Re: simple question about BSTR and CString
"Daniel James" <wastebasket@nospam.aaisp.org> wrote in message
news:VA.000015a2.2f3f80bb@nospam.aaisp.org...
In some cases, CString will actually point to the same memory.
This functionality has been added as an optimization. But it's okay
because CString will automatically ensure a new copy is allocated if
you need to change the contents of the string, and that everything
is cleaned up okay.
That used to happen when you assigned one CString object to another in
VC6; the two CStrings could share memory until one of them was altered.
This is a space-optimization known as "Copy On Write" or COW.
COW never happened when assigning a BSTR or simple C character array to
a CString; it could only happen with two (or more) CStrings as each
object sharing the data needs to be able to make a copy internally if
its value changes.
Yes, I know. I never meant to suggest that it would happen when assigning
non-CString values.
The trouble with COW is that it's very difficult to make it efficient,
especially if strings are used in more than one thread. For this reason
the libraries shipped with current Visual C++ products no longer use
COW, so each CString always has its own copy of the data.
The current runtimes do, however, use an optimization called the "small
string optimization" (in which heap allocation is not required for
strings below a certain size). This turns out to be a more effective
optimization in most cases, especially in multi-threaded applications.
I had to write my own CString class for a project. I also implemented the
optimization where a simple assignment would simply point to the same data
and increment an instance counter. It seemed like a good optimization to me
as I found that many times when strings are assigned, there is no reason to
make a copy. However, I didn't need to worry about multithreading in that
project.
It's not all that long ago that I examined the CString source. So, if
they've changes this as you've described, it must have been fairly recent.
--
Jonathan Wood
SoftCircuits Programming
http://www.softcircuits.com