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Abstract Class in C++ - Scaler Topics
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0.23-0.832 0.23z"></path></svg><span class="mobile_course__info__6TMCv">5</span></div></div></div></div><a class="tappable commonStyles_learning_button__JYLbC full-width" href="/topics/course/cpp-beginners/">Start Learning </a></div></div></div></div><div class="m-t-s article-outline_outline__0_P6T"><a class="Tappable-module_root__N7ll5 row full-width bold article-outline_outline__heading__YVedg"><svg fill="currentColor" class="icons_icon_chevron__cl99H icons_icon_chevron__right__YxLYH" version="1.1" xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 32 32"><path d="M9.875 20.563l-1.875-1.875 8-8 8 8-1.875 1.875-6.125-6.125z"></path></svg><div>Topics Covered</div></a></div><div class="markdown-body"><section class="main"><p>In C++, an abstract class is defined by having at least one pure virtual function, a function without a concrete definition. These classes are essential in object-oriented programming, structuring code to mirror real-life scenarios through inheritance and abstraction. Abstract classes, which cannot be instantiated, are pivotal for expressing broad concepts and upcasting, allowing derived classes to implement their interfaces. Utilize pointers and references for abstract class types, and remember that any subclass failing to define the pure virtual function becomes abstract itself. The virtual function is declared with the pure specifier (= 0).</p><h2 id="abstract-class-in-c---example" level="2">Abstract Class in C++ Example</h2><p>Let’s say we are making a calculator which returns the perimeter of the shape we put in. Think of what kind of code you would write for such a calculator. You might start with some initial shapes and hardcode the perimeter by making separate functions inside the Shape class.</p><p>The class might look something like this –</p><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p>This will work, but OOP suggests that we should try to stay close to real-world logic. So, we can make a class Shape as a parent class and then make separate classes like Square and Rectangle as child classes. This will make the code easier to maintain, and if you plan to add anything new in the future, then you can add that in the child class. To implement this feature, we need to use Abstract Classes. Abstract classes in C++ need at least one <strong>pure virtual function</strong> in a class. The classes that inherit the abstract class must define the pure virtual function; otherwise, the subclass will become an abstract class.</p></section> <section class="main"><h2 id="restrictions-on-abstract-classes" level="2">Restrictions on Abstract Classes</h2><p>There are some restrictions on abstract classes in C++.</p><p>Abstract classes cannot be used for the following –</p><ul> <li>Variables or member data</li> <li>Argument types</li> <li>Function return types</li> <li>Types of explicit conversions.</li> </ul><p>The constructors of the abstract class in c++ can call other member functions, but if they directly or indirectly call the pure virtual function, then the result is undefined.</p><p>But wait! What is a pure virtual function anyway?</p><p>To understand the <strong>pure virtual function</strong>, first, let’s understand virtual functions.</p><p>A <a href="https://www.scaler.com/topics/virtual-function-in-cpp/" target="_blank" title=""><strong>virtual function in C++</strong></a> is a member function declared within a base class and redefined by the derived class.</p><p>A <strong>pure virtual function (or abstract function)</strong> is a virtual function with no definition/logic. It is declared by assigning 0 at the time of declaration.</p><p>Let’s take a look at an example that will make things clear.</p><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p><strong>Output –</strong></p><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p>In the above code, you can see that the function <span class="highlight--red">perimeter()</span> is a pure virtual function, the <strong>“virtual”</strong> keyword is used, and it is assigned a value of 0.