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Somenath Mukhopadhyay
Somenath Mukhopadhyay

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Latches in C++ 20 concurrency - just like the CountdownLatch of Java concurrency package...

#cpp #concurrentprogramming #cpp20

Multithreaded programming is inherently difficult. One of the reasons is that we can't have control over how a thread will start and finish - in which order - it all depends upon the thread scheduling algorithm of the OS. This makes the reproduction of test cases difficult. Moreover, there are race conditions and deadlocks.

When I was teaching the Countdown latch - a thread synchronization technique used in the Java Concurrency package, there was none like that available in C++. I am happy to see that the concept of latch is introduced in C++20.

So...

What is a Latch in C++?

  • A synchronization primitive was introduced in C++20.
  • It allows one or more threads to wait for a certain number of operations to complete before proceeding.
  • Acts like a countdown counter that blocks threads until it reaches zero.

Good to know that the C++ team is trying to catch up with Java...

Here we go...

My experimentation with C++ latches.

I taught my young son Ridit about the Java Countdown latch three years ago.

Good to see the C++ team is making the standard library more powerful day by day.

Here is the C++ source code of my experimentation.

The following C++ code needs C++20 to compile and execute.

/* * Student.h * * Created on: Jan 7, 2024 * Author: som */ #ifndef STUDENT_H_ #define STUDENT_H_ #include <iostream> #include <string> #include <thread> #include <latch> using namespace std; class Student { private: string name; int timeToFinish; public: Student(string name, int timeToFinish){ this->name = name; this->timeToFinish = timeToFinish; } virtual ~Student(){ } void task(std::latch& l){ cout<< this->name << " is Starting the task"<<endl; this_thread::sleep_for(chrono::milliseconds(this->timeToFinish)); cout<< this->name <<" finished the task"<<endl; l.count_down(); } }; #endif /* STUDENT_H_ */
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/* * classETC.h * * Created on: Jan 7, 2024 * Author: som */ #ifndef CLASSETC_H_ #define CLASSETC_H_ #include <thread> #include <latch> #include "Student.h" class classETC { public: classETC(){ } virtual ~classETC(){ } void giveTaskToStudent(latch& l){ thread riditT(&Student::task, new Student("Ridit", 1000),ref(l)); thread ishanT(&Student::task, new Student("Ishan", 3000), ref(l)); thread rajdeepT(&Student::task, new Student("Rajdeep", 900), ref(l)); cout<<"Teacher is waiting for all the students to finish their task"<<endl; riditT.join(); ishanT.join(); rajdeepT.join(); l.wait(); cout<<"All students submitted their task... Teacher is leaving the class"<<endl; } }; #endif /* CLASSETC_H_ */
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Main: 

#include <iostream> #include <latch> #include <thread> #include <chrono> #include "classETC.h" using namespace std; int main() { cout << "C++ Version" << endl; // prints C++ Version if (__cplusplus == 202101L) cout << "C++23"; else if(__cplusplus == 202002L) cout << "C++20\n"; else if (__cplusplus == 201703L) cout << "C++17\n"; else if (__cplusplus == 201402L) cout << "C++14\n"; else if (__cplusplus == 201103L) cout << "C++11\n"; else if (__cplusplus == 199711L) cout << "C++98\n"; else cout << "pre-standard C++\n"; latch l(3); classETC etc; etc.giveTaskToStudent(l); return 0; }
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Explanation of the code:

Have a look at the Student class. Each student will do his work in a

background thread. As there are 3 students, the latch will be initiated with 3.

After completing the task, a student will reduce the latch counter by 1.

So when all the students will complete the task, the latch - on which

the main thread was blocked, will become zero and it will unblock the main thread.
The C++ latches work exactly the way the Java Countdown Latch
works.

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