In C++, there are two main ways to reuse existing classes: inheritance and composition. While both techniques allow you to reuse code and functionality, they differ in how they achieve this and have distinct advantages and disadvantages.

Inheritance involves creating a new class that is a child of an existing class. The new class, called the derived class or subclass, inherits the properties of the base class, called the parent class or superclass. In C++, inheritance is achieved using the public, private, or protected access specifiers. Public inheritance allows the derived class to access the public members of the base class, protected inheritance allows access to both protected and public members, and private inheritance restricts access to the base class's members.

Composition involves creating a new class that contains an object of an existing class as a member variable. The new class, called the composite class or wrapper class, has access to the public members of the contained class and can use them to provide its own functionality.

Here are some of the differences between inheritance and composition:

1. Code Reuse: Inheritance is primarily used to reuse code, whereas composition is used to reuse functionality. Inheritance allows the derived class to inherit all the public members of the base class, including its member variables and member functions, which can be used by the derived class as is or overridden. Composition allows the composite class to use the public members of the contained class to implement its own functionality.

2. Coupling: Inheritance can create tight coupling between the derived class and the base class, which can make the code harder to maintain and modify. Changes made to the base class can affect the behavior of the derived class, which can create unintended consequences. Composition, on the other hand, creates looser coupling between the composite class and the contained class, which makes it easier to modify and maintain the code.

3. Flexibility: Composition is generally more flexible than inheritance. With composition, you can choose which functionality to expose from the contained class to the composite class. This allows you to create a more specialized interface that fits the needs of the composite class. Inheritance, on the other hand, is less flexible because it forces the derived class to conform to the interface of the base class.

4. Polymorphism: Inheritance allows you to use polymorphism, which is the ability of objects of different classes to be treated as if they are of the same type. Polymorphism is achieved through virtual functions, which allow the derived class to override the base class's behavior. Composition does not provide built-in support for polymorphism, but it can be achieved through interface classes or abstract classes.

In summary, inheritance and composition are two different techniques for reusing code and functionality in C++. Inheritance is primarily used for code reuse and can create tight coupling between classes, while composition is used for functionality reuse and provides more flexibility and looser coupling. The choice between inheritance and composition depends on the specific requirements of the program and the design goals.