</p><p>In the derived classes, <strong>Rectangle</strong> and <strong>Shape</strong> redefine the pure virtual function.</p></section> <section class="main"><h2 id="characteristics-of-abstract-class-in-c--" level="2">Characteristics of Abstract Class in C++</h2><ol> <li>Abstract Classes must have at least one pure virtual function.</li> </ol><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><ol start="2"> <li>Abstract Classes cannot be instantiated, but pointers and references of Abstract Class types can be created. You cannot create an object of an abstract class. Here is an example of a pointer to an abstract class.</li> </ol><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p><strong>Output –</strong></p><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><ol start="3"> <li>Abstract Classes are mainly used for <strong>Upcasting</strong>, which means its derived classes can use its interface.</li> <li>Classes that inherit the Abstract Class must implement all pure virtual functions. If they do not, those classes will also be treated as abstract classes.</li> </ol></section> <section class="main"><h2 id="why-can’t-we-make-an-abstract-class-object-" level="2">Why Can’t We Make an Abstract Class Object?</h2><p>Abstract classes in C++ cannot be instantiated because they are “abstract”. It’s like someone is telling you to draw an animal without telling you which specific animal. You can only draw if the person asks you to draw a cat, dog, etc., but an animal is abstract.</p><p>The same is the case with abstract classes in C++, so we can’t create objects, but we can create a pointer of an abstract class.</p></section> <section class="summary"><h2 id="conclusion" level="2">Conclusion</h2><p>Through this article, you now know:</p><ul> <li>What is Abstract Class in C++?</li> <li>What is Inheritance?</li> <li>What is Abstraction?</li> <li>How to create and work with Abstract Class in C++?</li> <li>What are virtual functions, and what is the difference between virtual and pure virtual functions?</li> </ul><p>That’s all for now, folks!</p><p>Thanks for reading.</p></section></div></div></div></div><div class="view_right_section__M6YCz relative p-b-m"></div></div></div><footer><div class="full-width feedback_container__imOut"><div role="button" tabindex="0" class="signin-alert-popup_signInWrapper__Kr4eW"><div class="signin-alert-popup_overLay__Xq3de"></div><div 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Such classes are mainly used for Upcasting.\nauthor: Shubh Agarwal\ncategory: C++\n---\n\n\n\n\n\n:::section{.main}\nIn C++, an abstract class is defined by having at least one pure virtual function, a function without a concrete definition. These classes are essential in object-oriented programming, structuring code to mirror real-life scenarios through inheritance and abstraction. Abstract classes, which cannot be instantiated, are pivotal for expressing broad concepts and upcasting, allowing derived classes to implement their interfaces. Utilize pointers and references for abstract class types, and remember that any subclass failing to define the pure virtual function becomes abstract itself. The virtual function is declared with the pure specifier (= 0).\n\n## Abstract Class in C++ Example\nLet’s say we are making a calculator which returns the perimeter of the shape we put in. Think of what kind of code you would write for such a calculator. You might start with some initial shapes and hardcode the perimeter by making separate functions inside the Shape class.\n\nThe class might look something like this –\n\n```cpp\nclass Shape {\n public:\n // All the functions of both square and rectangle are clubbed together in a single class.\n void width(int w) {\n shape_width = w;\n }\n void height(int h) {\n shape_height = h;\n }\n int perimeterOfSquare(int s) {\n return 4 * s;\n }\n int perimeterOfRectange(int l, int b) {\n return 2 * (l + b);\n }\n \n protected:\n int shape_width;\n int shape_height;\n};\n```\nThis will work, but OOP suggests that we should try to stay close to real-world logic. So, we can make a class Shape as a parent class and then make separate classes like Square and Rectangle as child classes. This will make the code easier to maintain, and if you plan to add anything new in the future, then you can add that in the child class. To implement this feature, we need to use Abstract Classes. Abstract classes in C++ need at least one **pure virtual function** in a class. The classes that inherit the abstract class must define the pure virtual function; otherwise, the subclass will become an abstract class.