assignment operator just return immediately without doing any other work.

Because this is just a pointer comparison, it should be fast, and does not require operator== to be overloaded.

When not to handle self-assignment

Typically the self-assignment check is skipped for copy constructors. Because the object being copy constructed is newly created, the only case where the newly created object can be equal to the object being copied is when you try to initialize a newly defined object with itself:

In such cases, your compiler should warn you that c is an uninitialized variable.

Second, the self-assignment check may be omitted in classes that can naturally handle self-assignment. Consider this Fraction class assignment operator that has a self-assignment guard:

If the self-assignment guard did not exist, this function would still operate correctly during a self-assignment (because all of the operations done by the function can handle self-assignment properly).

Because self-assignment is a rare event, some prominent C++ gurus recommend omitting the self-assignment guard even in classes that would benefit from it. We do not recommend this, as we believe it’s a better practice to code defensively and then selectively optimize later.

The copy and swap idiom

A better way to handle self-assignment issues is via what’s called the copy and swap idiom. There’s a great writeup of how this idiom works on Stack Overflow .

The implicit copy assignment operator

Unlike other operators, the compiler will provide an implicit public copy assignment operator for your class if you do not provide a user-defined one. This assignment operator does memberwise assignment (which is essentially the same as the memberwise initialization that default copy constructors do).

Just like other constructors and operators, you can prevent assignments from being made by making your copy assignment operator private or using the delete keyword:

Note that if your class has const members, the compiler will instead define the implicit operator= as deleted. This is because const members can’t be assigned, so the compiler will assume your class should not be assignable.

If you want a class with const members to be assignable (for all members that aren’t const), you will need to explicitly overload operator= and manually assign each non-const member.

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Using-declaration.

Introduces a name that is defined elsewhere into the declarative region where this using-declaration appears. See using enum and (since C++20) using namespace for other related declarations.

[ edit ] Explanation

Using-declarations can be used to introduce namespace members into other namespaces and block scopes, or to introduce base class members into derived class definitions , or to introduce enumerators into namespaces, block, and class scopes (since C++20) .

[ edit ] In namespace and block scope

Using-declarations introduce a member of another namespace into the current namespace or block scope.

See namespace for details.

[ edit ] In class definition

Using-declaration introduces a member of a base class into the derived class definition, such as to expose a protected member of base as public member of derived. In this case, nested-name-specifier must name a base class of the one being defined. If the name is the name of an overloaded member function of the base class, all base class member functions with that name are introduced. If the derived class already has a member with the same name, parameter list, and qualifications, the derived class member hides or overrides (doesn't conflict with) the member that is introduced from the base class.

[ edit ] Notes

Only the name explicitly mentioned in the using-declaration is transferred into the declarative scope: in particular, enumerators are not transferred when the enumeration type name is using-declared.

A using-declaration cannot refer to a namespace , to a scoped enumerator (until C++20) , to a destructor of a base class or to a specialization of a member template for a user-defined conversion function.

A using-declaration cannot name a member template specialization ( template-id is not permitted by the grammar):

A using-declaration also can't be used to introduce the name of a dependent member template as a template-name (the template disambiguator for dependent names is not permitted).

If a using-declaration brings the base class assignment operator into derived class, whose signature happens to match the derived class's copy-assignment or move-assignment operator, that operator is hidden by the implicitly-declared copy/move assignment operator of the derived class. Same applies to a using-declaration that inherits a base class constructor that happens to match the derived class copy/move constructor (since C++11) .

[ edit ] Defect reports

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

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