• ... decouple classes that depend on each other.
• ... improve incremental build times considerably when changing the header file declaring the class where the forward declaration was used.
• ... increase the portability of your code.

AddressBook.h

//#include "Contact.h" // NO! Do this in the .cpp file instead. Unless you
                       // really need the definition of the class.

// Forward type declaration
class Contact;         // YES!

class AddressBook
{
public:
    // ...

/** Ways you can use the declaration without having the definition **/

    // Member functions input parameters.
    void addContact(unsigned int id, Contact const& newContact);
    void addContact(unsigned int id, Contact const* newContact);

    // Member functions output parameters.
    void getContact(unsigned int id, Contact& contact);
    void getContact(unsigned int id, Contact* contact);

    // These cases are a bit more obscure than the others, but it is legal and
    // still works.
    void addContact(unsigned int id, Contact newContact);
    Contact getContact(unsigned int id);

private:

    // Member variables
    Contact* m_contactPointer;
    Contact& m_contactRef;

/** These some cases where the compiler needs the definition. **/

    Contact::Address m_address;
    static const size_t ms_contactSize = sizeof(Contact);
    Contact m_contact;
    Contact m_contactArray[10];

/** Here are some cases that often work with different STL implementations and
    compilers, but are illegal. **/

    std::vector<Contact> m_contacts;
    std::auto_ptr<Contact> m_contactAutoPtr;
};

• Enums and typedefs types cannot be forward-declared.
• You will be able to use your forward-declared type as a template parameter if and only if the templated type doesn't need the full definition.
• You will need the full definition to get the size of a type, use any constructors or detructors, or access members (types scoped at class level).

Labels: c++, compiler, optimization, pay it forward

# posted by Jean-Yves Boudreau @ 2:45 p.m.