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Forward declaration is a technique used in programming languages, such as C++ or Java, to inform the compiler about the existence of a certain class, function, or variable. In this case, we are dealing with forward declaration of a class, which means informing the compiler that a particular class exists, before its actual definition is provided. This allows the use of the class, even if its full definition is not yet available.

Forward declarations are most commonly used in scenarios, where two or more classes have mutual dependencies on one other, i.e., they reference or use one another. In these cases, providing a forward declaration allows the compiler to understand the relationship between these classes, before their full implementation. This helps to avoid compilation errors that may arise due to circular dependencies.

Let's consider we have two classes, A and B, with mutual dependencies: class A { public: A(B *b) { this->b = b; } void useB(); private: B *b; }; class B { public: B(A *a) { this->a = a; } void useA(); private: A *a; }; In this case, the compiler will not be able to compile the code, because it cannot understand the relationship between A and B, and cannot determine the size of each class. To resolve this issue, we can use forward declaration: class B; // Forward declaration of class B class A { public: A(B *b) { this->b = b; } void useB(); private: B *b; }; class B { public: B(A *a) { this->a = a; } void useA(); private: A *a; }; Now, when the compiler encounters the forward declaration, it will know that class B exists, even though its full definition is yet to appear. This allows the code to compile successfully.

The purpose of a forward declaration of a class is to inform the compiler about the existence and structure of a class before its actual definition is provided. This is useful in situations where mutual dependencies between classes exist, and it helps to resolve potential circular dependencies and compilation issues.