Discussing the nuts and bolts of software development

Wednesday, May 30, 2007


Pay it Forward: Part 1

When writing C++ code, do your compiler a favor and use forward declarations whenever possible: it will pay you back later... and more.

Forward declarations will...


//#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
// ...

/** 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);


// 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;
If the user of this class doesn't use the member functions that require the undefined type, he will not be required to include the header that defines it. The user will only need to do this if he uses those member functions.

Here are some rules of thumb that will help you determine whether a full definition is necessary for a certain type:
Note that a small caveat of this technique is that it sometimes sacrifices some convenience in the cases where the user will almost certainly want to interact with the member functions which require the full definition of the type. In this case it is a judgment call whether to use forward declaration or not, otherwise...

Pay it forward... and wait to be paid back.

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