What is forward class declaration in C++?

Forward Declaration refers to the beforehand declaration of the syntax or signature of an identifier, variable, function, class, etc. prior to its usage (done later in the program). In this, the class is pre-defined before its use so that it can be called and used by other classes that are defined before this.

When can I forward declare C++?

In C++, classes can be forward-declared if you only need to use the pointer-to-that-class type (since all object pointers are the same size, and this is what the compiler cares about).

Can you forward declare reference?

As long as you don’t need definitions, such as pointers and references, you can avoid using forward declarations. That’s why most of the time you see them in the headers, and implementation files usually extract the corresponding defined headers.

When to use forward declaration in C + +?

In C++, classes can be forward-declared if you only need to use the pointer-to-that-class type (since all object pointers are the same size, and this is what the compiler cares about). This is especially useful inside class definitions, e.g. if a class contains a member that is a pointer (or a reference) to another class.

Can you forward declare bar to the base class?

Thus, you can’t forward declare Bar in any scenario where you then use it to help declare Foo, and it flat-out doesn’t make sense to have a forward declaration that includes the base class — what does that tell you besides nothing? Forward declarations are declarations, not definitions.

Do you need a forward declaration for pointers?

So, anything that requires the declaration of a class (like pointers to that class) need only the forward declaration. However, anything that would require the definition – i.e. would need to know the actual structure of the class – would not work with just the forward declaration.

What happens when a class is not forward declared?

Without the forward declaration, the compiler will produce an error message indicating that the identifier second has been used without being declared. In some object-oriented languages like C++ and Objective-C, it is sometimes necessary to forward-declare classes.