Put yourself in the compiler's position: when you forward declare a type, all the compiler knows is that this type exists; it knows nothing about its size, members, or methods. This is why it's called an incomplete type. Therefore, you cannot use the type to declare a member, or a base class, since the compiler would need to know the layout of the type.
Assuming the following forward declaration.
class X;
Here's what you can and cannot do.
What you can do with an incomplete type:
Declare a member to be a pointer or a reference to the incomplete type:
class Foo { X *p; X &r; };Declare functions or methods which accept/return incomplete types:
void f1(X); X f2();Define functions or methods which accept/return pointers/references to the incomplete type (but without using its members):
void f3(X*, X&) {} X& f4() {} X* f5() {}
What you cannot do with an incomplete type:
Use it as a base class
class Foo : X {} // compiler error!Use it to declare a member:
class Foo { X m; // compiler error! };Define functions or methods using this type
void f1(X x) {} // compiler error! X f2() {} // compiler error!Use its methods or fields, in fact trying to dereference a variable with incomplete type
class Foo { X *m; void method() { m->someMethod(); // compiler error! int i = m->someField; // compiler error! } };
When it comes to templates, there is no absolute rule: whether you can use an incomplete type as a template parameter is dependent on the way the type is used in the template.
For instance, std::vector<T> requires its parameter to be a complete type, while boost::container::vector<T> does not. Sometimes, a complete type is required only if you use certain member functions; this is the case for std::unique_ptr<T>, for example.
A well-documented template should indicate in its documentation all the requirements of its parameters, including whether they need to be complete types or not.
