Re: return string - no double allocation

"Alf P. Steinbach" <>
Tue, 24 Mar 2009 11:20:42 +0100
* Gernot Frisch:

Yes, you can do reference counting on the pointer. A simple
implementation would use boost::shared_ptr, but it does have some
overhead. A smarter implementation would use boost::intrusive_ptr, or
equivalent code.

I never used any of the above techniques. <ash on my head> Can you
briefly explain how I would implement that in my class:

static char* NO_DATA=""; // like NULL, but c_str() can return this
class DGStr
   DGStr() :m_len(0),m_dat(NO_DATA) {}
   DGStr(const DGStr& s):m_len(0),m_dat(NO_DATA) {*this=s;}
   DGStr& operator=(const DGStr& s)
       strcpy(m_dat, s.m_dat);
   ~DGStr() {if(m_dat!=NO_DATA) delete[] m_dat;}

   const char* c_str()const {return m_dat;}

   // very simple non optimized code
   void Alloc(int n)
       char* pD=new char[n+1];
       if(m_dat != NO_DATA)
           memcpy(pD, m_dat, m_len+1);
           delete[] m_dat;
       m_dat = pD;
// data
   char* m_dat;
   int m_len;

No explanation because it gets real complicated real fast and I've given a link
to an implementation, and since this is a learning exercise it's important to
focus on the most important thing to learn.

Namely, to first make the code correct, and only then (if at all) optimize.

Correctness issues: you're missing a constructor to create a DGStr with other
than empty string; operator= is missing a result specification; and Alloc
performs a needless copying, presumably to handle self-assignment but that
should be handled in some different way.

Design: it's generally a bad idea to implement a copy constructor in terms of
assignment. Instead implement assignment in terms of copy construction.

Style: never use all uppercase for non-macros (except for idiomatic usage),
always use it for macros.

Cheers & hth.,

- Alf

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