Incomplete class declarations (C++ only)

An incomplete class declaration is a class declaration that does not define any class members. You cannot declare any objects of the class type or refer to the members of a class until the declaration is complete. However, an incomplete declaration allows you to make specific references to a class prior to its definition as long as the size of the class is not required.

For example, you can define a pointer to the structure first in the definition of the structure second. Structure first is declared in an incomplete class declaration prior to the definition of second, and the definition of oneptr in structure second does not require the size of first:

struct first;           // incomplete declaration of struct first

struct second           // complete declaration of struct second
      first* oneptr;    // pointer to struct first refers to
                        // struct first prior to its complete
                        // declaration

      first one;        // error, you cannot declare an object of
                        // an incompletely declared class type
      int x, y;

struct first            // complete declaration of struct first
      second two;       // define an object of class type second
      int z;

However, if you declare a class with an empty member list, it is a complete class declaration. For example:

class X;                // incomplete class declaration
class Z {};             // empty member list
class Y
      X yobj;           // error, cannot create an object of an
                        // incomplete class type
      Z zobj;           // valid

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