{"id":4797,"date":"2016-06-21T06:16:17","date_gmt":"2016-06-21T06:16:17","guid":{"rendered":"https:\/\/www.modernescpp.com\/index.php\/the-atomic-boolean\/"},"modified":"2016-06-21T06:16:17","modified_gmt":"2016-06-21T06:16:17","slug":"the-atomic-boolean","status":"publish","type":"post","link":"https:\/\/www.modernescpp.com\/index.php\/the-atomic-boolean\/","title":{"rendered":"The Atomic Boolean"},"content":{"rendered":"

The remaining atomics – in contrast to std::atomic_flag – are partial or full specializations of the class template std::atomic<\/span>. Let’s start with std::atomic<bool>.<\/span><\/p>\n

<\/p>\n

 <\/p>\n

std::atomic<bool><\/h2>\n

std::atomic<bool><\/span> has a lot more to offer than std::atomic_flag<\/span>. It can explicitly be set to true<\/span> or false.<\/span> That’s enough to synchronize two threads. So I can simulate condition variables<\/a> with atomic variables.<\/p>\n

Let’s first have a look at condition variables.<\/p>\n

<\/p>\n

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 1\r\n 2\r\n 3\r\n 4\r\n 5\r\n 6\r\n 7\r\n 8\r\n 9\r\n10\r\n11\r\n12\r\n13\r\n14\r\n15\r\n16\r\n17\r\n18\r\n19\r\n20\r\n21\r\n22\r\n23\r\n24\r\n25\r\n26\r\n27\r\n28\r\n29\r\n30\r\n31\r\n32\r\n33\r\n34\r\n35\r\n36\r\n37\r\n38\r\n39\r\n40\r\n41\r\n42\r\n43\r\n44\r\n45\r\n46\r\n47\r\n48\r\n49<\/pre>\n<\/td>\n
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\/\/ conditionVariable.cpp<\/span>\r\n\r\n#include <condition_variable><\/span>\r\n#include <iostream><\/span>\r\n#include <thread><\/span>\r\n#include <vector><\/span>\r\n\r\nstd::vector<int<\/span>> mySharedWork;\r\nstd::mutex mutex_;\r\nstd::condition_variable condVar;\r\n\r\nbool<\/span> dataReady;\r\n\r\nvoid<\/span> waitingForWork(){\r\n    std::cout << \"Waiting \"<\/span> << std::endl;\r\n    std::unique_lock<std::mutex> lck(mutex_);\r\n    condVar.wait(lck,[]{return<\/span> dataReady;});\r\n    mySharedWork[1]= 2;\r\n    std::cout << \"Work done \"<\/span> << std::endl;\r\n}\r\n\r\nvoid<\/span> setDataReady(){\r\n    mySharedWork={1,0,3};\r\n    {\r\n        std::lock_guard<std::mutex> lck(mutex_);\r\n        dataReady=true;\r\n    }\r\n    std::cout << \"Data prepared\"<\/span> << std::endl;\r\n    condVar.notify_one();\r\n}\r\n\r\nint<\/span> main(){\r\n    \r\n  std::cout << std::endl;\r\n\r\n  std::thread<\/span> t1(waitingForWork);\r\n  std::thread<\/span> t2(setDataReady);\r\n\r\n  t1.join();\r\n  t2.join();\r\n  \r\n   for<\/span> (auto<\/span> v: mySharedWork){\r\n      std::cout << v << \" \"<\/span>;\r\n  }\r\n      \r\n  \r\n  std::cout << \"\\n\\n\"<\/span>;\r\n  \r\n}\r\n<\/pre>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n
 <\/p>\n
And now the pendant with atomic booleans.<\/p>\n
<\/p>\n
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 1\r\n 2\r\n 3\r\n 4\r\n 5\r\n 6\r\n 7\r\n 8\r\n 9\r\n10\r\n11\r\n12\r\n13\r\n14\r\n15\r\n16\r\n17\r\n18\r\n19\r\n20\r\n21\r\n22\r\n23\r\n24\r\n25\r\n26\r\n27\r\n28\r\n29\r\n30\r\n31\r\n32\r\n33\r\n34\r\n35\r\n36\r\n37\r\n38\r\n39\r\n40\r\n41\r\n42\r\n43\r\n44<\/pre>\n<\/td>\n
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\/\/ atomicCondition.cpp<\/span>\r\n\r\n#include <atomic><\/span>\r\n#include <chrono><\/span>\r\n#include <iostream><\/span>\r\n#include <thread><\/span>\r\n#include <vector><\/span>\r\n\r\nstd::vector<int<\/span>> mySharedWork;\r\nstd::atomic<bool<\/span>> dataReady(false);\r\n\r\nvoid<\/span> waitingForWork(){\r\n    std::cout << \"Waiting \"<\/span> << std::endl;\r\n    while<\/span> ( !dataReady.load() ){             \/\/ (3)<\/span><\/strong>\r\n        std::this_thread::sleep_for(std::chrono::milliseconds(5));\r\n    }\r\n    mySharedWork[1]= 2;                      \/\/ (4)<\/span><\/strong>\r\n    std::cout << \"Work done \"<\/span> << std::endl;\r\n}\r\n\r\nvoid<\/span> setDataReady(){\r\n    mySharedWork={1,0,3};                    \/\/ (1)<\/span><\/strong>\r\n    dataReady= true;                         \/\/ (2)<\/span><\/strong>\r\n    std::cout << \"Data prepared\"<\/span> << std::endl;\r\n}\r\n\r\nint<\/span> main(){\r\n    \r\n  std::cout << std::endl;\r\n\r\n  std::thread<\/span> t1(waitingForWork);\r\n  std::thread<\/span> t2(setDataReady);\r\n\r\n  t1.join();\r\n  t2.join();\r\n  \r\n  for<\/span> (auto<\/span> v: mySharedWork){\r\n      std::cout << v << \" \"<\/span>;\r\n  }\r\n      \r\n  \r\n  std::cout << \"\\n\\n\"<\/span>;\r\n  \r\n}\r\n<\/pre>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n
 <\/p>\n
What guarantees that line 17 will be executed after line 14? Or, to say it more generally, the thread t1 will execute mySharedWork[1]= 2<\/span> (line 17) after thread t2 had executed mySharedWork={1,0,3}<\/span> (line 22). Now it gets more formal.<\/p>\n