\n\n:::\n\n:::section{.main}\n## Restrictions on Abstract Classes\nThere are some restrictions on abstract classes in C++.\n\nAbstract classes cannot be used for the following –\n\n* Variables or member data\n* Argument types\n* Function return types\n* Types of explicit conversions.\n\nThe constructors of the abstract class in c++ can call other member functions, but if they directly or indirectly call the pure virtual function, then the result is undefined.\n\nBut wait! What is a pure virtual function anyway?\n\nTo understand the **pure virtual function**, first, let’s understand virtual functions.\n\nA [**virtual function in C++**](https://www.scaler.com/topics/virtual-function-in-cpp/) is a member function declared within a base class and redefined by the derived class.\n\nA **pure virtual function (or abstract function)** is a virtual function with no definition/logic. It is declared by assigning 0 at the time of declaration.\n\nLet’s take a look at an example that will make things clear.\n\n```cpp\n#include\u003ciostream\u003e\nusing namespace std;\n\n// Shape class.\nclass Shape {\n public:\n \n // Function to calculate the parameter, declared as pure virtual, so all the derived classes necessarily need to implement this.\n virtual int perimeter() = 0;\n void width(int w) {\n shape_width = w;\n }\n void height(int h) {\n shape_height = h;\n }\n \n protected:\n int shape_width;\n int shape_height;\n};\n\nclass Rectangle: public Shape {\n public: \n // Class Rectangle provided implementation of perimeter() function.\n int perimeter() {\n return (2 * (shape_width + shape_height));\n }\n};\n\nclass Square: public Shape {\n public: \n // Class Square provided implementation of perimeter() function.\n int perimeter() {\n return (4 * shape_width);\n }\n};\n\nint main() {\n Rectangle R;\n Square S;\n\n R.width(10);\n R.height(5);\n S.width(10);\n\n cout \u003c\u003c \"Perimeter of Rectangle: \" \u003c\u003c R.perimeter() \u003c\u003c endl;\n cout \u003c\u003c \"Perimeter of Square: \" \u003c\u003c S.perimeter() \u003c\u003c endl;\n return 0;\n}\n```\n**Output –** \n```plaintext\nPerimeter of Rectangle : 30\nPerimeter of Square: 40\n```\nIn the above code, you can see that the function `perimeter()` is a pure virtual function, the **“virtual”** keyword is used, and it is assigned a value of 0.\n\nIn the derived classes, **Rectangle** and **Shape** redefine the pure virtual function.\n\n:::\n\n:::section{.main}\n## Characteristics of Abstract Class in C++\n1. Abstract Classes must have at least one pure virtual function.\n\n```cpp\nvirtual int perimeter() = 0;\n```\n2. Abstract Classes cannot be instantiated, but pointers and references of Abstract Class types can be created. You cannot create an object of an abstract class.\nHere is an example of a pointer to an abstract class.\n\n```cpp\n#include\u003ciostream\u003e\nusing namespace std;\n\nclass Base {\n public:\n virtual void print() = 0;\n};\n\nclass Derived: public Base {\n public: \n void print() {\n cout \u003c\u003c \"This is from the derived class \\n\";\n }\n};\n\nint main(void) {\n Base* basePointer = new Derived();\n basePointer -\u003e print();\n return 0;\n}\n```\n**Output –** \n```plaintext\nThis is from the derived class\n```\n3. Abstract Classes are mainly used for **Upcasting**, which means its derived classes can use its interface.\n4. Classes that inherit the Abstract Class must implement all pure virtual functions. If they do not, those classes will also be treated as abstract classes.\n\n:::\n\n:::section{.main}\n## Why Can’t We Make an Abstract Class Object?\nAbstract classes in C++ cannot be instantiated because they are “abstract”. It’s like someone is telling you to draw an animal without telling you which specific animal. You can only draw if the person asks you to draw a cat, dog, etc., but an animal is abstract.