Today I am going to present you second article about new C++11 Multithreading feature. I am going to present you how we can pass parameters to thread
function (by value and by reference) as well as show you how to delay thread processing for selected amount of time.
In previous C++ Multithreading article we presented how to create thread function and run second thread in parallel to main thread. Now we are extending that example with usage of another multithreding features. Let take a look on the new example:
Output of that example could be (it can be little other output on your machine because we are working with Multithreading which is not protected by phenomenon called race condition which I will explain in one of another articles): In point I and II we are creating new thread which will invoke function print() from point Ia. Difference to the example from previus article is that we are passing two parameters to our function. First parameter firstParam is passed by value, second parameter secondParam is passed by reference.
Notice that while we are passing parameters by reference to thread function we need to use std::ref() function. It is because, std::thread constructor creates new function object based on names of our function, passed parameters and std::bind() function within its body. Because we are using std::bind function we are using copies of our parameters, so without using std::ref we would create reference to copied parameter which we passed to std::thread constructor.
In such situation our when we would change value of our secondParam in the body of print() second thread's function, it would not be visible in main thread's function because inside print() function we would operate on reference to copy of secondParam object. You can experiment with it on your machine. Try to remove std::ref function and pass secondParam as normal reference. Compare your and ours output. You should not see value: startSecondParam_modified for secondParam parameter inside main thread (in second thread this value is applied, of course).
In point IIa and III we are printing values of firstParam and secondParam for both threads with modifications during processing. You can see that changes to secondParam value done in second thread are visible in main thread as well (because we are properly passing secondParam to second thread by reference).
One more new feature of C++11 multithreading which is introduced in this example you can notice in point IV. Function std::thread::sleep_for() is being used for stoping working of thread which it is invoked on for selected amount of time. We are using std::chrono::milliseconds() function (new function for C++11 as well) to sleep our main thread for 10 ms (in order to wait for modification of secondParam in second thread). This is simple example of delaying processing inside particular thread.
Point V should be known from previous article. We are waiting for finishing second thread before leavin from our main application (main thread).
Code of this example can be found on our GitHub account here: https://github.com/xmementoit/CppAdventureExamples/tree/master/multithreading/passParamsToThreadFunction
In previous C++ Multithreading article we presented how to create thread function and run second thread in parallel to main thread. Now we are extending that example with usage of another multithreding features. Let take a look on the new example:
Output of that example could be (it can be little other output on your machine because we are working with Multithreading which is not protected by phenomenon called race condition which I will explain in one of another articles): In point I and II we are creating new thread which will invoke function print() from point Ia. Difference to the example from previus article is that we are passing two parameters to our function. First parameter firstParam is passed by value, second parameter secondParam is passed by reference.
Notice that while we are passing parameters by reference to thread function we need to use std::ref() function. It is because, std::thread constructor creates new function object based on names of our function, passed parameters and std::bind() function within its body. Because we are using std::bind function we are using copies of our parameters, so without using std::ref we would create reference to copied parameter which we passed to std::thread constructor.
In such situation our when we would change value of our secondParam in the body of print() second thread's function, it would not be visible in main thread's function because inside print() function we would operate on reference to copy of secondParam object. You can experiment with it on your machine. Try to remove std::ref function and pass secondParam as normal reference. Compare your and ours output. You should not see value: startSecondParam_modified for secondParam parameter inside main thread (in second thread this value is applied, of course).
In point IIa and III we are printing values of firstParam and secondParam for both threads with modifications during processing. You can see that changes to secondParam value done in second thread are visible in main thread as well (because we are properly passing secondParam to second thread by reference).
One more new feature of C++11 multithreading which is introduced in this example you can notice in point IV. Function std::thread::sleep_for() is being used for stoping working of thread which it is invoked on for selected amount of time. We are using std::chrono::milliseconds() function (new function for C++11 as well) to sleep our main thread for 10 ms (in order to wait for modification of secondParam in second thread). This is simple example of delaying processing inside particular thread.
Point V should be known from previous article. We are waiting for finishing second thread before leavin from our main application (main thread).
Code of this example can be found on our GitHub account here: https://github.com/xmementoit/CppAdventureExamples/tree/master/multithreading/passParamsToThreadFunction
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