Editing Copy-on-write (section)

===Examples===
The [[String (C++)|string]] class provided by the [[C++ standard library]] was specifically designed to allow copy-on-write implementations in the initial C++98 standard,<ref name="meyers">{{cite book |first=Scott |last=Meyers |author-link=Scott Meyers |date=2012 |title=Effective STL |publisher=Addison-Wesley |pages=64–65 |isbn=9780132979184 }}</ref> but not in the newer C++11 standard:<ref>{{cite web |title=Concurrency Modifications to Basic String |url=https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2534.html |website=Open Standards |access-date=10 November 2023 |archive-date=10 November 2023 |archive-url=https://web.archive.org/web/20231110024434/https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2534.html |url-status=live }}</ref>
<syntaxhighlight lang="cpp">
std::string x("Hello");

std::string y = x;  // x and y use the same buffer.

y += ", World!";    // Now y uses a different buffer; x still uses the same old buffer.
</syntaxhighlight>

In the [[PHP]] programming language, all types except references are implemented as copy-on-write. For example, strings and arrays are passed by reference, but when modified, they are duplicated if they have non-zero reference counts. This allows them to act as value types without the performance problems of copying on assignment or making them immutable.<ref>{{cite web |last1=Pauli |first1=Julien |last2=Ferrara |first2=Anthony |last3=Popov |first3=Nikita |title=Memory management |date=2013 |url=https://www.phpinternalsbook.com/php5/zvals/memory_management.html#reference-counting-and-copy-on-write |website=PhpInternalsBook.com |access-date=10 November 2023 |archive-date=10 November 2023 |archive-url=https://web.archive.org/web/20231110024435/https://www.phpinternalsbook.com/php5/zvals/memory_management.html#reference-counting-and-copy-on-write |url-status=live }}</ref>

In the [[Qt (software)|Qt]] framework, many types are copy-on-write ("implicitly shared" in Qt's terms). Qt uses atomic [[compare-and-swap]] operations to increment or decrement the internal reference counter. Since the copies are cheap, Qt types can often be safely used by [[Multithreading (computer architecture)|multiple threads]] without the need of [[Lock (computer science)|locking mechanisms]] such as [[Mutual exclusion|mutexes]]. The benefits of COW are thus valid in both single- and multithreaded systems.<ref>{{cite web |title=Threads and Implicitly Shared Classes |website=Qt Project |url=https://doc.qt.io/qt-5/threads-modules.html#threads-and-implicitly-shared-classes |access-date=10 November 2023 |archive-date=3 December 2023 |archive-url=https://web.archive.org/web/20231203002914/https://doc.qt.io/QT-5/threads-modules.html#threads-and-implicitly-shared-classes |url-status=live }}</ref>