\n\nThe same is the case with abstract classes in C++, so we can’t create objects, but we can create a pointer of an abstract class.\n\n:::\n\n:::section{.summary}\n## Conclusion\nThrough this article, you now know:\n\n* What is Abstract Class in C++?\n* What is Inheritance?\n* What is Abstraction?\n* How to create and work with Abstract Class in C++?\n* What are virtual functions, and what is the difference between virtual and pure virtual functions?\n\nThat’s all for now, folks! \n\nThanks for reading. \n\n\n:::\n","course_video_data":[],"collaborators":{"authors":[{"name":"Shubh Agrawal","slug":"57c1eae984a4","email":"shubhagrawal4457@gmail.com","image":"https://lh3.googleusercontent.com/a/ACg8ocKvUSQlg-znPsLLVFhkdJIbRbn-RTp3G3gaOho6dFqFrvU=s96-c","linkedin_profile":"https://www.linkedin.com/in/theshubhagrwl/","company":"Superlayer.io","designation":"Full Stack Developer Intern","validated_at":"2024-02-22T13:23:29.000Z","topics_slug":"shubh-agrawal","about":"Shubh Agrawal, a skilled Frontend Engineer, seamlessly combines creativity and technical prowess in crafting visually appealing and user-centric web interfaces. 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Such classes are mainly used for Upcasting.","author":"Shubh Agarwal","category":"C++","publish_date":"undefined"},"contentData":"\n\n\n\n\n:::section{.main}\nIn C++, an abstract class is defined by having at least one pure virtual function, a function without a concrete definition. These classes are essential in object-oriented programming, structuring code to mirror real-life scenarios through inheritance and abstraction. Abstract classes, which cannot be instantiated, are pivotal for expressing broad concepts and upcasting, allowing derived classes to implement their interfaces. Utilize pointers and references for abstract class types, and remember that any subclass failing to define the pure virtual function becomes abstract itself. The virtual function is declared with the pure specifier (= 0).\n\n## Abstract Class in C++ Example\nLet’s say we are making a calculator which returns the perimeter of the shape we put in. Think of what kind of code you would write for such a calculator. You might start with some initial shapes and hardcode the perimeter by making separate functions inside the Shape class.\n\nThe class might look something like this –\n\n```cpp\nclass Shape {\n public:\n // All the functions of both square and rectangle are clubbed together in a single class.\n void width(int w) {\n shape_width = w;\n }\n void height(int h) {\n shape_height = h;\n }\n int perimeterOfSquare(int s) {\n return 4 * s;\n }\n int perimeterOfRectange(int l, int b) {\n return 2 * (l + b);\n }\n \n protected:\n int shape_width;\n int shape_height;\n};\n```\nThis will work, but OOP suggests that we should try to stay close to real-world logic. So, we can make a class Shape as a parent class and then make separate classes like Square and Rectangle as child classes. This will make the code easier to maintain, and if you plan to add anything new in the future, then you can add that in the child class. To implement this feature, we need to use Abstract Classes. Abstract classes in C++ need at least one **pure virtual function** in a class. The classes that inherit the abstract class must define the pure virtual function; otherwise, the subclass will become an abstract class.\n\n:::\n\n:::section{.main}\n## Restrictions on Abstract Classes\nThere are some restrictions on abstract classes in C++.\n\nAbstract classes cannot be used for the following –\n\n* Variables or member data\n* Argument types\n* Function return types\n* Types of explicit conversions.\n\nThe constructors of the abstract class in c++ can call other member functions, but if they directly or indirectly call the pure virtual function, then the result is undefined.\n\nBut wait! What is a pure virtual function anyway?\n\nTo understand the **pure virtual function**, first, let’s understand virtual functions.\n\nA [**virtual function in C++**](https://www.scaler.com/topics/virtual-function-in-cpp/) is a member function declared within a base class and redefined by the derived class.