To implement the book, you might come up with a design like this:
class Image { // for image data public: Image(const string& imageDataFileName); ... }; class AudioClip { // for audio data public: AudioClip(const string& audioDataFileName); ... }; class PhoneNumber {... }; // for holding phone numbers class BookEntry { // for each entry in the public: // address book BookEntry(const string& name, const string& address = "", const string& imageFileName = "", const string& audioClipFileName = ""); ~BookEntry(); // phone numbers are added via this function void addPhoneNumber(const PhoneNumber& number); ... private: string theName; // person's name string theAddress; // their address list<PhoneNumber> thePhones; // their phone numbers Image *theImage; // their image AudioClip *theAudioClip; // an audio clip from them };
Each BookEntry must have name data, so you require that as a constructor argument (see Item 3), but the other fields -- the person's address and the names of files containing image and audio data -- are optional. Note the use of the list class to hold the person's phone numbers. This is one of several container classes that are part of the standard C++ library (see Item 35). A straightforward way to write the BookEntry constructor and destructor is as follows:
BookEntry:: BookEntry(const string& name, const string& address, const string& imageFileName, const string& audioClipFileName) : theName(name), theAddress(address), theImage(0), theAudioClip(0) { if (imageFileName != "") { theImage = new Image(imageFileName); } if (audioClipFileName != "") { theAudioClip = new AudioClip(audioClipFileName); } } BookEntry::~BookEntry() { delete theImage; delete theAudioClip; }
The constructor initializes the pointers theImage and theAudioClip to null, then makes them point to real objects if the corresponding arguments are non-null. The destructor deletes both pointers, thus ensuring that a BookEntry object doesn't give rise to a resource leak. Because C++ guarantees it's safe to delete null pointers, BookEntry's destructor need not check to see if the pointers actually point to something before deleting them.Everything looks fine here, and under normal conditions everything is fine, but under abnormal conditions -- under exceptional conditions -- things are not fine at all.
Consider what will happen if an exception is thrown during execution of this part of the BookEntry constructor:
if (audioClipFileName != "") { theAudioClip = new AudioClip(audioClipFileName); }
An exception might arise because operator new (see Item 8) is unable to allocate enough memory for an AudioClip object. One might also arise because the AudioClip constructor itself throws an exception. Regardless of the cause of the exception, if one is thrown within the BookEntry constructor, it will be propagated to the site where the BookEntry object is being created. Now, if an exception is thrown during creation of the object theAudioClip is supposed to point to (thus transferring control out of the BookEntry constructor), who deletes the object that theImage already points to? The obvious answer is that BookEntry's destructor does, but the obvious answer is wrong. BookEntry's destructor will never be called. Never.
C++ destroys only fully constructed objects, and an object isn't fully constructed until its constructor has run to completion. So if a BookEntry object b is created as a local object,
void testBookEntryClass() { BookEntry b("Addison-Wesley Publishing Company", "One Jacob Way, Reading, MA 01867"); ... }
and an exception is thrown during construction of b, b's destructor will not be called. Furthermore, if you try to take matters into your own hands by allocating b on the heap and then calling delete if an exception is thrown,
void testBookEntryClass() { BookEntry *pb = 0; try { pb = new BookEntry( "Addison-Wesley Publishing Company", "One Jacob Way, Reading, MA 01867"); ... } catch (...) { // catch all exceptions delete pb; // delete pb when an // exception is thrown throw; // propagate exception to } // caller delete pb; // delete pb normally }
you'll find that the Image object allocated inside BookEntry's constructor is still lost, because no assignment is made to pb unless the new operation succeeds. If BookEntry's constructor throws an exception, pb will be the null pointer, so deleting it in the code>catch block does nothing except make you feel better about yourself. Using the smart pointer class auto_ptr<BookEntry> (see Item 9) instead of a raw BookEntry* won't do you any good either, because the assignment to pb still won't be made unless the new operation succeeds.There is a reason why C++ refuses to call destructors for objects that haven't been fully constructed, and it's not simply to make your life more difficult. It's because it would, in many cases, be a nonsensical thing -- possibly a harmful thing -- to do. If a destructor were invoked on an object that wasn't fully constructed, how would the destructor know what to do? The only way it could know would be if bits had been added to each object indicating how much of the constructor had been executed. Then the destructor could check the bits and (maybe) figure out what actions to take. Such bookkeeping would slow down constructors, and it would make each object larger, too. C++ avoids this overhead, but the price you pay is that partially constructed objects aren't automatically destroyed.
Because C++ won't clean up after objects that throw exceptions during construction, you must design your constructors so that they clean up after themselves. Often, this involves simply catching all possible exceptions, executing some cleanup code, then rethrowing the exception so it continues to propagate. This strategy can be incorporated into the BookEntry constructor like this:
BookEntry:: BookEntry(const string& name, const string& address, const string& imageFileName, const string& audioClipFileName) : theName(name), theAddress(address), theImage(0), theAudioClip(0) { try { // this try block is new if (imageFileName != "") { theImage = new Image(imageFileName); } if (audioClipFileName != "") { theAudioClip = new AudioClip(audioClipFileName); } } catch (...) { // catch any exception delete