It's perfectly possible to write threadsafe fast COW containers. The trick is to separate mutatable and immutable objects. Right now I'm writing const_containers library (inspired by const_string). For example, I have two classes for strings: string_builder and const_string. string_builder (it is noncopyable, btw) is used to create strings which then are passed as refcounted const_strings: ========================================== const_string<char> doSomething() { string_builder<char> str; str+="blah-blah"+another_string; return const_string<char>(str.yield()); } string_builder str_again(doSomething().yieled()); str_again+="something"; ========================================== 'yield' method is used to transfer ownership of memory block (without any coping if block is owned exclusively). Robert Mathews wrote:
Hmm .... you know that COW implementations of strings tend not to be thread-safe, or if they are thread-safe, deadly slow?
For example DinkumWare switched from a COW implementation of strings to a small string optimization in the 7.1 release of VC. We run a million-plus transactions through our system every day, and monitor every crash. We've observed two things from this change: - crashes due to accidently passing strings between threads have disappeared. - speed has not changed detectably. In fact, if anything, seems to have increased slightly.
So, I'd never use a COW implementation again in a multithreaded environment - it's just simply impossible to make thread safe.
"Maksym Motornyy" <mmotorny@yandex.ru> wrote in message news:d7hvcu$usf$1@sea.gmane.org...
There's a pretty nice implementation of COW objects in the Adobe Open Source library (http://opensource.adobe.com). You might take a look at that. I saw it. Honestly, after I implemented COW myself :) Do you want to say it's no sence to double existing things?
A compiler that implements NRVO (Named Return Value Optimization) can return an object (essentially) by const reference without any special COW wrapper classes. Agree, this is not the best example for such a compiler. AFAIK VC++ doesn't implement this optimization.
Also, some C++ Standard Library implementations use a reference-counted std::string implementation, so this extra effort would be wasted. STLport and Dinkumware (I suppose most widely used) doesn't belong to that implementations. That's why personally I require COW-wrapper.
-- With respect, Alex Besogonov (alexy@izh.com)