I am looking for the definition of when I am allowed to do forward declaration of a class in another class's header file:
Am I allowed to do it for a base class, for a class held as a member, for a class passed to member function by reference, etc. ?
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.