Re: Add ... to long strings

"Doug Harrison [MVP]" <>
Thu, 06 Nov 2008 00:44:23 -0600
On Wed, 5 Nov 2008 21:09:37 -0700, "L.Allan" <>

"Tom Serface" <> wrote in message

Wow, from a percentage point of view that is quite a difference, although
unless you had to do it often you likely notice the difference, but it is
significant and I can see where in your case it might show up as a more
noticeable difference.

I was surprised at how much of a difference there was. Actually VERY

CString += seems to work like strcat, where it first has to find the end of
the buffer. strcat is VERY INEFFICIENT for a lot of appends to a long

That's why we old C hackers wrote strcpyEP:

char *
strcpyEP(char *dest, const char *src)
   while (*dest++ = *src++)

   return dest-1;
} /* strcpyEP */

This function returns a pointer to the terminating nul, great for

CString should be better, since it knows how long the actual string
is. There does seem to be a lot of unexpected overhead.

As Joe mentioned, the CString issue is different. It uses a naive linear
reallocation strategy, which is essentially O(N^2). This is pretty rampant
in MFC, but with care, it can be ameliorated. For example, I improved MFC's
CSharedFile by implementing an exponential growth policy in a derived class
HGLOBAL_FILE; the salient functions went like this:

HGLOBAL_File::GrowFile(DWORD dwNewLen)
   if (dwNewLen > m_nBufferSize && m_nBufferSize < DWORD(-1)/2)
      dwNewLen = max(dwNewLen, m_nBufferSize*2);

The comment in the file header was:

// This class implements an exponential regrowth policy, because CSharedFile is
// linear and has horrible performance even if the regrowth block size is the default
// 4096 bytes. We're talking half a second vs. > 10 sec to write 2 MB.

That was circa 1999, but things like this don't change in nature over time,
just in degree. IIRC, I used this to write shell folder ID lists to a
contiguous buffer for later use with shell clipboard operations.

Doug Harrison
Visual C++ MVP

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