What feature corresponds to 'synchronized' in Java?

synchronized in Java can guarantee thread-safety when accessing a shared object. What about C++?

Solution 1:

Use the following in C++:

#include <mutex>

std::mutex _mutex;

void f()
{
     std::unique_lock<std::mutex> lock(_mutex);
     // access your resource here.
}

Solution 2:

Despite this question has been already answered, by the idea of this article I made my version of synchronized keyword using just standard library (C++11) objects:

#include <mutex>
#define synchronized(m) \
    for(std::unique_lock<std::recursive_mutex> lk(m); lk; lk.unlock())

You can test it like:

#include <iostream>
#include <iomanip>
#include <mutex>
#include <thread>
#include <vector>

#define synchronized(m) \
    for(std::unique_lock<std::recursive_mutex> lk(m); lk; lk.unlock())

class Test {
    std::recursive_mutex m_mutex;
public:
    void sayHello(int n) {
        synchronized(m_mutex) {
            std::cout << "Hello! My number is: ";
            std::cout << std::setw(2) << n << std::endl;
        }
    }    
};

int main() {
    Test test;
    std::vector<std::thread> threads;
    std::cout << "Test started..." << std::endl;

    for(int i = 0; i < 10; ++i)
        threads.push_back(std::thread([i, &test]() {
            for(int j = 0; j < 10; ++j) {
                test.sayHello((i * 10) + j);
                std::this_thread::sleep_for(std::chrono::milliseconds(100));
            }
        }));    
    for(auto& t : threads) t.join(); 

    std::cout << "Test finished!" << std::endl;
    return 0;
}

This is just an approximation of synchonized keyword of Java but it works. Without it the sayHello method of the previous example can be implemented as the accepted answer says:

void sayHello(unsigned int n) {
    std::unique_lock<std::recursive_mutex> lk(m_mutex);

    std::cout << "Hello! My number is: ";
    std::cout << std::setw(2) << n << std::endl;
}

Solution 3:

There is no keyword in C++03 equivalent to synchronized in Java . But you can use Mutex to guarantee safety of thread.

Solution 4:

C++ does not have built-in threading or synchronization (yet), you have to use libraries for that. Boost.Thread is a good portable library that is designed to be compatible with the proposed threading facilities in C++0x.