\n\nA **pure virtual function (or abstract function)** is a virtual function with no definition/logic. It is declared by assigning 0 at the time of declaration.\n\nLet’s take a look at an example that will make things clear.\n\n```cpp\n#include\u003ciostream\u003e\nusing namespace std;\n\n// Shape class.\nclass Shape {\n public:\n \n // Function to calculate the parameter, declared as pure virtual, so all the derived classes necessarily need to implement this.\n virtual int perimeter() = 0;\n void width(int w) {\n shape_width = w;\n }\n void height(int h) {\n shape_height = h;\n }\n \n protected:\n int shape_width;\n int shape_height;\n};\n\nclass Rectangle: public Shape {\n public: \n // Class Rectangle provided implementation of perimeter() function.\n int perimeter() {\n return (2 * (shape_width + shape_height));\n }\n};\n\nclass Square: public Shape {\n public: \n // Class Square provided implementation of perimeter() function.\n int perimeter() {\n return (4 * shape_width);\n }\n};\n\nint main() {\n Rectangle R;\n Square S;\n\n R.width(10);\n R.height(5);\n S.width(10);\n\n cout \u003c\u003c \"Perimeter of Rectangle: \" \u003c\u003c R.perimeter() \u003c\u003c endl;\n cout \u003c\u003c \"Perimeter of Square: \" \u003c\u003c S.perimeter() \u003c\u003c endl;\n return 0;\n}\n```\n**Output –** \n```plaintext\nPerimeter of Rectangle : 30\nPerimeter of Square: 40\n```\nIn the above code, you can see that the function `perimeter()` is a pure virtual function, the **“virtual”** keyword is used, and it is assigned a value of 0.\n\nIn the derived classes, **Rectangle** and **Shape** redefine the pure virtual function.\n\n:::\n\n:::section{.main}\n## Characteristics of Abstract Class in C++\n1. Abstract Classes must have at least one pure virtual function.\n\n```cpp\nvirtual int perimeter() = 0;\n```\n2. Abstract Classes cannot be instantiated, but pointers and references of Abstract Class types can be created. You cannot create an object of an abstract class.\nHere is an example of a pointer to an abstract class.\n\n```cpp\n#include\u003ciostream\u003e\nusing namespace std;\n\nclass Base {\n public:\n virtual void print() = 0;\n};\n\nclass Derived: public Base {\n public: \n void print() {\n cout \u003c\u003c \"This is from the derived class \\n\";\n }\n};\n\nint main(void) {\n Base* basePointer = new Derived();\n basePointer -\u003e print();\n return 0;\n}\n```\n**Output –** \n```plaintext\nThis is from the derived class\n```\n3. Abstract Classes are mainly used for **Upcasting**, which means its derived classes can use its interface.\n4. Classes that inherit the Abstract Class must implement all pure virtual functions. If they do not, those classes will also be treated as abstract classes.\n\n:::\n\n:::section{.main}\n## Why Can’t We Make an Abstract Class Object?\nAbstract classes in C++ cannot be instantiated because they are “abstract”. It’s like someone is telling you to draw an animal without telling you which specific animal. You can only draw if the person asks you to draw a cat, dog, etc., but an animal is abstract.\n\nThe same is the case with abstract classes in C++, so we can’t create objects, but we can create a pointer of an abstract class.\n\n:::\n\n:::section{.summary}\n## Conclusion\nThrough this article, you now know:\n\n* What is Abstract Class in C++?\n* What is Inheritance?\n* What is Abstraction?\n* How to create and work with Abstract Class in C++?\n* What are virtual functions, and what is the difference between virtual and pure virtual functions?\n\nThat’s all for now, folks! \n\nThanks for reading. \n\n\n:::\n","mediaList":[],"pageClass":"articlePage","rootClass":"articleLayout","subProduct":"article","fallback":{"https://www.scaler.com/topics/api/v1/search/courses/1/6/":[{"slug":"python-for-beginners","title":"Python Course for Beginners With Certification: Mastering the Essentials","category_type":"topics_course","cover_image_thumbnail":"https://www.scaler.com/topics/images/course_card_image_pybeg.webp","featured_image":"https://www.scaler.com/topics/images/course_featured_image_pybeg.webp","modules_count":16,"description":"Welcome to the free Python course with certificate for beginners, designed to help you kickstart your programming journey. 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