Item 15. Lazy Optimization, Part 3: Iterators and References

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Difficulty: 6

In the third part of this miniseries, we examine the effect of references and iterators into a copy-on-write string. Can you spot the issues?

Consider the copy-on-write Optimized::String class from Item 14, but with two new functions: Length() and operator[]() .

 namespace Optimized {   class StringBuf   {   public:     StringBuf();              // start off empty    ~StringBuf();              // delete the buffer     void Reserve( size_t n ); // ensure len >= n     char*    buf;             // allocated buffer     size_t   len;             // length of buffer     size_t   used;            // # chars actually used     unsigned refs;            // reference count   private:     // No copying...     //     StringBuf( const StringBuf& );     StringBuf& operator=( const StringBuf& );   };   class String   {   public:     String();                 // start off empty      ~String();                 // decrement reference count                                 //  (delete buffer if refs==0)       String( const String& );  // point at same buffer and                                 //  increment reference count       void   Append( char );    // append one character  size_t Length() const;    // string length   char&  operator[](size_t);// element access   const char operator[](size_t) const;  // ... operator=() etc. omitted ...     private:       void AboutToModify( size_t n );                                 // lazy copy, ensure len>=n       StringBuf* data_;     };   } 

This allows code such as the following:

 if( s.Length() > 0 ) {   cout << s[0];   s[0] = 'a'; } 

Implement the new members of Optimized::String . Do any of the other members need to be changed because of the new member functions? Explain.

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