As an expert in the field of object-oriented programming, I can tell you that the concept of an abstract class is fundamental to the design and structure of many software systems. Abstract classes serve several important purposes in software development, and understanding these can lead to more robust, flexible, and maintainable code. Here's a detailed look at why abstract classes are essential:

1. Defining a Contract: The most significant role of an abstract class is to define a contract for its subclasses. It specifies a set of methods that any subclass must implement. This ensures a level of uniformity across different subclasses, which can be crucial for a system's integrity.

2. Encapsulation of Common Behavior: Abstract classes encapsulate common behavior that can be shared among multiple subclasses. This prevents code duplication and promotes the reuse of code, which is a fundamental principle of software development.

3. Providing a Common Interface: By defining a common interface through pure virtual functions, abstract classes allow for polymorphism. This means that different subclasses can be treated as instances of the abstract class, which can simplify the code that uses these subclasses.

4. Enforcing a Base Structure: Abstract classes can enforce a base structure for a group of related classes. This is particularly useful when the hierarchy of classes is complex, as it helps to maintain a clear and consistent structure.

5. Defining Partial Implementations: Sometimes, an abstract class can provide a partial implementation for certain methods, which can be shared among subclasses. This can reduce the amount of code that each subclass needs to write.

6. Supporting the Template Method Pattern: Abstract classes are often used to implement the template method design pattern, where the steps of an algorithm are defined in the abstract class, but some steps can be defined by the subclasses.
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Preventing Instantiation: Abstract classes cannot be instantiated on their own, which means they can't be created directly. This prevents misuse of the class as a standalone object and ensures that only fully implemented subclasses are used.
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Facilitating Interface Segregation: Abstract classes help in adhering to the interface segregation principle, which states that no client should be forced to depend on methods it does not use. By creating specific abstract classes for different clients, you can provide them with only the methods they need.
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Promoting Loose Coupling: By using abstract classes, you can promote loose coupling between classes. This is because the concrete classes that implement the abstract class are not tightly bound to the clients that use them, as long as they adhere to the abstract class interface.
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Evolution of Frameworks and Libraries: Abstract classes are crucial for the development of frameworks and libraries. They provide a foundation upon which other developers can build, extend, and customize the functionality to suit their specific needs.

In summary, abstract classes are a powerful tool in object-oriented programming that can lead to more organized, flexible, and maintainable code. They provide a way to define common behavior, enforce a structure, and ensure that all subclasses adhere to a certain set of rules.

Now, let's move on to the translation of the above explanation into Chinese.

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Only a subclass which defines all of the pure virtual functions can be instantiated. The purpose of an abstract class is to define some common behavior that can be inherited by multiple subclasses, without implementing the entire class.Sep 8, 2011read more >>