{"id":193,"date":"2013-01-13T08:51:29","date_gmt":"2013-01-13T16:51:29","guid":{"rendered":"http:\/\/martinecker.com\/martincodes\/?p=193"},"modified":"2013-01-13T08:51:49","modified_gmt":"2013-01-13T16:51:49","slug":"cplusplus11-wrap-pattern","status":"publish","type":"post","link":"http:\/\/martinecker.com\/martincodes\/cplusplus11-wrap-pattern\/","title":{"rendered":"C++11 Wrap Pattern"},"content":{"rendered":"

Herb Sutter gave a great talk at the C++ and Beyond 2012 conference on concurrency in C++11<\/a>.<\/p>\n

The talk had an interesting interlude on how to wrap all functions performed on an existing type T in order to inject any desired behavior, and that's what this post is all about.
\nThe pattern basically looks like this:<\/p>\n

\r\ntemplate <typename T>\r\nclass wrap\r\n{\r\npublic:\r\n\twrap(T wrapped = T{}) : m_wrapped{wrapped}\r\n\t{}\r\n\t\r\n\t\/\/ operator() takes any code that accepts a wrapped T\r\n\t\/\/ will usually be a lambda\r\n\ttemplate <typename F>\r\n\tauto operator()(F func) -> decltype(func(m_wrapped))\r\n\t{\r\n\t\t\/\/ do any pre-function call wrapper work \r\n\t\tauto result = func(m_wrapped);\r\n\t\t\/\/ do any post-function call wrapper work \r\n\t\treturn result;\r\n\t}\r\n\t\r\nprivate:\r\n\tT m_wrapped;\r\n\t\/\/ ... other state required by wrapper\r\n};\r\n<\/pre>\n
This wrap class template can now be used by instantiating it and calling operator() on it with an anonymous lambda function that accepts the wrapped type as single argument.<\/p>\n
\r\nwrap<X> wrapper;\r\nwrapper([](X& x)\r\n{\r\n\tx.call_something(foo, bar);\r\n\tx.do_something_else();\r\n\tstd::cout << x.print() << std::endl;\r\n});\r\n<\/pre>\n
A concrete example of using this pattern is a monitor wrapper class, which is similar to the synchronized keyword in Java.
\nA monitor wraps every function call (or a group of function calls) on an existing class in a mutex lock to make the class thread-safe.<\/p>\n
\r\ntemplate <typename T>\r\nclass monitor\r\n{\r\npublic:\r\n\tmonitor(T wrapped = T{}) : m_wrapped{wrapped}\r\n\t{}\r\n\t\r\n\ttemplate <typename F>\r\n\tauto operator()(F func) -> decltype(func(m_wrapped))\r\n\t{\r\n\t\tstd::lock_guard<mutex> lock{m_mutex};\r\n\t\treturn func(m_wrapped);\r\n\t}\r\n\r\nprivate:\r\n\tmutable T m_wrapped;\r\n\tmutable std::mutex m_mutex;\r\n};\r\n<\/pre>\n
Note that m_wrapped is mutable, which is required so that func(m_wrapped) works if func takes the T as a non-const reference T&.<\/p>\n
Mutable in C++11 means thread-safe, which means either bitwise const or internally synchronized. Our mutex lock guard guarantees that by the time the m_wrapped is used in func(m_wrapped) it will be internally sychronized, so making m_wrapped mutable is perfectly acceptable.<\/p>\n
\r\nmonitor<string> s = "start\\n";\r\nstd::vector<std::future<void>> tasks;\r\n\r\n\/\/ start a bunch of tasks\r\nfor (int i = 0; i < 5; ++i)\r\n{\r\n\ttasks.push_back(async([&,i]\r\n\t{\r\n\t\t\/\/ single transaction, synchronized modification of s\r\n\t\ts([=](string& s)\r\n\t\t{\r\n\t\t\ts += "transaction " + to_string(i) + " of 5";\r\n\t\t\ts += '\\n';\r\n\t\t});\r\n\t\t\r\n\t\t\/\/ do some more work\r\n\t\t\r\n\t\t\/\/ single transaction, synchronized read of s\r\n\t\ts([](string& s)\r\n\t\t{\r\n\t\t\tstd::cout << s;\r\n\t\t});\r\n\t});\r\n}\r\n\r\n\/\/ join all tasks\r\nfor (auto& task : tasks)\r\n\ttask.wait();\r\n<\/pre>\n
Note that T used in wrap or monitor could also be a reference, if you don't want to make a copy of T. This is useful for capturing stateful objects, such as std::cout.
\nSo it is perfectly valid to do:<\/p>\n
\r\nmonitor<ostream&> sync_cout{std::cout};\r\n\r\nsync_cout([=](ostream& cout)\r\n{\r\n\tcout << "Writing stuff to cout..." << std::endl;\r\n\tcout << "in a synchronized way" << std::endl;\r\n});\r\n<\/pre>\n
That's a wrap, folks! \ud83d\ude42<\/p>\n","protected":false},"excerpt":{"rendered":"
Herb Sutter gave a great talk at the C++ and Beyond 2012 conference on concurrency in C++11. The talk had an interesting interlude on how to wrap all functions performed on an existing type T in order to inject any … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_mi_skip_tracking":false,"footnotes":""},"categories":[10],"tags":[23,14,16,15],"class_list":["post-193","post","type-post","status-publish","format-standard","hentry","category-c","tag-c","tag-c11","tag-pattern","tag-wrap"],"yoast_head":"\nC++11 Wrap Pattern - Martin Codes<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"http:\/\/martinecker.com\/martincodes\/cplusplus11-wrap-pattern\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"C++11 Wrap Pattern - Martin Codes\" \/>\n<meta property=\"og:description\" content=\"Herb Sutter gave a great talk at the C++ and Beyond 2012 conference on concurrency in C++11. 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