When can I use a forward declaration?

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.

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