What is the purpose of a forward declaration of a class?

In programming languages like C++ and C#, forward declaration refers to the process of declaring the name and basic properties of a class, function, or other programming element, before providing its full definition. This can be useful in situations when two or more classes or functions depend on each other, and the compiler needs to know their basic structure before processing their full definitions.

The main purpose of using a forward declaration of a class is to resolve circular dependencies, which can occur when two classes need to reference each other, or when there's a hierarchy of classes that have mutual dependencies. By providing the forward declaration, the compiler will know the existence of the class and its basic structure, even before its full definition is provided.

Let's take an example in C++ to illustrate the purpose of forward declaration in a class: ```cpp class ClassB; // Forward declaration of ClassB class ClassA { public: ClassA(ClassB& b) : _b(b) {} private: ClassB& _b; }; class ClassB { public: ClassB(ClassA& a) : _a(a) {} private: ClassA& _a; }; int main() { ClassB someB(someA); ClassA someA(someB); } ``` In this example, ClassA has a reference to ClassB, and ClassB has a reference to ClassA. Since both classes need to know about each other, a forward declaration of ClassB at the beginning of the code is necessary. By declaring ClassB with its forward declaration, the compiler can correctly process the ClassA definition, even though the full definition of ClassB has not yet been provided.

It is important to note that forward declarations can only be used to declare the existence of a class and its basic structure, but they cannot be used to access or manipulate the full members and methods of the class, as the full implementation is not known in the forward declaration. Class implementations should be provided in the correct order or in separate header/source files to avoid issues related to circular dependencies and to enable full use of class methods and properties.